|Oracle9i Application Developer's Guide - Fundamentals
Release 1 (9.0.1)
Part Number A88876-02
This chapter discusses the procedures necessary to create and manage the different types of objects contained in a user's schema. The topics include:
A table is the data structure that holds data in a relational database. A table is composed of rows and columns.
A table can represent a single entity that you want to track within your system. This type of a table could represent a list of the employees within your organization, or the orders placed for your company's products.
A table can also represent a relationship between two entities. This type of a table could portray the association between employees and their job skills, or the relationship of products to orders. Within the tables, foreign keys are used to represent relationships.
Although some well designed tables could represent both an entity and describe the relationship between that entity and another entity, most tables should represent either an entity or a relationship. For example, the
EMP_TAB table describes the employees in a firm, but this table also includes a foreign key column,
DEPTNO, which represents the relationships of employees to departments.
The following sections explain how to create, alter, and drop tables. Some simple guidelines to follow when managing tables in your database are included.
The Oracle9i Database Administrator's Guide has more suggestions. You should also refer to a text on relational database or table design.
Consider the following guidelines when designing your tables:
Before creating a table, you should also determine whether to use integrity constraints. Integrity constraints can be defined on the columns of a table to enforce the business rules of your database automatically.
To create a table, use the SQL command
TABLE. For example, the following statement creates a non-clustered table named
Emp_tab that is physically stored in the
USERS tablespace. Notice that integrity constraints are defined on several columns of the table.
CREATE TABLE Emp_tab ( Empno NUMBER(5) PRIMARY KEY, Ename VARCHAR2(15) NOT NULL, Job VARCHAR2(10), Mgr NUMBER(5), Hiredate DATE DEFAULT (sysdate), Sal NUMBER(7,2), Comm NUMBER(7,2), Deptno NUMBER(3) NOT NULL, CONSTRAINT dept_afkey REFERENCES Dept_tab(Deptno)) PCTFREE 10 PCTUSED 40 TABLESPACE users STORAGE ( INITIAL 50K NEXT 50K MAXEXTENTS 10 PCTINCREASE 25 );
Oracle8i provides a special kind of table to hold temporary data. You specify whether the data is specific to a session or to a transaction. When the session or transaction finishes, the rows that it inserted are deleted. Multiple sessions or transactions can use the same temporary table, and each session or transaction only sees the rows that it created.
Temporary tables are useful any time you want to buffer a result set or construct a result set by running multiple DML operations. Here are a few specific examples:
During the session, the itinerary data is private. At the end of the session, the optional itineraries are dropped.
You create a temporary table by using special ANSI keywords. You specify the data as session-specific by using the
ROWS keywords. You specify the data as transaction-specific by using the
You can create indexes on temporary tables as you would on permanent tables.
For a session-specific temporary table, a session gets bound to the temporary table with the first insert in the table in the session. This binding goes away at the end of the session or by issuing a
TRUNCATE of the table in the session.
For a transaction-specific temporary table, a session gets bound to the temporary table with the first insert in the table in the transaction. The binding goes away at the end of the transaction.
DDL operations (except
TRUNCATE) are allowed on an existing temporary table only if no session is currently bound to that temporary table.
Unlike permanent tables, temporary tables and their indexes do not automatically allocate a segment when they are created. Instead, segments are allocated when the first
SELECT) is performed. This means that if a
DELETE is performed before the first
INSERT, the table appears to be empty.
Temporary segments are deallocated at the end of the transaction for transaction-specific temporary tables and at the end of the session for session-specific temporary tables.
If you rollback a transaction, the data you entered is lost, although the table definition persists.
You cannot create a table that is simultaneously both transaction- and session-specific.
A transaction-specific temporary table allows only one transaction at a time. If there are several autonomous transactions in a single transaction scope, each autonomous transaction can use the table only as soon as the previous one commits.
Because the data in a temporary table is, by definition, temporary, backup and recovery of a temporary table's data is not available in the event of a system failure. To prepare for such a failure, you should develop alternative methods for preserving temporary table data.
The following statement creates a session-specific temporary table,
FLIGHT_SCHEDULE, for use in an automated airline reservation scheduling system. Each client has its own session and can store temporary schedules. The temporary schedules are deleted at the end of the session.
CREATE GLOBAL TEMPORARY TABLE flight_schedule ( startdate DATE, enddate DATE, cost NUMBER) ON COMMIT PRESERVE ROWS;
You can use temporary tables to improve performance when you run complex queries. Running multiple such queries is relatively slow because the tables are accessed multiple times for each returned row. It is faster to cache the values from a complex query in a temporary table, then run the queries against the temporary table.
