Constraint Specifications

SQL Language Guide : 8. SQL Statements : ALTER TABLE : Constraint Specifications

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When a constraint is added to a table, the table is checked to ensure that its contents do not violate the constraint. If the contents of the table do violate the constraint, the DBMS Server returns an error and does not add the constraint.

The following table summarizes the elements of the constraint_clause:

Type	Keyword	Example
Referential	REFERENCES	ALTER TABLE dept_const ADD CONSTRAINT chkmgr FOREIGN KEY(mgr) REFERENCES emp(ename);
Unique	UNIQUE	ALTER TABLE emp_const ADD UNIQUE (eno, ename);
Primary key	PRIMARY KEY	ALTER TABLE emp_const ADD CONSTRAINT ekey PRIMARY KEY(eno);

For more information, see Constraints.