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Constraint- and Index-Related Views
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How do you find out what constraints exist on a table s columns When a process fails with the message Referential integrity constraint violated, what s the best way to find out what the constraint and the affected tables are The constraint- and index-related data dictionary views are critical for resolving problems similar to these. In the following sections, you ll examine the key constraint- and index-related views.
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DBA_CONSTRAINTS
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The DBA_CONSTRAINTS view provides information on all types of table constraints in the database. You can query this view when you need to figure out what type of constraints a table has. The view lists several types of constraints, as shown by the following query:
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CHAPTER 7 SCHEMA MAN AGEM ENT
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SQL> SELECT DISTINCT constraint_type FROM DBA_CONSTRAINTS; Constraint_type -------------------C P R U SQL>
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check constraints */ primary key constraint */ referential integrity (foreign key) constraint */ unique key constraint */
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The following query lets you know what, if any, constraints are in the TESTD table. The response indicates that the table has a single check constraint defined on it. The SYS prefix in the NAME column shows that CONSTRAINT_NAME is a default name, not one that was explicitly named by the owner of the table. SQL> SELECT constraint_name, constraint_type 2 FROM DBA_CONSTRAINTS 3* WHHERE table_name='TESTD'; CONSTRAINT_NAME ------------------SYS_C005263 SQL> CONSTRAINT_TYPE --------------C
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Note that if you want to see the particular referential constraints and the delete rule, you have to use a slight variation on the preceding query: SQL> SELECT constraint_name, constraint_type, R_constraint_name, delete_rule FROM dba_constraints WHERE table_name='ORDERS'; CONSTRAINT_NAME ---------------------ORDER_DATE_NN ORDER_CUSTOMER_ID_NN ORDER_MODE_LOV ORDER_TOTAL_MIN ORDER_PK ORDERS_SALES_REP_FK ORDERS_CUSTOMER_ID_FK 7 rows selected. SQL> TYPE -----C C C C P R R R_CONSTRAINT_NAME ----------------DELETE_RULE -----------
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EMP_EMP_ID_PK CUSTOMERS_PK
SET NULL SET NULL
DBA_CONS_COLUMNS
The DBA_CONS_COLUMNS view provides the column name and position in the table on which a constraint is defined. Here s the view: SQL> DESC DBA_CONS_COLUMNS Name ---------------OWNER CONSTRAINT_NAME TABLE_NAME COLUMN_NAME
CHAPTER 7 SCHEMA MAN AGEM ENT
POSITION SQL>
Using Views
A view is a virtual table it s a specific representation of a table or set of tables, and it is defined by using a SELECT statement. A view doesn t physically exist, like regular tables, as part of a tablespace. A view, in effect, creates a virtual table or subtable with only those rows and/or columns that you want the user to access. A view is the product of a stored query, so only the view definition is stored in the data dictionary. When you export the database, you ll see the statement exporting views, but that s referring only to the view definitions and not to any physical objects You can query views and even modify, remove, or add data using UPDATE, DELETE, or INSERT statements, provided the user has the appropriate privileges on the underlying base tables. For example, if you grant a user only the INSERT privilege on the base table underlying a view, that user can only insert new rows into that table but won t be able to select, update, or delete any rows. Views are used in applications for several reasons, including the following: Reduce complexity. Improve security. Increase convenience. Rename table columns. Customize the data for users. Protect data integrity. You create views by using a SQL statement that describes the composition of the view. When you invoke the view, the query by which the view is defined is executed, and the results are presented to you. A query on a view looks exactly like a regular query, but the database converts the query on the view into an identical query on the underlying tables. In order to create a view, you must have the CREATE VIEW system privilege, and to create a view in any schema, rather than just in your own, you need the CREATE ANY VIEW system privilege. In addition, you must either own the underlying tables or must be granted the SELECT, INSERT, UPDATE, and DELETE object privileges on all the tables underlying the view. You can use a view to add column-level or value-based security to a table. Columnlevel security is provided by creating views that give access to selected columns of base tables. Valuebased security involves using a WHERE clause in the view definition, which displays only selected rows of base tables. In order to use a view, a user needs privileges on the view itself, and not on the base tables underlying the view. The following statement creates a view called MY_EMPLOYEES that gives a specific manager s information only on the employees managed directly by her: SQL> CREATE VIEW my_employees AS SELECT employee_id, first_name, last_name, salary FROM employees WHERE manager_id=122; View created. SQL>
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