MySQL Constraint – UNIQUE

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Hello everyone in the previous article we learn about MySQL Constraint – NOT NULL and in this article, we will learn UNIQUE constraint with example. Let’s go.

What is a UNIQUE key?

The MySQL UNIQUE key used to specify that you don’t want to see duplicate data in a given field means all the values in a column are different. In MySQL, UNIQUE key is implemented through a constraint that is called UNIQUE Constraint.

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What is a UNIQUE Constraint?

MySQL UNIQUE constraint does not allow inserting a duplicate value in a column and each data value of a column that has the unique value. If you add to insert an already existing duplicate value in the unique column, then MySQL fetches an error.

Following is the CREATE TABLE statement with UNIQUE constraint syntax:

                              CREATE TABLE table_name (

                                             column1_name data type () UNIQUE,

                                             column2_name data type () UNIQUE

);

For define multiple column UNIQUE constraint then you can use following syntax:

CREATE TABLE table_name(

                              column1_name data type ()

                              column1_name data type ()

                              CONSRTAINT constraint_name UNIQUE (column1_name, column2_name)

);

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Example of CREATE TABLE statement with UNIQUE constraint:

                              CREATE TABLE EMPLOYEE (

                                            EID INT NOT NULL,

                                            First_Name VARCHAR (255),

                                            Last_Name VARCHAR (255),

                                            Address VARCHAR (255) UNIQUE,

                              );

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Example for define multiple column UNIQUE constraint:

                              CREATE TABLE EMPLOYEE (

                                            EID INT NOT NULL,

                                            First_Name VARCHAR (255),

                                            Last_Name VARCHAR (255),

                                            Address VARCHAR (255),

                                            CONSTRAINT UC_EMPLOYEE UNIQUE (EID, Address)

                              );

You can add UNIQUE constraint when the table is already created using the ALTER TABLE statement. Following is the syntax and example:

               Syntax:              

ALTER TABLE table_name

                              ADD CONSTRAINT constraint_name UNIQUE (column_name);

               Example

                              ALTER TABLE EMPLOYEE

                              ADD CONSTRAINT UC_EMPLOYEE UNIQUE (EID, Address);

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If you want to DROP created a UNIQUE constraint, then follow the steps:

ALTER TABLE EMPLOYEE

DROP CONSTRAINT UC_EMPLOYEE;

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