Referring to Schema Objects and Parts

This section tells you how to refer to schema objects and their parts in the context of a SQL statement. This section shows you:

• the general syntax for referring to an object

• how Oracle resolves a reference to an object

• how to refer to objects in schemas other than your own

• how to refer to objects in remote databases

The following diagram shows the general syntax for referring to an object or a part:

where:

object	is the name of the object.
schema	is the schema containing the object. The schema qualifier allows you to refer to an object in a schema other than your own. You must be granted privileges to refer to objects in other schemas. If you omit schema, Oracle assumes that you are referring to an object in your own schema.
	Only schema objects can be qualified with schema. Schema objects are shown with list item 9. Nonschema objects, also shown with list item 9, cannot be qualified with schema because they are not schema objects. (An exception is public synonyms, which can optionally be qualified with "PUBLIC". The quotation marks are required.)
part	is a part of the object. This identifier allows you to refer to a part of a schema object, such as a column or a partition of a table. Not all types of objects have parts.
dblink	applies only when you are using Oracle's distributed functionality. This is the name of the database containing the object. The dblink qualifier lets you refer to an object in a database other than your local database. If you omit dblink, Oracle assumes that you are referring to an object in your local database. Not all SQL statements allow you to access objects on remote databases.

You can include spaces around the periods separating the components of the reference to the object, but it is conventional to omit them.

How Oracle Resolves Schema Object References

When you refer to an object in a SQL statement, Oracle considers the context of the SQL statement and locates the object in the appropriate namespace. After locating the object, Oracle performs the statement's operation on the object. If the named object cannot be found in the appropriate namespace, Oracle returns an error.

The following example illustrates how Oracle resolves references to objects within SQL statements. Consider this statement that adds a row of data to a table identified by the name DEPT:

INSERT INTO dept 
   VALUES (50, 'SUPPORT', 'PARIS');

Based on the context of the statement, Oracle determines that DEPT can be:

• A table in your own schema

• A view in your own schema

• A private synonym for a table or view

• A public synonym

Oracle always attempts to resolve an object reference within the namespaces in your own schema before considering namespaces outside your schema. In this example, Oracle attempts to resolve the name DEPT as follows:

1. First, Oracle attempts to locate the object in the namespace in your own schema containing tables, views, and private synonyms. If the object is a private synonym, Oracle locates the object for which the synonym stands. This object could be in your own schema, another schema, or on another database. The object could also be another synonym, in which case Oracle locates the object for which this synonym stands.

2. If the object is in the namespace, Oracle attempts to perform the statement on the object. In this example, Oracle attempts to add the row of data to DEPT. If the object is not of the correct type for the statement, Oracle returns an error. In this example, DEPT must be a table, view, or a private synonym resolving to a table or view. If DEPT is a sequence, Oracle returns an error.

3. If the object is not in any namespace searched in thus far, Oracle searches the namespace containing public synonyms. If the object is in that namespace, Oracle attempts to perform the statement on it. If the object is not of the correct type for the statement, Oracle returns an error. In this example, if DEPT is a public synonym for a sequence, Oracle returns an error.

Referring to Objects in Other Schemas

To refer to objects in schemas other than your own, prefix the object name with the schema name:

schema.object

For example, this statement drops the EMP table in the schema SCOTT:

DROP TABLE scott.emp

Referring to Objects in Remote Databases

To refer to objects in databases other than your local database, follow the object name with the name of the database link to that database. A database link is a schema object that causes Oracle to connect to a remote database to access an object there. This section tells you:

• How to create database links

• How to use database links in your SQL statements

Creating Database Links

You create a database link with the CREATE DATABASE LINK statement described in Chapter 7, "SQL Statements". The statement allows you to specify this information about the database link:

• The name of the database link

• The database connect string to access the remote database

• The username and password to connect to the remote database

Oracle stores this information in the data dictionary.

Database Link Names

When you create a database link, you must specify its name. Database link names are different from names of other types of objects. They can be as long as 128 bytes and can contain periods (.) and the "at" sign (@).

