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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.
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); |
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; |
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