Constraints

Constraints offer additional data integrity by enforcing conditions on the data within a column. Whenever values are manipulated (inserted, deleted, or updated), constraints are checked and modifications that violate constraints are rejected.

For example, the UNIQUE constraint requires that all values in a column be unique from one another (except NULL values). If you attempt to write a duplicate value, the constraint rejects the entire statement.

Supported constraints

Constraint	Description
	Values must return TRUE or NULL for a Boolean expression.
	If a value is not defined for the constrained column in an INSERT statement, the DEFAULT value is written to the column.
	Values must exactly match existing values from the column it references.
	Values may not be NULL.
	Values must uniquely identify each row (one per table). This behaves as if the NOT NULL and UNIQUE constraints are applied, as well as automatically creates an for the table using the constrained columns.
	Each non-NULL value must be unique. This also automatically creates an for the table using the constrained columns.

Using constraints

Add constraints

How you add constraints depends on the number of columns you want to constrain, as well as whether or not the table is new.

• One column of a new table has its constraints defined after the column’s data type. For example, this statement applies the PRIMARY KEY constraint to foo.a:

  > CREATE TABLE foo (a INT PRIMARY KEY);
  

• Multiple columns of a new table have their constraints defined after the table’s columns. For example, this statement applies the PRIMARY KEY constraint to foo’s columns a and b:

  > CREATE TABLE bar (a INT, b INT, PRIMARY KEY (a,b));
  

The DEFAULT and NOT NULL constraints cannot be applied to multiple columns.

• Existing tables can have the following constraints added:

  • CHECK, FOREIGN KEY, and UNIQUE constraints can be added through . For example, this statement adds the UNIQUE constraint to baz.id:

    > ALTER TABLE baz ADD CONSTRAINT id_unique UNIQUE (id);
    

  • DEFAULT values and NOT NULL constraints can be added through . For example, this statement adds the to baz.bool:

    > ALTER TABLE baz ALTER COLUMN bool SET DEFAULT true;
    

  • constraints can be added with / in the following circumstances:

    • A statement precedes the ADD CONSTRAINT/ADD PRIMARY KEY statement in the same transaction. For examples, see and .
    • The current , the default primary key created if none is explicitly defined at table creation.
    • The ADD CONSTRAINT/ADD PRIMARY KEY is in the same transaction as a CREATE TABLE statement with no primary key defined.

Order of constraints

The order in which you list constraints is not important because constraints are applied to every modification of their respective tables or columns.

Name constraints on new tables

You can name constraints applied to new tables using the CONSTRAINT clause before defining the constraint:

> CREATE TABLE foo (a INT CONSTRAINT another_name PRIMARY KEY);

> CREATE TABLE bar (a INT, b INT, CONSTRAINT yet_another_name PRIMARY KEY (a,b));

View constraints

To view a table’s constraints, use or .

Remove constraints

The procedure for removing a constraint depends on its type:

Constraint Type	Procedure
	Use .
	Use .
	Use .
	Use .
	Primary key constraints can be dropped with if an statement follows the DROP CONSTRAINT statement in the same transaction.
	The UNIQUE constraint cannot be dropped directly. To remove the constraint, that was created by the constraint, e.g., DROP INDEX my_unique_constraint.

Change constraints

The procedure for changing a constraint depends on its type:

Constraint Type	Procedure
	that adds a new CHECK constraint (), and then remove the existing one ().
	The DEFAULT value can be changed through .
	that adds a new FOREIGN KEY constraint (), and then remove the existing one ().
	The NOT NULL constraint cannot be changed, only added and removed with .
	To change a primary key, use an statement. When you change a primary key with , the old primary key index becomes a secondary index. If you do not want the old primary key to become a secondary index, use / to change the primary key.
	that adds a new UNIQUE constraint (), and then remove the existing one ().

See also