MySQL Unique Key

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MySQL Unique key in MySQL is a single field or combination of fields that ensure all values going to store into the column will be unique. It means a column cannot stores duplicate values.

MySQL allows us to use more than one column with UNIQUE constraint in a table.

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Needs of Unique Key

1. It is useful in preventing the two records from storing identical values into the column.
2. Unique key stores only distinct values that maintain the integrity and reliability of the database for accessing the information in an organized way.
3. It also works with a foreign key in preserving the uniqueness of a table.
4. It can contain null value into the table.

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Syntax:

The following syntax is used to create a unique key in MySQL.

CREATE TABLE table_name(

    col1 datatype,

    col2 datatype UNIQUE,

    ...

);

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If we want to create more than one unique key column into a table, use the syntax as below:

CREATE TABLE table_name(

  col1 col_definition,

  col2 col_definition,

  ...

  [CONSTRAINT constraint_name]

  UNIQUE(column_name(s))

);

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Unique Key Example

CREATE TABLE Student2 (

    Stud_ID int NOT NULL UNIQUE, 

    Name varchar(45), 

    Email varchar(45),

    Age int, 

    City varchar(25)

);

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