Viewing Definitions of Integrity Constraints

specify this referential action, include the ON DELETE CASCADE option in the definition of the FOREIGN 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 to null. To specify this referential action, include the ON DELETE SET NULL option in the definition of the FOREIGN KEY constraint. For example:

CREATE TABLE Emp_tab (

FOREIGN KEY (Deptno) REFERENCES Dept_tab ON DELETE SET NULL);

Viewing Definitions of Integrity Constraints

The data dictionary contains the following views that relate to integrity constraints:

You can query these views to find the names of constraints, what columns they affect, and other information to help you manage constraints.

See Also: Oracle Database Reference for information on each view

Examples of Defining Integrity Constraints

The following CREATE TABLE statements define a number of integrity constraints:

CREATE TABLE Dept_tab (

Deptno NUMBER(3) PRIMARY KEY,

Dname VARCHAR2(15),

Loc VARCHAR2(15),

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CONSTRAINT Dname_ukey UNIQUE (Dname, Loc),

CONSTRAINT LOC_CHECK1

CHECK (Loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')));

CONSTRAINT Dept_fkey REFERENCES Dept_tab);

Example 1: Listing All of Your Accessible Constraints The following query lists all constraints defined on all tables accessible to the user:

SELECT Constraint_name, Constraint_type, Table_name,

R_constraint_name

FROM User_constraints;

Considering the example statements at the beginning of this section, a list similar to this is returned:

Notice the following:

■Some constraint names are user specified (such as DNAME_UKEY), while others are system specified (such as SYS_C00275).

■Each constraint type is denoted with a different character in the CONSTRAINT_ TYPE column. The following table summarizes the characters used for each constraint type.

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Note: An additional constraint type is indicated by the character "V" in the CONSTRAINT_TYPE column. This constraint type corresponds to constraints created using the WITH CHECK OPTION for views.

Example 2: Distinguishing NOT NULL Constraints from CHECK Constraints In the previous example, several constraints are listed with a constraint type of C. To distinguish which constraints are NOT NULL constraints and which are CHECK constraints in the EMP_TAB and DEPT_TAB tables, submit the following query:

SELECT Constraint_name, Search_condition

FROM User_constraints

WHERE (Table_name = 'DEPT_TAB' OR Table_name = 'EMP_TAB') AND

Constraint_type = 'C';

Considering the example CREATE TABLE statements at the beginning of this section, a list similar to this is returned:

CONSTRAINT_NAME SEARCH_CONDITION

Notice that the following are explicitly listed in the SEARCH_CONDITION column:

■NOT NULL constraints

■The conditions for user-defined CHECK constraints

Example 3: Listing Column Names that Constitute an Integrity Constraint The following query lists all columns that constitute the constraints defined on all tables accessible to you, the user:

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SELECT Constraint_name, Table_name, Column_name

FROM User_cons_columns;

Considering the example statements at the beginning of this section, a list similar to this is returned:

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