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SQLite - 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 cannot have two records with same age −
CREATE TABLE COMPANY( ID INT PRIMARY KEY NOT NULL, NAME TEXT NOT NULL, AGE INT NOT NULL UNIQUE, ADDRESS CHAR (25) , SALARY REAL , );
If COMPANY 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 COMPANY MODIFY AGE INT NOT NULL UNIQUE;
You can also use following syntax, which supports naming the constraint and multiple columns as well −
ALTER TABLE COMPANY ADD CONSTRAINT myUniqueConstraint UNIQUE(AGE, SALARY);
DROP a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL −
ALTER TABLE COMPANY DROP CONSTRAINT myUniqueConstraint;
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