13 Maintaining Data Integrity in Database Applications
In a database application, maintaining data integrity means ensuring that the data in the tables that the application manipulates conform to the appropriate business rules. A business rule specifies a condition or relationship that must always be true or must always be false. For example, a business rule might be that no employee can have a salary over $100,000 or that every employee in the EMPLOYEES
table must belong to a department in the DEPARTMENTS
table. Business rules vary from company to company, because each company defines its own policies about salaries, employee numbers, inventory tracking, and so on.
There are several ways to ensure data integrity, and the one to use whenever possible is the integrity constraint (or constraint).
This chapter supplements this information:
Note:
This chapter applies to only to constraints on tables. Constraints on views do not help maintain data integrity or have associated indexes. Instead, they enable query rewrites on queries involving views, thereby improving performance when using materialized views and other data warehousing features.
See Also:
-
Oracle Database Concepts for information about data integrity and constraints
-
Oracle Database Administrator's Guide for more information about managing constraints
-
Oracle Database SQL Language Reference for the syntactic and semantic information about constraints
-
Oracle Database SQL Language Reference for more information about constraints on views
-
Oracle Database Data Warehousing Guide for information about using constraints in data warehouses
-
How the Correct Data Type Increases Data Integrity for more information about the role that data type plays in data integrity
Topics:
13.1 Enforcing Business Rules with Constraints
Whenever possible, enforce business rules with constraints. Constraints have the advantage of speed: Oracle Database can check that all the data in a table obeys a constraint faster than application code can do the equivalent checking.
Example 13-1 creates a table of departments, a table of employees, a constraint to enforce the rule that all values in the department table are unique, and a constraint to enforce the rule that every employee must work for a valid department.
Example 13-1 Enforcing Business Rules with Constraints
Create table of departments:
DROP TABLE dept_tab; CREATE TABLE dept_tab ( deptname VARCHAR2(20), deptno INTEGER );
Create table of employees:
DROP TABLE emp_tab; CREATE TABLE emp_tab ( empname VARCHAR2(80), empno INTEGER, deptno INTEGER );
Create constraint to enforce rule that all values in department table are unique:
ALTER TABLE dept_tab ADD PRIMARY KEY (deptno);
Create constraint to enforce rule that every employee must work for a valid department:
ALTER TABLE emp_tab ADD FOREIGN KEY (deptno) REFERENCES dept_tab(deptno);
Now, whenever you insert an employee record into emp_tab
, Oracle Database checks that its deptno
value appears in dept_tab
.
Suppose that instead of using a constraint to enforce the rule that every employee must work for a valid department, you use a trigger that queries dept_tab
to check that it contains the deptno
value of the employee record to be inserted into emp_tab
. Because the query uses consistent read (CR), it might miss uncommitted changes from other transactions.
See Also:
-
Oracle Database SQL Language Reference for syntactic and semantic information about constraints
-
Oracle Database Concepts for the complete list of advantages of integrity constraints
-
Oracle Database Concepts for more information about using triggers to enforce business rules
13.2 Enforcing Business Rules with Both Constraints and Application Code
Enforcing business rules with both constraints and application code is recommended when application code can determine that data values are invalid without querying tables. The application code can provide immediate feedback to the user and reduce the load on the database by preventing attempts to insert invalid data into tables.
For Example 13-2, assume that Example 13-1 was run and then this column was added to the table emp_tab
:
empgender VARCHAR2(1)
The only valid values for empgender
are 'M'
and 'F'
. When someone tries to insert a row into emp_tab
or update the value of emp_tab
.empgender
, application code can determine whether the new value for emp_tab
.empgender
is valid without querying a table. If the value is invalid, the application code can notify the user instead of trying to insert the invalid value, as in Example 13-2.
Example 13-2 Enforcing Business Rules with Both Constraints and Application Code
CREATE OR REPLACE PROCEDURE add_employee ( e_name emp_tab.empname%TYPE, e_gender emp_tab.empgender%TYPE, e_number emp_tab.empno%TYPE, e_dept emp_tab.deptno%TYPE ) AUTHID DEFINER IS BEGIN IF UPPER(e_gender) IN ('M','F') THEN INSERT INTO emp_tab VALUES (e_name, e_gender, e_number, e_dept); ELSE DBMS_OUTPUT.PUT_LINE('Gender must be M or F.'); END IF; END; / BEGIN add_employee ('Smith', 'H', 356, 20); END; /
Result:
Gender must be M or F.
13.3 Creating Indexes for Use with Constraints
When a unique or primary key constraint is enabled, Oracle Database creates an index automatically, but Oracle recommends that you create these indexes explicitly. If you want to use an index with a foreign key constraint, then you must create the index explicitly.
When a constraint can use an existing index, Oracle Database does not create an index for that constraint. Note that:
-
A unique or primary key constraint can use either a unique index, an entire nonunique index, or the first few columns of a nonunique index.
-
If a unique or primary key constraint uses a nonunique index, then no other unique or primary key constraint can use that nonunique index.
-
The column order in the constraint and index need not match.
-
The object number of the index used by a unique or primary key constraint is stored in
CDEF$
.ENABLED
for that constraint. No static data dictionary view or dynamic performance view shows this information.If an enabled unique or primary key constraint is using an index, you cannot drop only the index. To drop the index, you must either drop the constraint itself or disable the constraint and then drop the index.
See Also:
-
Oracle Database Administrator's Guide for more information about indexes associated with constraints
-
Oracle Database Administrator's Guide for information about disabling and dropping constraints
-
Oracle Database Administrator's Guide for information about creating indexes explicitly
-
Oracle Database SQL Language Reference for information about creating indexes explicitly
13.4 When to Use NOT NULL Constraints
By default, a column can contain a NULL
value. To ensure that the column never contains a NULL
value, use the NOT
NULL
constraint.
Use a NOT
NULL
constraint in both of these situations:
-
A column must contain a non-
NULL
value.For example, in the table
HR
.EMPLOYEES
, each employee must have an employee ID. Therefore, the columnHR
.EMPLOYEES
.EMPLOYEE_ID
has aNOT
NULL
constraint, and nobody can insert a new employee record intoHR
.EMPLOYEES
without specifying a non-NULL
value forEMPLOYEE_ID
. You can insert a new employee record intoHR
.EMPLOYEES
without specifying a salary; therefore, the columnHR
.EMPLOYEES
.SALARY
does not have aNOT
NULL
constraint. -
You want to allow index scans of the table, or allow an operation that requires indexing all rows.
