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;