SQL ADD CONSTRAINT Keyword

The ADD CONSTRAINT command is used to add a constraint to an existing table after it has been created. Constraints help enforce rules for the data in a table.

SQL ADD CONSTRAINT Common Constraint Types

• PRIMARY KEY
• FOREIGN KEY
• UNIQUE
• CHECK
• DEFAULT

SQL ADD CONSTRAINT Syntax

ALTER TABLE table_name
ADD CONSTRAINT constraint_name constraint_type (column1, column2, ...);

SQL ADD CONSTRAINT Example

This query adds a primary key constraint named PK_Patient on the patient_id and last_name columns of the Patients table:

ALTER TABLE Patients
ADD CONSTRAINT PK_Patient PRIMARY KEY (patient_id, last_name);

Note: This syntax may not be supported in our playground. If you receive a syntax error near "CONSTRAINT", it means our playground does not support adding named constraints using ADD CONSTRAINT.

As a workaround, constraints like primary keys should be defined at the time of table creation, or use simplified constraint syntax if available.

SQL ADD CONSTRAINT Labs