Oracle Database indexes do not store keys whose values are all
NULL
. Therefore, for the preceding kinds of operations, at least one indexed column must have aNOT
NULL
constraint.
Example 13-3 uses the SQL*Plus command DESCRIBE
to show which columns of the DEPARTMENTS
table have NOT
NULL
constraints, and then shows what happens if you try to insert NULL
values in columns that have NOT
NULL
constraints.
Example 13-3 Inserting NULL Values into Columns with NOT NULL Constraints
DESCRIBE DEPARTMENTS;
Result:
Name Null? Type ----------------------------------------- -------- ------------ DEPARTMENT_ID NOT NULL NUMBER(4) DEPARTMENT_NAME NOT NULL VARCHAR2(30) MANAGER_ID NUMBER(6) LOCATION_ID NUMBER(4)
Try to insert NULL
into DEPARTMENT_ID
column:
INSERT INTO DEPARTMENTS ( DEPARTMENT_ID, DEPARTMENT_NAME, MANAGER_ID, LOCATION_ID ) VALUES (NULL, 'Sales', 200, 1700);
Result:
VALUES (NULL, 'Sales', 200, 1700) * ERROR at line 4: ORA-01400: cannot insert NULL into ("HR"."DEPARTMENTS"."DEPARTMENT_ID")
Omitting a value for a column that cannot be NULL
is the same as assigning it the value NULL
:
INSERT INTO DEPARTMENTS ( DEPARTMENT_NAME, MANAGER_ID, LOCATION_ID ) VALUES ('Sales', 200, 1700);
Result:
INSERT INTO DEPARTMENTS ( * ERROR at line 1: ORA-01400: cannot insert NULL into ("HR"."DEPARTMENTS"."DEPARTMENT_ID")
You can prevent the preceding error by giving DEPARTMENT_ID
a non-NULL
default value.
You can combine NOT
NULL
constraints with other constraints to further restrict the values allowed in specific columns. For example, the combination of NOT
NULL
and UNIQUE
constraints forces the input of values in the UNIQUE
key, eliminating the possibility that data in a new conflicts with data in an existing row.
See Also:
-
Oracle Database SQL Language Reference for more information about
NOT
NULL
constraint -
When to Use Default Column Values for more information about the usage of default column values
13.5 When to Use Default Column Values
When an INSERT
statement does not specify a value for a specific column, that column receives its default value. By default, that default value is NULL
. You can change the default value when you define the column while creating the table or when you alter the column using the ALTER
TABLE
statement.
Note:
Giving a column a non-NULL
default value does not ensure that the value of the column will never have the value NULL
, as the NOT
NULL
constraint does.
Use a default column value in these situations:
-
The column has a
NOT
NULL
constraint.Giving the column a non-
NULL
default value prevents the error that would occur if someone inserted a row without specifying a value for the column. -
There is a most common value for the column.
For example, if most departments in the company are in New York, then set the default value of the column
DEPARTMENTS
.LOCATION
to'NEW YORK'
. -
There is a non-
NULL
value that signifies no entry.For example, if the value zero in the column
EMPLOYEES
.SALARY
means that the salary has not yet been determined, then set the default value of that column to zero.A default column value that signifies no entry can simplify testing. For example, it lets you change this test:
IF (employees.salary IS NOT NULL) AND (employees.salary < 50000)
To this test:
IF employees.salary < 50000
-
You want to automatically record the names of users who modify a table.
For example, suppose that you allow users to insert rows into a table through a view. You give the base table a column named
inserter
(which need not be included in the definition of the view), to store the name of the user who inserted the row. To record the user name automatically, define a default value that invokes theUSER
function. For example:CREATE TABLE audit_trail ( value1 NUMBER, value2 VARCHAR2(32), inserter VARCHAR2(30)
DEFAULT USER
);
See Also:
-
Oracle Database SQL Language Reference for more information about the
INSERT
statement -
Oracle Database SQL Language Reference for more information about the
CREATE
TABLE
statement -
Oracle Database SQL Language Reference for more information about the
ALTER
TABLE
statement -
When to Use NOT NULL Constraints for information about the
NOT
NULL
constraint
13.6 Choosing a Primary Key for a Table (PRIMARY KEY Constraint)
The primary key of a table uniquely identifies each row and ensures that no duplicate rows exist (and typically, this is its only purpose). Therefore, a primary key value cannot be NULL
.
A table can have at most one primary key, but that key can have multiple columns (that is, it can be a composite key). To designate a primary key, use the PRIMARY
KEY
constraint.
Whenever practical, choose as the primary key a single column whose values are generated by a sequence.
The second-best choice for a primary key is a single column whose values are all of the following:
-
Unique
-
Never changed
-
Never
NULL
-
Short and numeric (and therefore easy to type)
Minimize your use of composite primary keys, whose values are long and cannot be generated by a sequence.
See Also:
-
Oracle Database Concepts for general information about primary key constraints
-
Oracle Database SQL Language Reference for complete information about primary key constraints, including restrictions
-
Oracle Database SQL Language Reference for information about sequences
13.7 When to Use UNIQUE Constraints
Use a UNIQUE
constraint (which designates a unique key) on any column or combination of columns (except the primary key) where duplicate non-NULL
values are not allowed. For example:
Unique Key | Primary Key |
---|---|
Employee Social Security Number |
Employee number |
Truck license plate number |
Truck number |
Customer phone number (country code column, area code column, and local phone number column) |
Customer number |
Department name column and location column |
Department number |
Figure 13-1 shows a table with a UNIQUE
constraint, a row that violates the constraint, and a row that satisfies it.
Figure 13-1 Rows That Violate and Satisfy a UNIQUE Constraint
Description of "Figure 13-1 Rows That Violate and Satisfy a UNIQUE Constraint"
See Also:
-
Oracle Database Concepts for general information about
UNIQUE
constraints -
Oracle Database SQL Language Reference for complete information about
UNIQUE
constraints, including restrictions
13.8 Enforcing Referential Integrity with FOREIGN KEY Constraints
When two tables share one or more columns, you use can use a FOREIGN
KEY
constraint to enforce referential integrity—that is, to ensure that the shared columns always have the same values in both tables.
