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ADD CONSTRAINT	Use the ADD CONSTRAINT statement to add constraints to columns.	false

The ADD CONSTRAINT statement is part of ALTER TABLE and can add the following constraints to columns:

{{site.data.alerts.callout_info}} The Primary Key and Not Null constraints can only be applied through CREATE TABLE. The Default constraint is managed through ALTER COLUMN.{{site.data.alerts.end}}

Synopsis

{% include sql/diagrams/add_constraint.html %}

Required Privileges

The user must have the CREATE privilege on the table.

Parameter	Description
`table_name`	The name of the table containing the column you want to constrain.
`name`	The name of the constraint, which must be unique to its table and follow these identifier rules.
`constraint_elem`	The Check, Foreign Keys, Unique constraint you want to add. Adding/changing a Default constraint is done through `ALTER COLUMN`. Adding/changing the table's Primary Key is not supported through `ALTER TABLE`; it can only be specified during table creation.

Adding the Unique constraint requires that all of a column's values be distinct from one another (except for NULL values).

> ALTER TABLE orders ADD CONSTRAINT id_customer_unique UNIQUE (id, customer);

Adding the Check constraint requires that all of a column's values evaluate to TRUE for a Boolean expression.

> ALTER TABLE orders ADD CONSTRAINT total_0_check CHECK (total > 0);

Adding the Foreign Key constraint requires that all of a column's values are equal to an existing value in the column it references.

> ALTER TABLE orders ADD CONSTRAINT customer_fk FOREIGN KEY (customer) REFERENCEs customers (id);