The name that you give to a database link must correspond to the name of the database to which the database link refers and the location of that database in the hierarchy of database names. The following syntax diagram shows the form of the name of a database link:

dblink::=

where:

database	should specify name portion of the global name of the remote database to which the database link connects. This global name is stored in the data dictionary of the remote database; you can see this name in the GLOBAL_NAME view.
domain	should specify the domain portion of the global name of the remote database to which the database link connects. If you omit domain from the name of a database link, Oracle qualifies the database link name with the domain of your local database as it currently exists in the data dictionary.
connect_descriptor	allows you to further qualify a database link. Using connect descriptors, you can create multiple database links to the same database. For example, you can use connect descriptors to create multiple database links to different instances of the Oracle Parallel Server that access the same database.

The combination database.domain is sometimes called the "service name".

See Also: Net8 Administrator's Guide.

Username and Password

Oracle uses the username and password to connect to the remote database. The username and password for a database link are optional.

Database Connect String

The database connect string is the specification used by Net8 to access the remote database. For information on writing database connect strings, see the Net8 documentation for your specific network protocol. The database string for a database link is optional.

Referring to Database Links

Database links are available only if you are using Oracle's distributed functionality. When you issue a SQL statement that contains a database link, you can specify the database link name in one of these forms:

complete	is the complete database link name as stored in the data dictionary, including the database, domain, and optional connect_descriptor components.
partial	is the database and optional connect_descriptor components, but not the domain component.

Oracle performs these tasks before connecting to the remote database:

1. If the database link name specified in the statement is partial, Oracle expands the name to contain the domain of the local database as found in the global database name stored in the data dictionary. (You can see the current global database name in the GLOBAL_NAME data dictionary view.)

2. Oracle first searches for a private database link in your own schema with the same name as the database link in the statement. Then, if necessary, it searches for a public database link with the same name.
   • Oracle always determines the username and password from the first matching database link (either private or public). If the first matching database link has an associated username and password, Oracle uses it. If it does not have an associated username and password, Oracle uses your current username and password.

   • If the first matching database link has an associated database string, Oracle uses it. If not, Oracle searches for the next matching (public) database link. If no matching database link is found, or if no matching link has an associated database string, Oracle returns an error.

3. Oracle uses the database string to access the remote database. After accessing the remote database, if the value of the GLOBAL_NAMES parameter is TRUE, Oracle verifies that the database.domain portion of the database link name matches the complete global name of the remote database. If this condition is true, Oracle proceeds with the connection, using the username and password chosen in Step 2. If not, Oracle returns an error.

4. If the connection using the database string, username, and password is successful, Oracle attempts to access the specified object on the remote database using the rules for resolving object references and referring to objects in other schemas discussed earlier in this section.

You can disable the requirement that the database.domain portion of the database link name must match the complete global name of the remote database by setting to FALSE the initialization parameter GLOBAL_NAMES or the GLOBAL_NAMES parameter of the ALTER SYSTEM or ALTER SESSION statement.

See Also: Oracle8i Distributed Database Systems for more information on remote name resolution.

Referencing Object Type Attributes and Methods

To reference object type attributes or methods in a SQL statement, you must fully qualify the reference with a table alias. Consider the following example:

CREATE TYPE person AS OBJECT 
   (ssno VARCHAR(20), 
    name VARCHAR (10));

CREATE TABLE emptab (pinfo person);

In a SQL statement, reference to the SSNO attribute must be fully qualified using a table alias, as illustrated below:

SELECT e.pinfo.ssno FROM emptab e;

UPDATE emptab e SET e.pinfo.ssno = '510129980' 
   WHERE e.pinfo.name = 'Mike';

To reference an object type's member method that does not accept arguments, you must provide "empty" parentheses. For example, assume that AGE is a method in the person type that does not take arguments. In order to call this method in a SQL statement, you must provide empty parentheses as shows in this example:

SELECT e.pinfo.age() FROM emptab e 
   WHERE e.pinfo.name = 'Mike';

See Also: Oracle8i Concepts for more information on user-defined datatypes.