Note:
A FOREIGN
KEY
constraint is also called a referential integrity constraint, and its CONSTRAINT_TYPE
is R
in the static data dictionary views *_CONSTRAINTS
.
Designate one table as the referenced or parent table and the other as the dependent or child table. In the parent table, define either a PRIMARY
KEY
or UNIQUE
constraint on the shared columns. In the child table, define a FOREIGN
KEY
constraint on the shared columns. The shared columns now comprise a foreign key. Defining additional constraints on the foreign key affects the parent-child relationship.
Figure 13-2 shows a foreign key defined on the department number. It guarantees that every value in this column must match a value in the primary key of the department table. This constraint prevents erroneous department numbers from getting into the employee table.
Figure 13-2 shows parent and child tables that share one column, a row that violates the FOREIGN
KEY
constraint, and a row that satisfies it.
Figure 13-2 Rows That Violate and Satisfy a FOREIGN KEY Constraint
Description of "Figure 13-2 Rows That Violate and Satisfy a FOREIGN KEY Constraint"
Topics:
See Also:
-
Oracle Database Concepts for general information about foreign key constraints
-
Oracle Database SQL Language Reference for complete information about foreign key constraints, including restrictions
13.8.1 FOREIGN KEY Constraints and NULL Values
Foreign keys allow key values that are all NULL
, even if there are no matching PRIMARY
or UNIQUE
keys.
-
By default (without any
NOT
NULL
orCHECK
clauses), theFOREIGN
KEY
constraint enforces the match none rule for composite foreign keys in the ANSI/ISO standard. -
To enforce the match full rule for
NULL
values in composite foreign keys, which requires that all components of the key beNULL
or all be non-NULL
, define aCHECK
constraint that allows only allNULL
or all non-NULL
values in the composite foreign key. For example, with a composite key comprised of columnsA
,B
, andC
:CHECK ((A IS NULL AND B IS NULL AND C IS NULL) OR (A IS NOT NULL AND B IS NOT NULL AND C IS NOT NULL))
-
In general, it is not possible to use declarative referential integrity to enforce the match partial rule for
NULL
values in composite foreign keys, which requires the non-NULL
portions of the key to appear in the corresponding portions in the primary or unique key of a single row in the referenced table. You can often use triggers to handle this case.See Also:
Oracle Database PL/SQL Language Reference for more information about triggers
13.8.2 Defining Relationships Between Parent and Child Tables
Several relationships between parent and child tables can be determined by the other types of constraints defined on the foreign key in the child table.
No Constraints on the Foreign Key
When no other constraints are defined on the foreign key, any number of rows in the child table can reference the same parent key value. This model allows nulls in the foreign key.
This model establishes a one-to-many relationship between the parent and foreign keys that allows undetermined values (nulls) in the foreign key. An example of such a relationship is shown in Figure 13-2 between the employee
and department
tables. Each department (parent key) has many employees (foreign key), and some employees might not be in a department (nulls in the foreign key).
NOT NULL Constraint on the Foreign Key
When nulls are not allowed in a foreign key, each row in the child table must explicitly reference a value in the parent key because nulls are not allowed in the foreign key.
Any number of rows in the child table can reference the same parent key value, so this model establishes a one-to-many relationship between the parent and foreign keys. However, each row in the child table must have a reference to a parent key value; the absence of a value (a null) in the foreign key is not allowed. The same example in the previous section illustrates such a relationship. However, in this case, employees must have a reference to a specific department.
UNIQUE Constraint on the Foreign Key
When a UNIQUE
constraint is defined on the foreign key, only one row in the child table can reference a given parent key value. This model allows nulls in the foreign key.
This model establishes a one-to-one relationship between the parent and foreign keys that allows undetermined values (nulls) in the foreign key. For example, assume that the employee table had a column named MEMBERNO
, referring to an employee membership number in the company insurance plan. Also, a table named INSURANCE
has a primary key named MEMBERNO
, and other columns of the table keep respective information relating to an employee insurance policy. The MEMBERNO
in the employee table must be both a foreign key and a unique key:
-
To enforce referential integrity rules between the
EMP_TAB
andINSURANCE
tables (theFOREIGN KEY
constraint) -
To guarantee that each employee has a unique membership number (the
UNIQUE
key constraint)
UNIQUE and NOT NULL Constraints on the Foreign Key
When both UNIQUE
and NOT
NULL
constraints are defined on the foreign key, only one row in the child table can reference a given parent key value, and because NULL
values are not allowed in the foreign key, each row in the child table must explicitly reference a value in the parent key.
This model establishes a one-to-one relationship between the parent and foreign keys that does not allow undetermined values (nulls) in the foreign key. If you expand the previous example by adding a NOT
NULL
constraint on the MEMBERNO
column of the employee table, in addition to guaranteeing that each employee has a unique membership number, you also ensure that no undetermined values (nulls) are allowed in the MEMBERNO
column of the employee table.
13.8.3 Rules for Multiple FOREIGN KEY Constraints
Oracle Database allows a column to be referenced by multiple FOREIGN
KEY
constraints; there is no limit on the number of dependent keys. This situation might be present if a single column is part of two different composite foreign keys.
13.8.4 Deferring Constraint Checks
When Oracle Database checks a constraint, it signals an error if the constraint is not satisfied. To defer checking constraints until the end of the current transaction, use the SET
CONSTRAINTS
statement.
Note:
You cannot use the SET
CONSTRAINTS
statement inside a trigger.
When deferring constraint checks:
-
Select appropriate data.
You might want to defer constraint checks on
UNIQUE
andFOREIGN
keys if the data you are working with has any of these characteristics:-
Tables are snapshots.
-
Some tables contain a large amount of data being manipulated by another application, which might not return the data in the same order.
-
-
Update cascade operations on foreign keys.
-
Ensure that constraints are deferrable.
After identifying the appropriate tables, ensure that their
FOREIGN
,UNIQUE
andPRIMARY
key constraints are createdDEFERRABLE
. -
Within the application that manipulates the data, set all constraints deferred before you begin processing any data, as follows:
SET CONSTRAINTS ALL DEFERRED;
-
(Optional) Check for constraint violations immediately before committing the transaction.
Immediately before the
COMMIT
statement, run theSET
CONSTRAINTS
ALL
IMMEDIATE
statement. If there are any problems with a constraint, this statement fails, and identifies the constraint that caused the error. If you commit while constraints are violated, the transaction rolls back and you get an error message.
