UNIQUE in SQL

Adding UNIQUE Constraints in table:

                                                UNIQUE allow only distinct value in database table. It used to avoid redundant information in table. It allow NULL value for unknown data.

How to add UNIQUE Constraints:

    CREATE TABLE Employee(id int UNIQUE, Emp_name nvarchar(20));

In this statement the id is unique value, ie value can't be repeated

Field	Data Type	Null	key	Default
id	INT	NO	UNIQUE
emp_name	NVARCHAR(18)	NO

CREATE TABLE Course(id int UNIQUE, Course_name nvarchar(20));

Field	Data Type	Null	key	Default
id	INT		UNIQUE
course_name	NVARCHAR(20)

UNIQUE using ALTER

CREATE TABLE Employee(id int, emp_name nvarchar(20),Age int, dept_id int)

 

Field	Data Type	Null	key	Default
id	INT
emp_name	NVARCHAR(20)
Age	INT
Dept_id	INT

ALTER TABLE Employee ADD  UNIQUE(ID); for MSSQL Server, Oracle, Access.

Field	Data Type	Null	key	Default
id	INT		UNIQUE
emp_name	NVARCHAR(20)
Age	INT
Dept_id	INT

To add UNIQUE to Multiple Column.

ALTER TABLE Employee ADD   UNIQUE(Emp_name, age); for MSSQL Server, Oracle, Access.

Field	Data Type	Null	key	Default
id	INT		UNIQUE
emp_name	NVARCHAR(20)		UNIQUE
Age	INT		UNIQUE
Dept_id	INT

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