Check Constraints

What is a check constraint?

A check constraint allows you to specify a condition on each row in a table.

Note

• A check constraint can NOT be defined on an SQL View.
• The check constraint defined on a table must refer to only columns in that table. It can not refer to columns in other tables.
• A check constraint can NOT include an SQL Subquery.

A check constraint can be defined in either an SQL CREATE TABLE statement or an SQL ALTER TABLE statement.

Using a CREATE TABLE statement

The syntax for creating a check constraint using a CREATE TABLE statement is:

CREATE TABLE table_name
(
  column1 datatype null/not null,
  column2 datatype null/not null,

  ...

  CONSTRAINT constraint_name CHECK (column_name condition) [DISABLE]

);

The DISABLE keyword is optional. If you create a check constraint using the DISABLE keyword, the constraint will be created, but the condition will not be enforced.

For Example

CREATE TABLE suppliers
(
  supplier_id numeric(4),
  supplier_name varchar2(50),
  CONSTRAINT check_supplier_id
  CHECK (supplier_id BETWEEN 100 and 9999)
);

In this first example, we've created a check constraint on the suppliers table called check_supplier_id. This constraint ensures that the supplier_id field contains values between 100 and 9999.

CREATE TABLE suppliers
(
  supplier_id numeric(4),
  supplier_name varchar2(50),
  CONSTRAINT check_supplier_name
  CHECK (supplier_name = upper(supplier_name))
);

In this second example, we've created a check constraint called check_supplier_name. This constraint ensures that the supplier_name column always contains uppercase characters.

Using an ALTER TABLE statement

The syntax for creating a check constraint in an ALTER TABLE statement is:

ALTER TABLE table_name
add CONSTRAINT constraint_name CHECK (column_name condition) [DISABLE];

The DISABLE keyword is optional. If you create a check constraint using the DISABLE keyword, the constraint will be created, but the condition will not be enforced.

For Example

ALTER TABLE suppliers
add CONSTRAINT check_supplier_name
  CHECK (supplier_name IN ('IBM', 'Microsoft', 'NVIDIA'));

In this example, we've created a check constraint on the existing suppliers table called check_supplier_name. It ensures that the supplier_name field only contains the following values: IBM, Microsoft, or NVIDIA.

Drop a Check Constraint

The syntax for dropping a check constraint is:

ALTER TABLE table_name
drop CONSTRAINT constraint_name;

For Example

ALTER TABLE suppliers
drop CONSTRAINT check_supplier_id;

In this example, we're dropping a check constraint on the suppliers table called check_supplier_id.

Enable a Check Constraint

The syntax for enabling a check constraint is:

ALTER TABLE table_name
enable CONSTRAINT constraint_name;

For Example

ALTER TABLE suppliers
enable CONSTRAINT check_supplier_id;

In this example, we're enabling a check constraint on the suppliers table called check_supplier_id.

Disable a Check Constraint

The syntax for disabling a check constraint is:

ALTER TABLE table_name
disable CONSTRAINT constraint_name;

For Example

ALTER TABLE suppliers
disable CONSTRAINT check_supplier_id;

In this example, we're disabling a check constraint on the suppliers table called check_supplier_id.