In Example 13-4, the PRIMARY
and FOREIGN
keys of the table emp
are created DEFERRABLE
and then deferred.
Example 13-4 Deferring Constraint Checks
DROP TABLE dept; CREATE TABLE dept ( deptno NUMBER PRIMARY KEY, dname VARCHAR2 (30) ); DROP TABLE emp; CREATE TABLE emp ( empno NUMBER, ename VARCHAR2(30), deptno NUMBER, CONSTRAINT pk_emp_empno PRIMARY KEY (empno) DEFERRABLE, CONSTRAINT fk_emp_deptno FOREIGN KEY (deptno) REFERENCES dept(deptno) DEFERRABLE); INSERT INTO dept (deptno, dname) VALUES (10, 'Accounting'); INSERT INTO dept (deptno, dname) VALUES (20, 'SALES'); INSERT INTO emp (empno, ename, deptno) VALUES (1, 'Corleone', 10); INSERT INTO emp (empno, ename, deptno) VALUES (2, 'Costanza', 20); COMMIT; SET CONSTRAINTS ALL DEFERRED; UPDATE dept SET deptno = deptno + 10 WHERE deptno = 20;
Query:
SELECT * from dept ORDER BY deptno;
Result:
DEPTNO DNAME ---------- ------------------------------ 10 Accounting 30 SALES 2 rows selected.
Update:
UPDATE emp SET deptno = deptno + 10 WHERE deptno = 20;
Result:
1 row updated.
Query:
SELECT * from emp ORDER BY deptno;
Result:
EMPNO ENAME DEPTNO ---------- ------------------------------ ---------- 1 Corleone 10 2 Costanza 30 2 rows selected.
The SET
CONSTRAINTS
applies only to the current transaction, and its setting lasts for the duration of the transaction, or until another SET
CONSTRAINTS
statement resets the mode. The ALTER
SESSION
SET
CONSTRAINTS
statement applies only for the current session. The defaults specified when you create a constraint remain while the constraint exists.
See Also:
Oracle Database SQL Language Reference for more information about the SET
CONSTRAINTS
statement
13.9 Minimizing Space and Time Overhead for Indexes Associated with Constraints
When you create a UNIQUE
or PRIMARY
key, Oracle Database checks to see if an existing index enforces uniqueness for the constraint. If there is no such index, the database creates one.
When Oracle Database uses a unique index to enforce a constraint, and constraints associated with the unique index are dropped or disabled, the index is dropped. To preserve the statistics associated with the index (which would take a long time to re-create), specify the KEEP
INDEX
clause on the DROP
CONSTRAINT
statement.
While enabled foreign keys reference a PRIMARY
or UNIQUE
key, you cannot disable or drop the PRIMARY
or UNIQUE
key constraint or the index.
Note:
UNIQUE
and PRIMARY
keys with deferrable constraints must all use nonunique indexes.
To use existing indexes when creating unique and primary key constraints, include USING
INDEX
in the CONSTRAINT
clause.
See Also:
Oracle Database SQL Language Reference for more details and examples of integrity constraints
13.10 Guidelines for Indexing Foreign Keys
Index foreign keys unless the matching unique or primary key is never updated or deleted.
See Also:
Oracle Database Concepts for more information about indexing foreign keys
13.11 Referential Integrity in a Distributed Database
The declaration of a referential constraint cannot specify a foreign key that references a primary or unique key of a remote table.
However, you can maintain parent/child table relationships across nodes using triggers.
See Also:
Oracle Database PL/SQL Language Reference for more information about triggers that enforce referential integrity
Note:
If you decide to define referential integrity across the nodes of a distributed database using triggers, be aware that network failures can make both the parent table and the child table inaccessible.
For example, assume that the child table is in the SALES
database, and the parent table is in the HQ
database.
If the network connection between the two databases fails, then some data manipulation language (DML) statements against the child table (those that insert rows or update a foreign key value) cannot proceed, because the referential integrity triggers must have access to the parent table in the HQ
database.
13.12 When to Use CHECK Constraints
Use CHECK
constraints when you must enforce integrity rules based on logical expressions, such as comparisons. Never use CHECK
constraints when any of the other types of constraints can provide the necessary checking.
Examples of CHECK
constraints include:
-
A
CHECK
constraint on employee salaries so that no salary value is greater than 10000. -
A
CHECK
constraint on department locations so that only the locations "BOSTON
", "NEW
YORK
", and "DALLAS
" are allowed. -
A
CHECK
constraint on the salary and commissions columns to prevent the commission from being larger than the salary.
13.12.1 Restrictions on CHECK Constraints
A CHECK
constraint requires that a condition be true or unknown for every row of the table. If a statement causes the condition to evaluate to false, then the statement is rolled back. The condition of a CHECK
constraint has these limitations:
-
The condition must be a Boolean expression that can be evaluated using the values in the row being inserted or updated.
-
The condition cannot contain subqueries or sequences.
-
The condition cannot include the
SYSDATE
,UID
,USER
, orUSERENV
SQL functions. -
The condition cannot contain the pseudocolumns
LEVEL
orROWNUM
. -
The condition cannot contain the
PRIOR
operator. -
The condition cannot contain a user-defined function.
See Also:
-
Oracle Database SQL Language Reference for information about the
LEVEL
pseudocolumn -
Oracle Database SQL Language Reference for information about the
ROWNUM
pseudocolumn -
Oracle Database SQL Language Reference for information about the
PRIOR
operator (used in hierarchical queries)
13.12.2 Designing CHECK Constraints
When using CHECK
constraints, remember that a CHECK
constraint is violated only if the condition evaluates to false; true and unknown values (such as comparisons with nulls) do not violate a check condition. Ensure that any CHECK
constraint that you define is specific enough to enforce the rule.
For example, consider this CHECK
constraint:
CHECK (Sal > 0 OR Comm >= 0)
At first glance, this rule may be interpreted as "do not allow a row in the employee table unless the employee salary is greater than zero or the employee commission is greater than or equal to zero." But if a row is inserted with a null salary, that row does not violate the CHECK
constraint, regardless of whether the commission value is valid, because the entire check condition is evaluated as unknown. In this case, you can prevent such violations by placing NOT
NULL
constraints on both the SAL
and COMM
columns.
