Both PRIMARY KEY and UNIQUE KEY enforces the Uniqueness of the values (i.e. avoids duplicate values) on the column[s] on which it is defined. Also these key’s can Uniquely identify each row in database table.
[ALSO READ] Difference Between Sql Server VARCHAR and NVARCHAR Data Type
Below table lists out the major difference between PRIMARY KEY and UNIQUE KEY:
| PRIMARY KEY | UNIQUE KEY | |
| NULL | It doesn’t allow Null values. Because of this we refer PRIMARY KEY = UNIQUE KEY + Not Null CONSTRAINT |
Allows Null value. But only one Null value. |
| INDEX | By default it adds a clustered index | By default it adds a UNIQUE non-clustered index |
| LIMIT | A table can have only one PRIMARY KEY Column[s] | A table can have more than one UNIQUE Key Column[s] |
| CREATE SYNTAX | Below is the sample example for defining a single column as a PRIMARY KEY column while creating a table:CREATE TABLE dbo.Customer ( Id INT NOT NULL PRIMARY KEY, FirstName VARCHAR(100), LastName VARCHAR(100), City VARCHAR(50) ) Below is the Sample example for defining multiple columns as PRIMARY KEY. It also shows how we can give name for the PRIMARY KEY: CREATE TABLE dbo.Customer |
Below is the sample example for defining a single column as a UNIQUE KEY column while creating a table:CREATE TABLE dbo.Customer ( Id INT NOT NULL UNIQUE, FirstName VARCHAR(100), LastName VARCHAR(100), City VARCHAR(50) ) Below is the Sample example for defining multiple columns as UNIQUE KEY. It also shows how we can give name for the UNIQUE KEY: CREATE TABLE dbo.Customer |
| ALTER SYNTAX | Below is the Syntax for adding PRIMARY KEY CONSTRAINT on a column when the table is already created and doesn’t have any primary key:ALTER TABLE dbo.Customer ADD CONSTRAINT PK_CUSTOMER PRIMARY KEY (Id) |
Below is the Syntax for adding UNIQUE KEY CONSTRAINT on a column when the table is already created:ALTER TABLE dbo.Customer ADD CONSTRAINT UK_CUSTOMER UNIQUE (Id) |
| DROP SYNTAX | Below is the Syntax for dropping a PRIMARY KEY:ALTER TABLE dbo.Customer DROP CONSTRAINT PK_CUSTOMER |
Below is the Syntax for dropping a UNIQUE KEY:ALTER TABLE dbo.Customer DROP CONSTRAINT UK_CUSTOMER |
ALSO READ
- Varchar vs NVarchar
- Varchar vs Varchar(MAX)
- Char vs Varchar
- Text vs Varchar(Max)
- Union vs Union All
- DateTime vs DateTime2
- SET QUOTED_IDENTIFIER ON vs SET QUOTED_IDENTIFIER OFF
- Stored Procedure vs User Defined Function
- Primary Key vs Unique Key
- RAISERROR vs THROW
- Temporary Table vs Table Variable
- Len() vs Datalength()
- Sequence vs Identity
- DATEDIFF vs DATEDIFF_BIG
- LEFT JOIN vs LEFT OUTER JOIN
- RIGHT JOIN vs RIGHT OUTER JOIN
- JOIN vs INNER JOIN
- LEFT OUTER JOIN vs RIGHT OUTER JOIN
- SMALLDATETIME vs DATETIME