Syntax for Schema Objects and Parts in SQL Statements

How Oracle Database 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 operation specified by the statement on the object. If the named object cannot be found in the appropriate namespace, then 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 departments:

INSERT INTO departments VALUES (
   280, 'ENTERTAINMENT_CLERK', 206, 1700);

Based on the context of the statement, Oracle determines that departments 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 departments 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, then 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, then Oracle attempts to perform the statement on the object. In this example, Oracle attempts to add the row of data to departments. If the object is not of the correct type for the statement, then Oracle returns an error. In this example, departments must be a table, view, or a private synonym resolving to a table or view. If departments is a sequence, then Oracle returns an error.

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

If a public synonym has any dependent tables or user-defined types, then you cannot create an object with the same name as the synonym in the same schema as the dependent objects.

If a synonym does not have any dependent tables or user-defined types, then you can create an object with the same name in the same schema as the dependent objects. Oracle invalidates any dependent objects and attempts to revalidate them when they are next accessed.

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 employees table in the sample schema hr:

DROP TABLE hr.employees

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 statement CREATE DATABASE LINK. The statement lets you 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::=

Description of the illustration dblink.gif

where:

• database should specify the 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 data dictionary 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, then Oracle qualifies the database link name with the domain of your local database as it currently exists in the data dictionary.

• connect_descriptor lets you 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 Real Application Clusters that access the same database.

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

See Also: Oracle Net Services 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 Oracle Net to access the remote database. For information on writing database connect strings, see the Oracle Net documentation for your specific network protocol. The database string for a database link is optional.

Referring to Partitioned Tables and Indexes

Tables and indexes can be partitioned. When partitioned, these schema objects consist of a number of parts called partitions, all of which have the same logical attributes. For example, all partitions in a table share the same column and constraint definitions, and all partitions in an index share the same index columns.

Partition-extended and subpartition-extended names let you perform some partition-level and subpartition-level operations, such as deleting all rows from a partition or subpartition, on only one partition or subpartition. Without extended names, such operations would require that you specify a predicate (WHERE clause). For range- and list-partitioned tables, trying to phrase a partition-level operation with a predicate can be cumbersome, especially when the range partitioning key uses more than one column. For hash partitions and subpartitions, using a predicate is more difficult still, because these partitions and subpartitions are based on a system-defined hash function.

Partition-extended names let you use partitions as if they were tables. An advantage of this method, which is most useful for range-partitioned tables, is that you can build partition-level access control mechanisms by granting (or revoking) privileges on these views to (or from) other users or roles.To use a partition as a table, create a view by selecting data from a single partition, and then use the view as a table.

You can specify partition-extended or subpartition-extended table names for the following DML statements:

• DELETE

• INSERT

• LOCK TABLE

• SELECT

• UPDATE

Syntax  The basic syntax for using partition-extended and subpartition-extended table names is:

partition_extended_name::=

Restrictions on Extended Names Currently, the use of partition-extended and subpartition-extended table names has the following restrictions:

• No remote tables: A partition-extended or subpartition-extended table name cannot contain a database link (dblink) or a synonym that translates to a table with a dblink. To use remote partitions and subpartitions, create a view at the remote site that uses the extended table name syntax and then refer to the remote view.

• No synonyms: A partition or subpartition extension must be specified with a base table. You cannot use synonyms, views, or any other objects.

Example In the following statement, sales is a partitioned table with partition sales_q1_2000. You can create a view of the single partition sales_q1_2000, and then use it as if it were a table. This example deletes rows from the partition.

CREATE VIEW Q1_2000_sales AS
   SELECT * FROM sales PARTITION (SALES_Q1_2000);

DELETE FROM Q1_2000_sales WHERE amount_sold < 0; 

Referring to Object Type Attributes and Methods

To refer to object type attributes or methods in a SQL statement, you must fully qualify the reference with a table alias. Consider the following example from the sample schema oe, which contains a type cust_address_typ and a table customers with a cust_address column based on the cust_address_typ:

CREATE TYPE cust_address_typ
  OID '82A4AF6A4CD1656DE034080020E0EE3D'
  AS OBJECT
    ( street_address     VARCHAR2(40)
    , postal_code        VARCHAR2(10)
    , city               VARCHAR2(30)
    , state_province     VARCHAR2(10)
    , country_id         CHAR(2)
    );
/
CREATE TABLE customers
    ( customer_id        NUMBER(6)     
    , cust_first_name    VARCHAR2(20) CONSTRAINT cust_fname_nn NOT NULL
    , cust_last_name     VARCHAR2(20) CONSTRAINT cust_lname_nn NOT NULL
    , cust_address       cust_address_typ
.
.
.

In a SQL statement, reference to the postal_code attribute must be fully qualified using a table alias, as illustrated in the following example:

SELECT c.cust_address.postal_code FROM customers c;

UPDATE customers c SET c.cust_address.postal_code = 'GU13 BE5' 
   WHERE c.cust_address.city = 'Fleet';

To reference a member method that does not accept arguments, you must provide empty parentheses. For example, the sample schema oe contains an object table categories_tab, based on catalog_typ, which contains the member function getCatalogName. In order to call this method in a SQL statement, you must provide empty parentheses as shown in this example:

SELECT TREAT(VALUE(c) AS catalog_typ).getCatalogName() "Catalog Type"
   FROM categories_tab c
   WHERE category_id = 90;

Catalog Type
------------------------------------
online catalog