Note:
If you are not sure when unknown values result in NULL
conditions, review the truth tables for the logical conditions in Oracle Database SQL Language Reference
13.12.3 Rules for Multiple CHECK Constraints
A single column can have multiple CHECK
constraints that reference the column in its definition. There is no limit to the number of CHECK
constraints that can be defined that reference a column.
The order in which the constraints are evaluated is not defined, so be careful not to rely on the order or to define multiple constraints that conflict with each other.
13.12.4 Choosing Between CHECK and NOT NULL Constraints
According to the ANSI/ISO standard, a NOT
NULL
constraint is an example of a CHECK
constraint, where the condition is:
CHECK (column_name IS NOT NULL)
Therefore, you can write NOT
NULL
constraints for a single column using either a NOT
NULL
constraint or a CHECK
constraint. The NOT
NULL
constraint is easier to use than the CHECK
constraint.
In the case where a composite key can allow only all NULL
or all non-NULL
values, you must use a CHECK
constraint. For example, this CHECK
constraint allows a key value in the composite key made up of columns C1
and C2
to contain either all nulls or all values:
CHECK ((C1 IS NULL AND C2 IS NULL) OR (C1 IS NOT NULL AND C2 IS NOT NULL))
13.13 Examples of Defining Constraints
Example 13-5 and Example 13-6 show how to create simple constraints during the prototype phase of your database design. In these examples, each constraint is given a name. Naming the constraints prevents the database from creating multiple copies of the same constraint, with different system-generated names, if the data definition language (DDL) statement runs multiple times.
Example 13-5 creates tables and their constraints at the same time, using the CREATE
TABLE
statement.
Example 13-5 Defining Constraints with the CREATE TABLE Statement
DROP TABLE DeptTab; CREATE TABLE DeptTab ( Deptno NUMBER(3) CONSTRAINT pk_DeptTab_Deptno PRIMARY KEY, Dname VARCHAR2(15), Loc VARCHAR2(15), CONSTRAINT u_DeptTab_Dname_Loc UNIQUE (Dname, Loc), CONSTRAINT c_DeptTab_Loc CHECK (Loc IN ('NEW YORK', 'BOSTON', 'CHICAGO'))); DROP TABLE EmpTab; CREATE TABLE EmpTab ( Empno NUMBER(5) CONSTRAINT pk_EmpTab_Empno PRIMARY KEY, Ename VARCHAR2(15) NOT NULL, Job VARCHAR2(10), Mgr NUMBER(5) CONSTRAINT r_EmpTab_Mgr REFERENCES EmpTab, Hiredate DATE, Sal NUMBER(7,2), Comm NUMBER(5,2), Deptno NUMBER(3) NOT NULL CONSTRAINT r_EmpTab_DeptTab REFERENCES DeptTab ON DELETE CASCADE);
Example 13-6 creates constraints for existing tables, using the ALTER
TABLE
statement.
You cannot create a validated constraint on a table if the table contains rows that violate the constraint.
Example 13-6 Defining Constraints with the ALTER TABLE Statement
-- Create tables without constraints: DROP TABLE DeptTab; CREATE TABLE DeptTab ( Deptno NUMBER(3), Dname VARCHAR2(15), Loc VARCHAR2(15) ); DROP TABLE EmpTab; CREATE TABLE EmpTab ( Empno NUMBER(5), Ename VARCHAR2(15), Job VARCHAR2(10), Mgr NUMBER(5), Hiredate DATE, Sal NUMBER(7,2), Comm NUMBER(5,2), Deptno NUMBER(3) ); --Define constraints with the ALTER TABLE statement: ALTER TABLE DeptTab ADD CONSTRAINT pk_DeptTab_Deptno PRIMARY KEY (Deptno); ALTER TABLE EmpTab ADD CONSTRAINT fk_DeptTab_Deptno FOREIGN KEY (Deptno) REFERENCES DeptTab; ALTER TABLE EmpTab MODIFY (Ename VARCHAR2(15) NOT NULL);
See Also:
Oracle Database Administrator's Guide for information about creating and maintaining constraints for a large production database
13.13.1 Privileges Needed to Define Constraints
If you have the CREATE
TABLE
or CREATE
ANY
TABLE
system privilege, then you can define constraints on the tables that you create.
If you have the ALTER
ANY
TABLE
system privilege, then you can define constraints on any existing table.
If you have the ALTER
object privilege for a specific table, then you can define constraints on that table.
UNIQUE
and PRIMARY
KEY
constraints require that the table owner has either the UNLIMITED
TABLESPACE
system privilege or a quota for the tablespace that contains the associated index.
You can define FOREIGN
KEY
constraints if the parent table or view is in your schema or you have the REFERENCES
privilege on the columns of the referenced key in the parent table or view.
13.13.2 Naming Constraints
Assign names to constraints NOT
NULL
, UNIQUE
, PRIMARY
KEY
, FOREIGN
KEY
, and CHECK
using the CONSTRAINT
option of the constraint clause. This name must be unique among the constraints that you own. If you do not specify a constraint name, one is assigned automatically by Oracle Database.
Choosing your own name makes error messages for constraint violations more understandable, and prevents the creation of duplicate constraints with different names if the SQL statements are run more than once.
See the previous examples of the CREATE
TABLE
and ALTER
TABLE
statements for examples of the CONSTRAINT
option of the constraint
clause. The name of each constraint is included with other information about the constraint in the data dictionary.
See Also:
"Viewing Information About Constraints" for examples of static data dictionary views
13.14 Enabling and Disabling Constraints
This section explains the mechanisms and procedures for manually enabling and disabling constraints.
enabled constraint. When a constraint is enabled, the corresponding rule is enforced on the data values in the associated columns. The definition of the constraint is stored in the data dictionary.
disabled constraint. When a constraint is disabled, the corresponding rule is not enforced. The definition of the constraint is still stored in the data dictionary.
An integrity constraint represents an assertion about the data in a database. This assertion is always true when the constraint is enabled. The assertion might not be true when the constraint is disabled, because data that violates the integrity constraint can be in the database.
Topics:
13.14.1 Why Disable Constraints?
During day-to-day operations, keep constraints enabled. In certain situations, temporarily disabling the constraints of a table makes sense for performance reasons. For example:
-
When loading large amounts of data into a table using SQL*Loader
-
When performing batch operations that make massive changes to a table (such as changing each employee number by adding 1000 to the existing number)
-
When importing or exporting one table at a time
Temporarily turning off constraints can speed up these operations.