For example, even with a view like this defined to simplify further queries, the queries against the view may be slow because the contents of the view are recalculated each time:
CREATE OR REPLACE VIEW Profile_values_view AS SELECT d.Profile_option_name, d.Profile_option_id, Profile_option_value, u.User_name, Level_id, Level_code FROM Profile_definitions d, Profile_values v, Profile_users u WHERE d.Profile_option_id = v.Profile_option_id AND ((Level_code = 'USER' AND Level_id = U.User_id) OR (Level_code = 'DEPARTMENT' AND Level_id = U.Department_id) OR (Level_code = 'SITE')) AND NOT EXISTS (SELECT 1 FROM PROFILE_VALUES P WHERE P.PROFILE_OPTION_ID = V.PROFILE_OPTION_ID AND ((Level_code = 'USER' AND level_id = u.User_id) OR (Level_code = 'DEPARTMENT' AND level_id = u.Department_id) OR (Level_code = 'SITE')) AND INSTR('USERDEPARTMENTSITE', v.Level_code) > INSTR('USERDEPARTMENTSITE', p.Level_code));
A temporary table allows us to run the computation once, and cache the result in later SQL queries and joins:
CREATE GLOBAL TEMPORARY TABLE Profile_values_temp ( Profile_option_name VARCHAR(60) NOT NULL, Profile_option_id NUMBER(4) NOT NULL, Profile_option_value VARCHAR2(20) NOT NULL, Level_code VARCHAR2(10) , Level_id NUMBER(4) , CONSTRAINT Profile_values_temp_pk PRIMARY KEY (Profile_option_id) ) ON COMMIT PRESERVE ROWS ORGANIZATION INDEX; INSERT INTO Profile_values_temp (Profile_option_name, Profile_option_id, Profile_option_value, Level_code, Level_id) SELECT Profile_option_name, Profile_option_id, Profile_option_value, Level_code, Level_id FROM Profile_values_view; COMMIT;
Now the temporary table can be used to speed up queries, and the results cached in the temporary table are freed automatically by the database when the session ends.
In complex queries that process the same subquery multiple times, you might be tempted to store the subquery results in a temporary table and perform additional queries against the temporary table. The
WITH clause lets you factor out the subquery, give it a name, then reference that name multiple times within the original complex query.
This technique lets the optimizer choose how to deal with the subquery results -- whether to create a temporary table or inline it as a view.
For example, the following query joins two tables and computes the aggregate
SUM(SAL) more than once. The bold text represents the parts of the query that are repeated.
SELECT dname, SUM(sal) AS dept_total FROM emp, dept WHERE emp.deptno = dept.deptno GROUP BY dname HAVING SUM(sal) > ( SELECT SUM(sal) * 1/3 FROM emp, dept WHERE emp.deptno = dept.deptno ) ORDER BY SUM(sal) DESC;
You can improve the query by doing the subquery once, and referencing it at the appropriate points in the main query. The bold text represents the common parts of the subquery, and the places where the subquery is referenced.
WITH summary AS ( SELECT dname, SUM(sal) AS dept_total FROM emp, dept WHERE emp.deptno = dept.deptno GROUP BY dname ) SELECT dname, dept_total FROM summary WHERE dept_total > ( SELECT SUM(dept_total) * 1/3 FROM summary ) ORDER BY dept_total DESC;
A view is a logical representation of another table or combination of tables. A view derives its data from the tables on which it is based. These tables are called base tables. Base tables might in turn be actual tables or might be views themselves.
All operations performed on a view actually affect the base table of the view. You can use views in almost the same way as tables. You can query, update, insert into, and delete from views, just as you can standard tables.
Views can provide a different representation (such as subsets or supersets) of the data that resides within other tables and views. Views are very powerful because they allow you to tailor the presentation of data to different types of users.
The following sections explain how to create, replace, and drop views using SQL commands.
Use the SQL command
VIEW to create a view. For example, the following statement creates a view on a subset of data in the
CREATE VIEW Sales_staff AS SELECT Empno, Ename, Deptno FROM Emp_tab WHERE Deptno = 10 WITH CHECK OPTION CONSTRAINT Sales_staff_cnst;
The object names are resolved when the view is created or when the program containing the SQL is compiled, relative to the schema of the view owner.
You can define views with any query that references tables, snapshots, or other views.
The query that defines the
SALES_STAFF view references only rows in department 10. Furthermore,
OPTION creates the view with the constraint that
UPDATE statements issued against the view are not allowed to create or result in rows that the view cannot select.
Considering the example above, the following
INSERT statement successfully inserts a row into the
EMP_TAB table through the
However, the following
INSERT statement is rolled back and returns an error because it attempts to insert a row for department number 30, which could not be selected using the
The following statement creates a view that joins data from the
CREATE VIEW Division1_staff AS SELECT Ename, Empno, Job, Dname FROM Emp_tab, Dept_tab WHERE Emp_tab.Deptno IN (10, 30) AND Emp_tab.Deptno = Dept_tab.Deptno;
Division1_staff view is defined by a query that joins information from the
Dept_tab tables. The
OPTION is not specified in the
VIEW statement because rows cannot be inserted into or updated in a view defined with a query that contains a join that uses the
In accordance with the ANSI/ISO standard, Oracle expands any wildcard in a top-level view query into a column list when a view is created and stores the resulting query in the data dictionary; any subqueries are left intact. The column names in an expanded column list are enclosed in quote marks to account for the possibility that the columns of the base object were originally entered with quotes and require them for the query to be syntactically correct.
