You can use UNIQUE constraints to make sure that no duplicate values are entered in specific columns that do not participate in a primary key. Although both a UNIQUE constraint and a PRIMARY KEY constraint enforce uniqueness, use a UNIQUE constraint instead of a PRIMARY KEY constraint when you want to enforce the uniqueness of a column, or combination of columns, that is not the primary key.
Multiple UNIQUE constraints can be defined on a table, whereas only one PRIMARY KEY constraint can be defined on a table.
Also, unlike PRIMARY KEY constraints, UNIQUE constraints allow for the value NULL. However, as with any value participating in a UNIQUE constraint, only one null value is allowed per column.
A UNIQUE constraint can be referenced by a FOREIGN KEY constraint.
When you declare a UNIQUE constraint, SQL Server creates a UNIQUE index to speed up the process of searching for duplicates. In this case the index defaults to NONCLUSTERED index, because you can have only one CLUSTERED index per table.
Easy way to understand
Multiple UNIQUE constraints can be defined on a table, whereas only one PRIMARY KEY constraint can be defined on a table.
Also, unlike PRIMARY KEY constraints, UNIQUE constraints allow for the value NULL. However, as with any value participating in a UNIQUE constraint, only one null value is allowed per column.
A UNIQUE constraint can be referenced by a FOREIGN KEY constraint.
When you declare a UNIQUE constraint, SQL Server creates a UNIQUE index to speed up the process of searching for duplicates. In this case the index defaults to NONCLUSTERED index, because you can have only one CLUSTERED index per table.
Easy way to understand
- Primary Key column represents the table in terms of uniqueness.
- Unique key constraint on column is due to a business need or scenario
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