13.14.2 Creating Enabled Constraints (Default)
When you define an integrity constraint (using either CREATE
TABLE
or ALTER
TABLE
), Oracle Database enables the constraint by default. For code clarity, you can explicitly enable the constraint by including the ENABLE
clause in its definition, as in Example 13-7.
Example 13-7 Creating Enabled Constraints
/* Use CREATE TABLE statement to create enabled constraint (ENABLE keyword is optional): */ DROP TABLE t1; CREATE TABLE t1 (Empno NUMBER(5) PRIMARY KEY ENABLE); /* Create table without constraint and then use ALTER TABLE statement to add enabled constraint (ENABLE keyword is optional): */ DROP TABLE t2; CREATE TABLE t2 (Empno NUMBER(5)); ALTER TABLE t2 ADD PRIMARY KEY (Empno) ENABLE;
Include the ENABLE
clause when defining a constraint for a table to be populated a row at a time by individual transactions. This ensures that data is always consistent, and reduces the performance overhead of each DML statement.
An ALTER
TABLE
statement that tries to enable an integrity constraint fails if an existing row of the table violates the integrity constraint. The statement rolls back and the constraint definition is neither stored nor enabled.
See Also:
Fixing Constraint Exceptions, for more information about rows that violate constraints
13.14.3 Creating Disabled Constraints
You define and disable an integrity constraint (using either CREATE
TABLE
or ALTER
TABLE
), by including the DISABLE
clause in its definition, as in Example 13-8.
Example 13-8 Creating Disabled Constraints
/* Use CREATE TABLE statement to create disabled constraint */ DROP TABLE t1; CREATE TABLE t1 (Empno NUMBER(5) PRIMARY KEY DISABLE); /* Create table without constraint and then use ALTER TABLE statement to add disabled constraint */ DROP TABLE t2; CREATE TABLE t2 (Empno NUMBER(5)); ALTER TABLE t2 ADD PRIMARY KEY (Empno) DISABLE;
Include the DISABLE
clause when defining a constraint for a table to have large amounts of data inserted before anybody else accesses it, particularly if you must cleanse data after inserting it, or must fill empty columns with sequence numbers or parent/child relationships.
An ALTER
TABLE
statement that defines and disables a constraint never fails, because its rule is not enforced.
13.14.4 Enabling Existing Constraints
After you have cleansed the data and filled the empty columns, you can enable constraints that were disabled during data insertion.
To enable an existing constraint, use the ALTER
TABLE
statement with the ENABLE
clause, as in Example 13-9.
Example 13-9 Enabling Existing Constraints
-- Create table with disabled constraints: DROP TABLE DeptTab; CREATE TABLE DeptTab ( Deptno NUMBER(3) PRIMARY KEY DISABLE, Dname VARCHAR2(15), Loc VARCHAR2(15), CONSTRAINT uk_DeptTab_Dname_Loc UNIQUE (Dname, Loc) DISABLE, CONSTRAINT c_DeptTab_Loc CHECK (Loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')) DISABLE ); -- Enable constraints: ALTER TABLE DeptTab ENABLE PRIMARY KEY ENABLE CONSTRAINT uk_DeptTab_Dname_Loc ENABLE CONSTRAINT c_DeptTab_Loc;
An ALTER
TABLE
statement that attempts to enable an integrity constraint fails if any of the table rows violate the integrity constraint. The statement is rolled back and the constraint is not enabled.
See Also:
Fixing Constraint Exceptions, for more information about rows that violate constraints
13.14.5 Disabling Existing Constraints
If you must perform a large insert or update when a table contains data, you can temporarily disable constraints to improve performance of the bulk operation.
To disable an existing constraint, use the ALTER
TABLE
statement with the DISABLE
clause, as in Example 13-10.
Example 13-10 Disabling Existing Constraints
-- Create table with enabled constraints: DROP TABLE DeptTab; CREATE TABLE DeptTab ( Deptno NUMBER(3) PRIMARY KEY ENABLE, Dname VARCHAR2(15), Loc VARCHAR2(15), CONSTRAINT uk_DeptTab_Dname_Loc UNIQUE (Dname, Loc) ENABLE, CONSTRAINT c_DeptTab_Loc CHECK (Loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')) ENABLE ); -- Disable constraints: ALTER TABLE DeptTab DISABLE PRIMARY KEY DISABLE CONSTRAINT uk_DeptTab_Dname_Loc DISABLE CONSTRAINT c_DeptTab_Loc;
13.14.6 Guidelines for Enabling and Disabling Key Constraints
When enabling or disabling UNIQUE
, PRIMARY
KEY
, and FOREIGN
KEY
constraints, be aware of several important issues and prerequisites. UNIQUE
key and PRIMARY
KEY
constraints are usually managed by the database administrator.
13.14.7 Fixing Constraint Exceptions
If a row of a table disobeys an integrity constraint, then this row is in violation of the constraint and is called an exception to the constraint. If any exceptions exist, then the constraint cannot be enabled. The rows that violate the constraint must be updated or deleted before the constraint can be enabled.
You can identify exceptions for a specific integrity constraint as you try to enable the constraint.
See Also:
Fixing Constraint Exceptions, for more information about this procedure
When you try to create or enable a constraint, and the statement fails because integrity constraint exceptions exist, the statement is rolled back. You cannot enable the constraint until all exceptions are either updated or deleted. To determine which rows violate the integrity constraint, include the EXCEPTIONS
option in the ENABLE
clause of a CREATE
TABLE
or ALTER
TABLE
statement.
See Also:
Oracle Database Administrator's Guide for more information about responding to constraint exceptions
13.15 Modifying Constraints
Starting with Oracle8i, you can modify an existing constraint with the MODIFY
CONSTRAINT
clause, as in Example 13-11.
