SQL -UNIQUE Constraint

The UNIQUE Constraint prevents two records from having identical values in a particular column. In the CUSTOMERS table, for example, you might want to prevent two or more people from having identical age.

Example:

For example, the following SQL creates a new table called CUSTOMERS and adds five columns. Here AGE column is set to UNIQUE, so that you can not have two records with same age:

CREATE TABLE CUSTOMERS( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL UNIQUE, ADDRESS CHAR (25) , SALARY DECIMAL (18, 2), PRIMARY KEY (ID) );

If CUSTOMERS table has already been created, then to add a UNIQUE constraint to AGE column, you would write a statement similar to the following:

ALTER TABLE CUSTOMERS MODIFY AGE INT NOT NULL UNIQUE;

You can also use following syntax, which supports naming the constraint and multiple columns as well:

ALTER TABLE CUSTOMERS ADD CONSTRAINT myUniqueConstraint UNIQUE(AGE, SALARY);

DROP a UNIQUE Constraint:

To drop a UNIQUE constraint, use the following SQL:

ALTER TABLE CUSTOMERS DROP CONSTRAINT myUniqueConstraint;

If you are using MySQL then you can use following syntax:

ALTER TABLE CUSTOMERS
   DROP INDEX myUniqueConstraint;