As an example, assume that the
Dept_view view is created as follows:
Oracle stores the defining query of the
Dept_view view as
Views created with errors do not have wildcards expanded. However, if the view is eventually compiled without errors, then wildcards in the defining query are expanded.
A view can be created even if the defining query of the view cannot be executed, as long as the
VIEW command has no syntax errors. We call such a view a view with errors. For example, if a view refers to a non-existent table or an invalid column of an existing table, or if the owner of the view does not have the required privileges, then the view can still be created and entered into the data dictionary.
You can only create a view with errors by using the
FORCE option of the
When a view is created with errors, Oracle returns a message and leaves the status of the view as
INVALID. If conditions later change so that the query of an invalid view can be executed, then the view can be recompiled and become valid. Oracle dynamically compiles the invalid view if you attempt to use it.
To create a view, you must have been granted the following privileges:
VIEWsystem privilege to create a view in your schema, or the
VIEWsystem privilege to create a view in another user's schema. These privileges can be acquired explicitly or through a role.
INSERTprivilege for Scott's
EMP_TABtable, then you can create a view on his
EMP_TABtable, but you can only use this view to insert new rows into the
OPTIONor the system privileges with the
OPTION; if not, then the view owner has insufficient privileges to grant access to the view to other users.
To alter the definition of a view, you must replace the view using one of the following methods:
VIEWstatement that contains the
REPLACEoption. This option replaces the current definition of a view, but preserves the present security authorizations.
For example, assume that you create the
SALES_STAFF view, as given in a previous example. You also grant several object privileges to roles and other users. However, now you realize that you must redefine the
SALES_STAFF view to correct the department number specified in the
WHERE clause of the defining query, because it should have been 30. To preserve the grants of object privileges that you have made, you can replace the current version of the
SALES_STAFF view with the following statement:
CREATE OR REPLACE VIEW Sales_staff AS SELECT Empno, Ename, Deptno FROM Emp_tab WHERE Deptno = 30 WITH CHECK OPTION CONSTRAINT Sales_staff_cnst;
Replacing a view has the following effects:
OPTIONfor a view's definition is dropped.
To replace a view, you must have all of the privileges necessary to drop the view, as well as all of those required to create the view.
Views can be queried in the same manner as tables. For example, to query the
Division1_staff view, enter a valid
SELECT statement that references the view:
SELECT * FROM Division1_staff; ENAME EMPNO JOB DNAME ------------------------------------------------------ CLARK 7782 MANAGER ACCOUNTING KING 7839 PRESIDENT ACCOUNTING MILLER 7934 CLERK ACCOUNTING ALLEN 7499 SALESMAN SALES WARD 7521 SALESMAN SALES JAMES 7900 CLERK SALES TURNER 7844 SALESMAN SALES MARTIN 7654 SALESMAN SALES BLAKE 7698 MANAGER SALES
With some restrictions, rows can be inserted into, updated in, or deleted from a base table using a view. The following statement inserts a new row into the
EMP_TAB table using the
Restrictions on DML operations for views use the following criteria in the order listed:
BYclause, or a group function, then rows cannot be inserted into, updated in, or deleted from the base tables using the view.
OPTION, then a row cannot be inserted into, or updated in, the base table (using the view), if the view cannot select the row from the base table.
NULLcolumn that does not have a
DEFAULTclause is omitted from the view, then a row cannot be inserted into the base table using the view.
DECODE(deptno, 10, "
SALES", ...), then rows cannot be inserted into or updated in the base table using the view.
The constraint created by
OPTION of the
SALES_STAFF view only allows rows that have a department number of 10 to be inserted into, or updated in, the
EMP_TAB table. Alternatively, assume that the
SALES_STAFF view is defined by the following statement (that is, excluding the
CREATE VIEW Sales_staff AS SELECT Empno, Ename FROM Emp_tab WHERE Deptno = 10 WITH CHECK OPTION CONSTRAINT Sales_staff_cnst;
Considering this view definition, you can update the
ENAME fields of existing records, but you cannot insert rows into the
EMP_TAB table through the
SALES_STAFF view because the view does not let you alter the
DEPTNO field. If you had defined a
DEFAULT value of 10 on the
DEPTNO field, then you could perform inserts.
When a user attempts to reference an invalid view, Oracle returns an error message to the user:
This error message is returned when a view exists but is unusable due to errors in its query (whether it had errors when originally created or it was created successfully but became unusable later because underlying objects were altered or dropped).
To issue a query or an
DELETE statement against a view, you must have the
DELETE object privilege for the view, respectively, either explicitly or through a role.
Use the SQL command
VIEW to drop a view. For example:
You can drop any view contained in your schema. To drop a view in another user's schema, you must have the
VIEW system privilege.