See Also:
Oracle Database SQL Language Reference for information about the parameters you can modify
Example 13-11 Modifying Constraints
/* Create & then modify a CHECK constraint: */ DROP TABLE X1Tab; CREATE TABLE X1Tab ( a1 NUMBER CONSTRAINT c_X1Tab_a1 CHECK (a1>3) DEFERRABLE DISABLE ); ALTER TABLE X1Tab MODIFY CONSTRAINT c_X1Tab_a1 ENABLE; ALTER TABLE X1Tab MODIFY CONSTRAINT c_X1Tab_a1 RELY; ALTER TABLE X1Tab MODIFY CONSTRAINT c_X1Tab_a1 INITIALLY DEFERRED; ALTER TABLE X1Tab MODIFY CONSTRAINT c_X1Tab_a1 ENABLE NOVALIDATE; /* Create & then modify a PRIMARY KEY constraint: */ DROP TABLE t1; CREATE TABLE t1 (a1 INT, b1 INT); ALTER TABLE t1 ADD CONSTRAINT pk_t1_a1 PRIMARY KEY(a1) DISABLE; ALTER TABLE t1 MODIFY PRIMARY KEY INITIALLY IMMEDIATE USING INDEX PCTFREE = 30 ENABLE NOVALIDATE; ALTER TABLE t1 MODIFY PRIMARY KEY ENABLE NOVALIDATE;
13.16 Renaming Constraints
One property of a constraint that you can modify is its name. Situations in which you would rename a constraint include:
-
You want to clone a table and its constraints.
Constraint names must be unique, even across multiple schemas. Therefore, the constraints in the original table cannot have the same names as those in the cloned table.
-
You created a constraint with a default system-generated name, and now you want to give it a name that is easy to remember, so that you can easily enable and disable it.
Example 13-12 shows how to find the system-generated name of a constraint and change it.
Example 13-12 Renaming a Constraint
DROP TABLE T; CREATE TABLE T ( C1 NUMBER PRIMARY KEY, C2 NUMBER );
Query:
SELECT CONSTRAINT_NAME FROM USER_CONSTRAINTS WHERE TABLE_NAME = 'T' AND CONSTRAINT_TYPE = 'P';
Result (system-generated name of constraint name varies):
CONSTRAINT_NAME
------------------------------
SYS_C0013059
1 row selected.
Rename constraint from name reported in preceding query to T_C1_PK
:
ALTER TABLE T RENAME CONSTRAINT SYS_C0013059 TO T_C1_PK;
Query:
SELECT CONSTRAINT_NAME FROM USER_CONSTRAINTS WHERE TABLE_NAME = 'T' AND CONSTRAINT_TYPE = 'P';
Result:
CONSTRAINT_NAME ------------------------------ T_C1_PK 1 row selected.
13.17 Dropping Constraints
You can drop a constraint using the DROP
clause of the ALTER
TABLE
statement. Situations in which you would drop a constraint include:
-
The constraint enforces a rule that is no longer true.
-
The constraint is no longer needed.
To drop a constraint and all other integrity constraints that depend on it, specify CASCADE
.
Example 13-13 Dropping Constraints
-- Create table with constraints: DROP TABLE DeptTab; CREATE TABLE DeptTab ( Deptno NUMBER(3) PRIMARY KEY, Dname VARCHAR2(15), Loc VARCHAR2(15), CONSTRAINT uk_DeptTab_Dname_Loc UNIQUE (Dname, Loc), CONSTRAINT c_DeptTab_Loc CHECK (Loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')) ); -- Drop constraints: ALTER TABLE DeptTab DROP PRIMARY KEY DROP CONSTRAINT uk_DeptTab_Dname_Loc DROP CONSTRAINT c_DeptTab_Loc;
When dropping UNIQUE
, PRIMARY
KEY
, and FOREIGN
KEY
constraints, be aware of several important issues and prerequisites. UNIQUE
and PRIMARY
KEY
constraints are usually managed by the database administrator.
See Also:
-
Oracle Database SQL Language Reference for more information about the
DROP
clause of theALTER
TABLE
statement. -
Oracle Database Administrator's Guide for more information about dropping constraints.
-
Oracle Database SQL Language Reference for information about the
CASCADE
CONSTRAINTS
clause of theDROP
TABLE
statement, which drops all referential integrity constraints that refer to primary and unique keys in the dropped table
13.18 Managing FOREIGN KEY Constraints
FOREIGN
KEY
constraints enforce relationships between columns in different tables. Therefore, they cannot be enabled if the constraint of the referenced primary or unique key is not present or not enabled.
13.18.1 Data Types and Names for Foreign Key Columns
You must use the same data type for corresponding columns in the dependent and referenced tables. The column names need not match.
13.18.2 Limit on Columns in Composite Foreign Keys
Because foreign keys reference primary and unique keys of the parent table, and PRIMARY
KEY
and UNIQUE
key constraints are enforced using indexes, composite foreign keys are limited to 32 columns.
13.18.3 Foreign Key References Primary Key by Default
If the column list is not included in the REFERENCES
option when defining a FOREIGN
KEY
constraint (single column or composite), then Oracle Database assumes that you intend to reference the primary key of the specified table. Alternatively, you can explicitly specify the column(s) to reference in the parent table within parentheses. Oracle Database automatically checks to verify that this column list references a primary or unique key of the parent table. If it does not, then an informative error is returned.
13.18.4 Privileges Required to Create FOREIGN KEY Constraints
To create a FOREIGN
KEY
constraint, the creator of the constraint must have privileged access to the parent and child tables.
-
Parent Table The creator of the referential integrity constraint must own the parent table or have
REFERENCES
object privileges on the columns that constitute the parent key of the parent table. -
Child Table The creator of the referential integrity constraint must have the ability to create tables (that is, the
CREATE
TABLE
orCREATE
ANY
TABLE
system privilege) or the ability to alter the child table (that is, theALTER
object privilege for the child table or theALTER
ANY
TABLE
system privilege).
In both cases, necessary privileges cannot be obtained through a role; they must be explicitly granted to the creator of the constraint.
