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UNIQUE Constraint in SQL - SQL UNIQUE Constraint
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UNIQUE Constraint in SQL - How to use UNIQUE Constraint in SQL?
In SQL UNIQUE constraint ensures that all values in a column are different to each other. While creating a table if the developer defines a column with a UNIQUE constraint this means that the Column will not accept the duplicate value.
Syntax - Syntax of UNIQUE Constraint is given below
UNIQUE Constraint on CREATE TABLE
SQL Server / Oracle / MS Access:
CREATE TABLE Student ( ID int NOT NULL UNIQUE, LastName varchar(250) NOT NULL, FirstName varchar(250), Age int );
MySQL:
CREATE TABLE Student ( ID int NOT NULL, LastName varchar(250) NOT NULL, FirstName varchar(250), Age int, UNIQUE (ID) );
The Above SQL query will create a unique constraint for the ID column when the Student table is created. The mean ID column of the student table will always accept a unique value.
For Multiple columns - To make Multiple columns Unique use the below SQL query.
SQL Server / Oracle / MS Access/MySQL :
CREATE TABLE Student ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, CONSTRAINT UC_Student UNIQUE (ID,LastName) );
The above query will create a unique constraint for ID and LastName Column when the student table is created.
UNIQUE Constraint on ALTER TABLE
Syntax
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons ADD UNIQUE (ID);
For Multiple Column
SQL Server / Oracle / MS Access/MySQL :
ALTER TABLE Persons ADD CONSTRAINT UC_Student UNIQUE (ID,LastName);
DROP a UNIQUE Constraint
SQL Server / Oracle / MS Access:
ALTER TABLE Persons DROP CONSTRAINT UC_Student;
MySQL:
ALTER TABLE Persons DROP INDEX UC_Student;
Hope!!! The above Tutorial on SQL UNIQUE Constraint is helpful for you...
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