Oracle allows you, with some restrictions, to modify views that involve joins. Consider the following simple view:
This view does not involve a join operation. If you issue the SQL statement:
EMP_TAB base table that underlies the view changes, and employee 7839's name changes from
CAESAR in the
However, if you create a view that involves a join operation, such as:
CREATE VIEW Emp_dept_view AS SELECT e.Empno, e.Ename, e.Deptno, e.Sal, d.Dname, d.Loc FROM Emp_tab e, Dept_tab d /* JOIN operation */ WHERE e.Deptno = d.Deptno AND d.Loc IN ('DALLAS', 'NEW YORK', 'BOSTON');
then there are restrictions on modifying either the
EMP_TAB or the
DEPT_TAB base table through this view, for example, using a statement such as:
A modifiable join view is a view that contains more than one table in the top-level
FROM clause of the
SELECT statement, and that does not contain any of the following:
A further restriction on which join views are modifiable is that if a view is a join on other nested views, then the other nested views must be mergeable into the top level view.
The examples in this section use the
DEPT_TAB tables. However, the examples work only if you explicitly define the primary and foreign keys in these tables, or define unique indexes. Here are the appropriately constrained table definitions for
CREATE TABLE Dept_tab ( Deptno NUMBER(4) PRIMARY KEY, Dname VARCHAR2(14), Loc VARCHAR2(13)); CREATE TABLE Emp_tab ( Empno NUMBER(4) PRIMARY KEY, Ename VARCHAR2(10), Job varchar2(9), Mgr NUMBER(4), Hiredate DATE, Sal NUMBER(7,2), Comm NUMBER(7,2), Deptno NUMBER(2), FOREIGN KEY (Deptno) REFERENCES Dept_tab(Deptno));
You could also omit the primary and foreign key constraints listed above, and create a
(DEPTNO) to make the following examples work.
The concept of a key-preserved table is fundamental to understanding the restrictions on modifying join views. A table is key preserved if every key of the table can also be a key of the result of the join. So, a key-preserved table has its keys preserved through a join.
SELECT all rows from
EMP_DEPT_VIEW defined in the "Modifying a Join View" section, then the results are:
EMPNO ENAME DEPTNO DNAME LOC --------------------------------------------------------- 7782 CLARK 10 ACCOUNTING NEW YORK 7839 KING 10 ACCOUNTING NEW YORK 7934 MILLER 10 ACCOUNTING NEW YORK 7369 SMITH 20 RESEARCH DALLAS 7876 ADAMS 20 RESEARCH DALLAS 7902 FORD 20 RESEARCH DALLAS 7788 SCOTT 20 RESEARCH DALLAS 7566 JONES 20 RESEARCH DALLAS 8 rows selected.
In this view,
EMP_TAB is a key-preserved table, because
EMPNO is a key of the
EMP_TAB table, and also a key of the result of the join.
DEPT_TAB is not a key-preserved table, because although
DEPTNO is a key of the
DEPT_TAB table, it is not a key of the join.
DELETE statement on a join view can modify only one underlying base table.
The following example shows an
UPDATE statement that successfully modifies the
UPDATE statement would be disallowed on the
This statement fails with an
ORA-01779 error ("cannot modify a column which maps to a non key-preserved table"), because it attempts to modify the underlying
DEPT_TAB table, and the
DEPT_TAB table is not key preserved in the
In general, all modifiable columns of a join view must map to columns of a key-preserved table. If the view is defined using the
OPTION clause, then all join columns and all columns of repeated tables are not modifiable.
So, for example, if the
EMP_DEPT view were defined using
OPTION, then the following
UPDATE statement would fail:
The statement fails because it is trying to update a join column.
You can delete from a join view provided there is one and only one key-preserved table in the join.
DELETE statement works on the
DELETE statement on the
EMP_DEPT view is legal because it can be translated to a
DELETE operation on the base
EMP_TAB table, and because the
EMP_TAB table is the only key-preserved table in the join.
In the following view, a
DELETE operation cannot be performed on the view because both
E2 are key-preserved tables:
CREATE VIEW emp_emp AS SELECT e1.Ename, e2.Empno, e1.Deptno FROM Emp_tab e1, Emp_tab e2 WHERE e1.Empno = e2.Empno; WHERE e1.Empno = e2.Empno;
If a view is defined using the
OPTION clause and the key-preserved table is repeated, then rows cannot be deleted from such a view. For example:
CREATE VIEW Emp_mgr AS SELECT e1.Ename, e2.Ename Mname FROM Emp_tab e1, Emp_tab e2 WHERE e1.mgr = e2.Empno WITH CHECK OPTION;
No deletion can be performed on this view because the view involves a self-join of the table that is key preserved.
INSERT statement on the
EMP_DEPT view succeeds, because only one key-preserved base table is being modified (
EMP_TAB), and 40 is a valid
DEPTNO in the
DEPT_TAB table (thus satisfying the
KEY integrity constraint on the
INSERT statement fails for the same reason: This
UPDATE on the base
EMP_TAB table would fail: the
KEY integrity constraint on the
EMP_TAB table is violated.