These restrictions allow:
-
The owner of the child table to explicitly decide which constraints are enforced and which other users can create constraints
-
The owner of the parent table to explicitly decide if foreign keys can depend on the primary and unique keys in her tables
13.18.5 Choosing How Foreign Keys Enforce Referential Integrity
Oracle Database allows different types of referential integrity actions to be enforced, as specified with the definition of a FOREIGN
KEY
constraint:
-
Prevent Delete or Update of Parent Key The default setting prevents the deletion or update of a parent key if there is a row in the child table that references the key. For example:
CREATE TABLE Emp_tab ( FOREIGN KEY (Deptno) REFERENCES Dept_tab);
-
Delete Child Rows When Parent Key Deleted The
ON
DELETE
CASCADE
action allows parent key data that is referenced from the child table to be deleted, but not updated. When data in the parent key is deleted, all rows in the child table that depend on the deleted parent key values are also deleted. To specify this referential action, include theON
DELETE
CASCADE
option in the definition of theFOREIGN
KEY
constraint. For example:CREATE TABLE Emp_tab ( FOREIGN KEY (Deptno) REFERENCES Dept_tab ON DELETE CASCADE);
-
Set Foreign Keys to Null When Parent Key Deleted The
ON
DELETE
SET
NULL
action allows data that references the parent key to be deleted, but not updated. When referenced data in the parent key is deleted, all rows in the child table that depend on those parent key values have their foreign keys set toNULL
. To specify this referential action, include theON
DELETE
SET
NULL
option in the definition of theFOREIGN
KEY
constraint. For example:CREATE TABLE Emp_tab ( FOREIGN KEY (Deptno) REFERENCES Dept_tab ON DELETE SET NULL);
13.19 Viewing Information About Constraints
To find the names of constraints, what columns they affect, and other information to help you manage them, query the static data dictionary views *_CONSTRAINTS
and *_CONS_COLUMNS
, as in Example 13-14.
See Also:
Oracle Database Reference for information about *_CONSTRAINTS
and *_CONS_COLUMNS
Example 13-14 Viewing Information About Constraints
DROP TABLE DeptTab; CREATE TABLE DeptTab ( Deptno NUMBER(3) PRIMARY KEY, Dname VARCHAR2(15), Loc VARCHAR2(15), CONSTRAINT uk_DeptTab_Dname_Loc UNIQUE (Dname, Loc), CONSTRAINT c_DeptTab_Loc CHECK (Loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')) ); DROP TABLE EmpTab; CREATE TABLE EmpTab ( Empno NUMBER(5) PRIMARY KEY, Ename VARCHAR2(15) NOT NULL, Job VARCHAR2(10), Mgr NUMBER(5) CONSTRAINT r_EmpTab_Mgr REFERENCES EmpTab ON DELETE CASCADE, Hiredate DATE, Sal NUMBER(7,2), Comm NUMBER(5,2), Deptno NUMBER(3) NOT NULL CONSTRAINT r_EmpTab_Deptno REFERENCES DeptTab ); -- Format columns (optional): COLUMN CONSTRAINT_NAME FORMAT A20; COLUMN CONSTRAINT_TYPE FORMAT A4 HEADING 'TYPE'; COLUMN TABLE_NAME FORMAT A10; COLUMN R_CONSTRAINT_NAME FORMAT A17; COLUMN SEARCH_CONDITION FORMAT A40; COLUMN COLUMN_NAME FORMAT A12;
List accessible constraints in DeptTab and EmpTab:
SELECT CONSTRAINT_NAME, CONSTRAINT_TYPE, TABLE_NAME, R_CONSTRAINT_NAME FROM USER_CONSTRAINTS WHERE (TABLE_NAME = 'DEPTTAB' OR TABLE_NAME = 'EMPTAB') ORDER BY CONSTRAINT_NAME;
Result:
CONSTRAINT_NAME TYPE TABLE_NAME R_CONSTRAINT_NAME -------------------- ---- ---------- ----------------- C_DEPTTAB_LOC C DEPTTAB R_EMPTAB_DEPTNO R EMPTAB SYS_C006286 R_EMPTAB_MGR R EMPTAB SYS_C006290 SYS_C006286 P DEPTTAB SYS_C006288 C EMPTAB SYS_C006289 C EMPTAB SYS_C006290 P EMPTAB UK_DEPTTAB_DNAME_LOC U DEPTTAB 8 rows selected.
Distinguish between NOT
NULL
and CHECK
constraints in DeptTab
and EmpTab
:
SELECT CONSTRAINT_NAME, SEARCH_CONDITION FROM USER_CONSTRAINTS WHERE (TABLE_NAME = 'DEPTTAB' OR TABLE_NAME = 'EMPTAB') AND CONSTRAINT_TYPE = 'C' ORDER BY CONSTRAINT_NAME;
Result:
CONSTRAINT_NAME SEARCH_CONDITION -------------------- ---------------------------------------- C_DEPTTAB_LOC Loc IN ('NEW YORK', 'BOSTON', 'CHICAGO') SYS_C006288 "ENAME" IS NOT NULL SYS_C006289 "DEPTNO" IS NOT NULL 3 rows selected.
For DeptTab
and EmpTab
, list columns that constitute constraints:
SELECT CONSTRAINT_NAME, TABLE_NAME, COLUMN_NAME FROM USER_CONS_COLUMNS WHERE (TABLE_NAME = 'DEPTTAB' OR TABLE_NAME = 'EMPTAB') ORDER BY CONSTRAINT_NAME;
Result:
CONSTRAINT_NAME TABLE_NAME COLUMN_NAME -------------------- ---------- ------------ C_DEPTTAB_LOC DEPTTAB LOC R_EMPTAB_DEPTNO EMPTAB DEPTNO R_EMPTAB_MGR EMPTAB MGR SYS_C006286 DEPTTAB DEPTNO SYS_C006288 EMPTAB ENAME SYS_C006289 EMPTAB DEPTNO SYS_C006290 EMPTAB EMPNO UK_DEPTTAB_DNAME_LOC DEPTTAB LOC UK_DEPTTAB_DNAME_LOC DEPTTAB DNAME 9 rows selected.
Note that:
-
Some constraint names are user specified (such as
UK_DEPTTAB_DNAME_LOC
), while others are system specified (such asSYS_C006290
). -
Each constraint type is denoted with a different character in the
CONSTRAINT_TYPE
column. This table summarizes the characters used for each constraint type:Constraint Type Character PRIMARY
KEY
P
UNIQUE
KEY
U
FOREIGN
KEY
R
CHECK
,NOT
NULL
C
Note:
An additional constraint type is indicated by the character "
V
" in theCONSTRAINT_TYPE
column. This constraint type corresponds to constraints created using theWITH
CHECK
OPTION
for views.
These constraints are explicitly listed in the SEARCH_CONDITION
column:
-
NOT
NULL
constraints -
The conditions for user-defined
CHECK
constraints