Examples of Defining Integrity Constraints

Note: If you are not sure when unknown values result in NULL conditions, review the truth tables for the logical operators AND and

OR in Oracle Database SQL Reference

Rules for Multiple CHECK Constraints

A single column can have multiple CHECK constraints that reference the column in its definition. There is no limit to the number of CHECK constraints that can be defined that reference a column.

The order in which the constraints are evaluated is not defined, so be careful not to rely on the order or to define multiple constraints that conflict with each other.

Choosing Between CHECK and NOT NULL Integrity Constraints

According to the ANSI/ISO standard, a NOT NULL integrity constraint is an example of a CHECK integrity constraint, where the condition is the following:

CHECK (Column_name IS NOT NULL)

Therefore, NOT NULL integrity constraints for a single column can, in practice, be written in two forms: using the NOT NULL constraint or a CHECK constraint. For ease of use, you should always choose to define NOT NULL integrity constraints, instead of CHECK constraints with the IS NOT NULL condition.

In the case where a composite key can allow only all nulls or all values, you must use a CHECK integrity constraint. For example, the following expression of a CHECK integrity constraint allows a key value in the composite key made up of columns C1 and C2 to contain either all nulls or all values:

CHECK ((C1 IS NULL AND C2 IS NULL) OR (C1 IS NOT NULL AND C2 IS NOT NULL))

Examples of Defining Integrity Constraints

Here are some examples showing how to create simple constraints during the prototype phase of your database design.

Each constraint is given a name in these examples. Naming the constraints prevents the database from creating multiple copies of the same constraint, with different system-generated names, if the DDL is run multiple times.

Maintaining Data Integrity Through Constraints 3-17

Examples of Defining Integrity Constraints

See Also: Oracle Database Administrator's Guide for information on creating and maintaining constraints for a large production database

Example: Defining Integrity Constraints with the CREATE TABLE Command

The following examples of CREATE TABLE statements show the definition of several integrity constraints:

Example: Defining Constraints with the ALTER TABLE Command

You can also define integrity constraints using the constraint clause of the ALTER TABLE command. For example:

CREATE UNIQUE INDEX I_dept ON Dept_tab(deptno);

ALTER TABLE Dept_tab

ADD CONSTRAINT Dept_pkey PRIMARY KEY (deptno);

ALTER TABLE Emp_tab

ADD CONSTRAINT Dept_fkey FOREIGN KEY (Deptno) REFERENCES Dept_tab;

ALTER TABLE Emp_tab MODIFY (Ename VARCHAR2(15) NOT NULL);

You cannot create a validated constraint on a table if the table already contains any rows that would violate the constraint.

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