INSERT statement fails with an
ORA-01776 error ("cannot modify more than one base table through a view").
INSERT cannot, implicitly or explicitly, refer to columns of a non-key-preserved table. If the join view is defined using the
OPTION clause, then you cannot perform an
INSERT to it.
Three views you can use for modifying join views are shown in Table 2-1.
Shows all columns in all tables and views in the user's schema that are modifiable
Shows all columns in all tables and views in the DBA schema that are modifiable
Shows all columns in all tables and views that are modifiable
Views that involve outer joins are modifiable in some cases. For example:
CREATE VIEW Emp_dept_oj1 AS SELECT Empno, Ename, e.Deptno, Dname, Loc FROM Emp_tab e, Dept_tab d WHERE e.Deptno = d.Deptno (+);
EMPNO ENAME DEPTNO DNAME LOC ------- ---------- ------- -------------- ------------- 7369 SMITH 40 OPERATIONS BOSTON 7499 ALLEN 30 SALES CHICAGO 7566 JONES 20 RESEARCH DALLAS 7654 MARTIN 30 SALES CHICAGO 7698 BLAKE 30 SALES CHICAGO 7782 CLARK 10 ACCOUNTING NEW YORK 7788 SCOTT 20 RESEARCH DALLAS 7839 KING 10 ACCOUNTING NEW YORK 7844 TURNER 30 SALES CHICAGO 7876 ADAMS 20 RESEARCH DALLAS 7900 JAMES 30 SALES CHICAGO 7902 FORD 20 RESEARCH DALLAS 7934 MILLER 10 ACCOUNTING NEW YORK 7521 WARD 30 SALES CHICAGO 14 rows selected.
Columns in the base
EMP_TAB table of
EMP_DEPT_OJ1 are modifiable through the view, because
EMP_TAB is a key-preserved table in the join.
The following view also contains an outer join:
CREATE VIEW Emp_dept_oj2 AS SELECT e.Empno, e.Ename, e.Deptno, d.Dname, d.Loc FROM Emp_tab e, Dept_tab d WHERE e.Deptno (+) = d.Deptno;
EMPNO ENAME DEPTNO DNAME LOC ---------- ---------- --------- -------------- ---- 7782 CLARK 10 ACCOUNTING NEW YORK 7839 KING 10 ACCOUNTING NEW YORK 7934 MILLER 10 ACCOUNTING NEW YORK 7369 SMITH 20 RESEARCH DALLAS 7876 ADAMS 20 RESEARCH DALLAS 7902 FORD 20 RESEARCH DALLAS 7788 SCOTT 20 RESEARCH DALLAS 7566 JONES 20 RESEARCH DALLAS 7499 ALLEN 30 SALES CHICAGO 7698 BLAKE 30 SALES CHICAGO 7654 MARTIN 30 SALES CHICAGO 7900 JAMES 30 SALES CHICAGO 7844 TURNER 30 SALES CHICAGO 7521 WARD 30 SALES CHICAGO OPERATIONS BOSTON 15 rows selected.
In this view,
EMP_TAB is no longer a key-preserved table, because the
EMPNO column in the result of the join can have nulls (the last row in the
SELECT above). So,
INSERT operations cannot be performed on this view.
In the case of views containing an outer join on other nested views, a table is key preserved if the view or views containing the table are merged into their outer views, all the way to the top. A view which is being outer-joined is currently merged only if it is "simple." For example:
The select list of the view has no expressions, and there is no
Consider the following set of views:
CREATE VIEW Emp_v AS SELECT Empno, Ename, Deptno FROM Emp_tab; CREATE VIEW Emp_dept_oj1 AS SELECT e.*, Loc, d.Dname FROM Emp_v e, Dept_tab d WHERE e.Deptno = d.Deptno (+);
In these examples,
EMP_V is merged into
EMP_V is a simple view, and so
EMP_TAB is a key-preserved table. But if
EMP_V is changed as follows:
Then, because of the presence of the
EMP_V_2 cannot be merged into
EMP_DEPT_OJ1, and hence
EMP_TAB is no longer a key-preserved table.
If you are in doubt whether a view is modifiable, then you can
SELECT from the view
USER_UPDATABLE_COLUMNS to see if it is. For example:
This might return:
OWNER TABLE_NAME COLUMN_NAM UPD ---------- ---------- ---------- --- SCOTT EMP_DEPT_V EMPNO NO SCOTT EMP_DEPT_V ENAME NO SCOTT EMP_DEPT_V DEPTNO NO SCOTT EMP_DEPT_V DNAME NO SCOTT EMP_DEPT_V LOC NO 5 rows selected.
The sequence generator generates sequential numbers, which can help to generate unique primary keys automatically, and to coordinate keys across multiple rows or tables.
