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SQL UNIQUE Constraint

SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.

Both UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or a set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

Note that each table can have multiple UNIQUE constraints, but each table can only have one PRIMARY KEY constraint.

SQL UNIQUE Constraint When Creating a Table

The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons" table is created:

MySQL:

SQL Server / Oracle / MS Access:

To name a UNIQUE constraint and define a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

SQL UNIQUE Constraint When Altering a Table

When a table has already been created, to create a UNIQUE constraint on the "P_Id" column, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

To name a UNIQUE constraint and define a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

Dropping a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:

MySQL:

SQL Server / Oracle / MS Access:

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