Without sequences, sequential values can only be produced programmatically. A new primary key value can be obtained by selecting the most recently produced value and incrementing it. This method requires a lock during the transaction and causes multiple users to wait for the next value of the primary key; this waiting is known as serialization. If you have such constructs in your applications, then you should replace them with access to sequences. Sequences eliminate serialization and improve the concurrency of your application.
The following sections explain how to create, alter, use, and drop sequences using SQL commands.
Use the SQL command
SEQUENCE to create a sequence. The following statement creates a sequence used to generate employee numbers for the
EMPNO column of the
Notice that several parameters can be specified to control the function of sequences. You can use these parameters to indicate whether the sequence is ascending or descending, the starting point of the sequence, the minimum and maximum values, and the interval between sequence values. The
NOCYCLE option indicates that the sequence cannot generate more values after reaching its maximum or minimum value.
CACHE option of the
SEQUENCE command pre-allocates a set of sequence numbers and keeps them in memory so that they can be accessed faster. When the last of the sequence numbers in the cache have been used, another set of numbers is read into the cache.
For additional implications for caching sequence numbers when using Oracle Real Application Clusters, see Oracle9i Real Application Clusters Deployment and Performance.
General information about caching sequence numbers is included in "Caching Sequence Numbers".
To create a sequence in your schema, you must have the
SEQUENCE system privilege. To create a sequence in another user's schema, you must have the
You can change any of the parameters that define how corresponding sequence numbers are generated; however, you cannot alter a sequence to change the starting number of a sequence. To do this, you must drop and re-create the sequence.
Use the SQL command
SEQUENCE to alter a sequence. For example:
To alter a sequence, your schema must contain the sequence, or you must have the
SEQUENCE system privilege.
Once a sequence is defined, it can be accessed and incremented by multiple users with no waiting. Oracle does not wait for a transaction that has incremented a sequence to complete before that sequence can be incremented again.
The examples outlined in the following sections show how sequences can be used in master/detail table relationships. Assume an order entry system is partially comprised of two tables,
ORDERS_TAB (master table) and
LINE_ITEMS_TAB (detail table), that hold information about customer orders. A sequence named
ORDER_SEQ is defined by the following statement:
A sequence is referenced in SQL statements with the
CURRVAL pseudocolumns; each new sequence number is generated by a reference to the sequence's pseudocolumn
NEXTVAL, while the current sequence number can be repeatedly referenced using the pseudo-column
CURRVAL are not reserved words or keywords and can be used as pseudo-column names in SQL statements such as
To generate and use a sequence number, reference seq_name.
NEXTVAL. For example, assume a customer places an order. The sequence number can be referenced in a values list. For example:
Or, the sequence number can be referenced in the
SET clause of an
UPDATE statement. For example:
The sequence number can also be referenced outermost
SELECT of a query or subquery. For example:
As defined, the first reference to
ORDER_SEQ.NEXTVAL returns the value 1. Each subsequent statement that references
ORDER_SEQ.NEXTVAL generates the next sequence number (2, 3, 4,. . .). The pseudo-column
NEXTVAL can be used to generate as many new sequence numbers as necessary. However, only a single sequence number can be generated for each row. In other words, if
NEXTVAL is referenced more than once in a single statement, then the first reference generates the next number, and all subsequent references in the statement return the same number.
Once a sequence number is generated, the sequence number is available only to the session that generated the number. Independent of transactions committing or rolling back, other users referencing
ORDER_SEQ.NEXTVAL obtain unique values. If two users are accessing the same sequence concurrently, then the sequence numbers each user receives might have gaps because sequence numbers are also being generated by the other user.
To use or refer to the current sequence value of your session, reference seq_name.
CURRVAL can only be used if seq_name.
NEXTVAL has been referenced in the current user session (in the current or a previous transaction).
CURRVAL can be referenced as many times as necessary, including multiple times within the same statement. The next sequence number is not generated until
NEXTVAL is referenced. Continuing with the previous example, you would finish placing the customer's order by inserting the line items for the order:
INSERT INTO Line_items_tab (Orderno, Partno, Quantity) VALUES (Order_seq.CURRVAL, 20321, 3); INSERT INTO Line_items_tab (Orderno, Partno, Quantity) VALUES (Order_seq.CURRVAL, 29374, 1);
INSERT statement given in the previous section generated a new sequence number of 347, both rows inserted by the statements in this section insert rows with order numbers of 347.
NEXTVAL can be used in the following places:
SELECTlist of a
SETclause of an
NEXTVAL cannot be used in these places:
SELECTstatement with the
SELECTstatement with a
SELECTstatement that is combined with another
SELECTstatement with the
WHEREclause of a
DEFAULTvalue of a column in a
Sequence numbers can be kept in the sequence cache in the System Global Area (SGA). Sequence numbers can be accessed more quickly in the sequence cache than they can be read from disk.
The sequence cache consists of entries. Each entry can hold many sequence numbers for a single sequence.
Follow these guidelines for fast access to all sequence numbers:
When an application accesses a sequence in the sequence cache, the sequence numbers are read quickly. However, if an application accesses a sequence that is not in the cache, then the sequence must be read from disk to the cache before the sequence numbers are used.
If your applications use many sequences concurrently, then your sequence cache might not be large enough to hold all the sequences. In this case, access to sequence numbers might often require disk reads. For fast access to all sequences, be sure your cache has enough entries to hold all the sequences used concurrently by your applications.
The number of entries in the sequence cache is determined by the initialization parameter
SEQUENCE_CACHE_ENTRIES. The default value for this parameter is 10 entries. Oracle creates and uses sequences internally for auditing, grants of system privileges, grants of object privileges, profiles, debugging stored procedures, and labels. Be sure your sequence cache has enough entries to hold these sequences as well as sequences used by your applications.
If the value for your
SEQUENCE_CACHE_ENTRIES parameter is too low, then it is possible to skip sequence values. For example, assume that this parameter is set to 4, and that you currently have four cached sequences. If you create a fifth sequence, then it will replace the least recently used sequence in the cache. All of the remaining values in this displaced sequence are lost. In other words, if the displaced sequence originally held 10 cached sequence values, and only one had been used, then nine would be lost when the sequence was displaced.
When a sequence is read into the sequence cache, sequence values are generated and stored in a cache entry. These values can then be accessed quickly. The number of sequence values stored in the cache is determined by the
CACHE parameter in the
SEQUENCE statement. The default value for this parameter is 20.
SEQUENCE statement creates the
SEQ2 sequence so that 50 values of the sequence are stored in the
The first 50 values of
SEQ2 can then be read from the cache. When the 51st value is accessed, the next 50 values will be read from disk.
Choosing a high value for
CACHE lets you access more successive sequence numbers with fewer reads from disk to the sequence cache. However, if there is an instance failure, then all sequence values in the cache are lost. Cached sequence numbers also could be skipped after an export and import if transactions continue to access the sequence numbers while the export is running.
If you use the
NOCACHE option in the
SEQUENCE statement, then the values of the sequence are not stored in the sequence cache. In this case, every access to the sequence requires a disk read. Such disk reads slow access to the sequence. This
SEQUENCE statement creates the
SEQ3 sequence so that its values are never stored in the cache:
To use a sequence, your schema must contain the sequence or you must have been granted the
SELECT object privilege for another user's sequence.
To drop a sequence, use the SQL command
SEQUENCE. For example, the following statement drops the
When you drop a sequence, its definition is removed from the data dictionary. Any synonyms for the sequence remain, but return an error when referenced.
You can drop any sequence in your schema. To drop a sequence in another schema, you must have the
SEQUENCE system privilege.
A synonym is an alias for a table, view, snapshot, sequence, procedure, function, or package. The following sections explain how to create, use, and drop synonyms using SQL commands.
Use the SQL command
SYNONYM to create a synonym. The following statement creates a public synonym named
PUBLIC_EMP on the
EMP_TAB table contained in the schema of
You must have the
SYNONYM system privilege to create a private synonym in your schema, or the
SYNONYM system privilege to create a private synonym in another user's schema. To create a public synonym, you must have the
SYNONYM system privilege.
A synonym can be referenced in a DML statement the same way that the underlying object of the synonym can be referenced. For example, if a synonym named
EMP_TAB refers to a table or view, then the following statement is valid:
If the synonym named
FIRE_EMP refers to a standalone procedure or package procedure, then you could execute it in SQL*Plus or Enterprise Manager with the command
You can also use synonyms for GRANT and REVOKE statements, but not with other DML statements.
You can successfully use any private synonym contained in your schema or any public synonym, assuming that you have the necessary privileges to access the underlying object, either explicitly, from an enabled role, or from
PUBLIC. You can also reference any private synonym contained in another schema if you have been granted the necessary object privileges for the private synonym. You can only reference another user's synonym using the object privileges that you have been granted. For example, if you have the
SELECT privilege for the
EMP_TAB synonym, then you can query the
EMP_TAB synonym, but you cannot insert rows using the synonym for
To drop a synonym, use the SQL command
SYNONYM. To drop a private synonym, omit the
PUBLIC keyword; to drop a public synonym, include the
PUBLIC keyword. The following statement drops the private synonym named
The following statement drops the public synonym named
When you drop a synonym, its definition is removed from the data dictionary. All objects that reference a dropped synonym remain (for example, views and procedures) but become invalid.
You can drop any private synonym in your own schema. To drop a private synonym in another user's schema, you must have the
SYNONYM system privilege. To drop a public synonym, you must have the
SYNONYM system privilege.
You can create several tables and views and grant privileges in one operation using the SQL command
SCHEMA command is useful if you want to guarantee the creation of several tables and views and grants in one operation; if an individual table or view creation fails or a grant fails, then the entire statement is rolled back, and none of the objects are created or the privileges granted.
For example, the following statement creates two tables and a view that joins data from the two tables:
CREATE SCHEMA AUTHORIZATION scott CREATE VIEW Sales_staff AS SELECT Empno, Ename, Sal, Comm FROM Emp_tab WHERE Deptno = 30 WITH CHECK OPTION CONSTRAINT Sales_staff_cnst CREATE TABLE Dept_tab ( Deptno NUMBER(3) PRIMARY KEY, Dname VARCHAR2(15), Loc VARCHAR2(25)) CREATE TABLE Emp_tab ( Empno NUMBER(5) PRIMARY KEY, Ename VARCHAR2(15) NOT NULL, Job VARCHAR2(10), Mgr NUMBER(5), Hiredate DATE DEFAULT (sysdate), Sal NUMBER(7,2), Comm NUMBER(7,2), Deptno NUMBER(3) NOT NULL CONSTRAINT Dept_fkey REFERENCES Dept_tab(Deptno)) GRANT SELECT ON Sales_staff TO human_resources;
SCHEMA command does not support Oracle extensions to the ANSI
VIEW commands (for example, the
To create schema objects, such as multiple tables, using the
SCHEMA command, you must have the required privileges for any included operation.
You should decide when you want to use partial and complete global object names in the definition of views, synonyms, and procedures. Keep in mind that database names should be stable, and databases should not be unnecessarily moved within a network.
In a distributed database system, each database should have a unique global name. The global name is composed of the database name and the network domain that contains the database. Each schema object in the database then has a global object name consisting of the schema object name and the global database name.
Because Oracle ensures that the schema object name is unique within a database, you can ensure that it is unique across all databases by assigning unique global database names. You should coordinate with your database administrator on this task, because it is usually the DBA who is responsible for assigning database names.
An object name takes the following form:
Some examples include:
A session is established when a user logs onto a database. Object names are resolved relative to the current user session. The username of the current user is the default schema. The database to which the user has directly logged-on is the default database.
Oracle has separate namespaces for different classes of objects. All objects in the same namespace must have distinct names, but two objects in different namespaces can have the same name. Tables, views, snapshots, sequences, synonyms, procedures, functions, and packages are in a single namespace. Triggers, indexes, and clusters each have their own individual namespace. For example, there can be a table, trigger, and index all named
Based on the context of an object name, Oracle searches the appropriate namespace when resolving the name to an object. For example, in the following statement:
Oracle looks up
TEST in the cluster namespace.
Rather than supplying an object name directly, you can also refer to an object using a synonym. A private synonym name has the same syntax as an ordinary object name. A public synonym is implicitly in the
PUBLIC schema, but users cannot explicitly qualify a synonym with the schema
Synonyms can only be used to reference objects in the same namespace as tables. Due to the possibility of synonyms, the following rules are used to resolve a name in a context that requires an object in the table namespace:
When global object names are used in a distributed database (either explicitly or indirectly within a synonym), the local Oracle session resolves the reference as is locally required (for example, resolving a synonym to a remote table's global object name). After the partially resolved statement is shipped to the remote database, the remote Oracle session completes the resolution of the object as above.
See Oracle9i Database Concepts for more information about name resolution in a distributed database.
If necessary, you can rename some schema objects using two different methods: drop and re-create the object, or rename the object using the SQL command
If you use the
RENAME command to rename a table, view, sequence, or a private synonym of a table, view, or sequence, then grants made for the object are carried forward for the new name, and the next statement renames the
You cannot rename a stored PL/SQL program unit, public synonym, index, or cluster. To rename such an object, you must drop and re-create it.
Renaming a schema object has the following effects:
To rename an object, you must be the owner of the object.
The following statement sets the current schema of the session to the schema name given in the statement.
Subsequent SQL statements use this schema name for the schema qualifier when the qualifier is missing. Note that the session still has only the privileges of the current user and does not acquire any extra privileges by the above
emp_tab is not schema-qualified, the table name is resolved under schema
joe. But if
scott does not have select privilege on table
scott cannot execute the
The data dictionary provides many views that provide information about schema objects. The following is a summary of the views associated with schema objects:
The following query lists all of the objects owned by the user issuing the query:
The query above might return results similar to the following:
Column information, such as name, datatype, length, precision, scale, and default data values, can be listed using one of the views ending with the
_COLUMNS suffix. For example, the following query lists all of the default column values for the
SELECT Table_name, Column_name, Data_default FROM User_tab_columns WHERE Table_name = 'DEPT_TAB' OR Table_name = 'EMP_TAB';
Considering the example statements at the beginning of this section, a display similar to the one below is displayed:
TABLE_NAME COLUMN_NAME DATA_DEFAULT
---------- --------------- --------------------
DEPT_TAB LOC ('NEW YORK')
EMP_TAB HIREDATE (sysdate)
When you create a view or a synonym, the view or synonym is based on its underlying base object. The
_DEPENDENCIES data dictionary views can be used to reveal the dependencies for a view and the
_SYNONYMS data dictionary views can be used to list the base object of a synonym. For example, the following query lists the base objects for the synonyms created by the user
This query could return information similar to the following: