In This Article, You Will Know About SQL DEFAULT Constraint.
Before moving ahead, let’s know a bit about the SQL CHECK Constraint.
Table of Contents
SQL DEFAULT Constraint
The SQL DEFAULT Constraint is used to assign a default value to the column.
If not any other value is specified, then the default value is applied to all new records.
SQL DEFAULT on CREATE TABLE
My SQL / SQL Server
Example: Use the DEFAULT Constraint to return the data based on a rule.
CREATE TABLE Student_info (
Serial Number int NOT NULL,
Name char(10),
Class char(10),
Subject char(10) DEFAULT 'Eco'
);
The DEAFULT Constarint value is also used for inserting value by using GETDATE():
Example: Use the DEFAULT Constraint to return the data based on a rule.
CREATE TABLE Student_info (
Serial Number int NOT NULL,
Name char(10),
Class char(10),
Date date DEFAULT GETDATE()
);
SQL DEFAULT on ALTER TABLE
Apply DEFAULT after the table is created.
SQL Server
Syntax:
ALTER TABLE Table_Name
ADD CONSTRAINT Constraint_Table_Name
DEFAULT value FOR Table_Name;
Example: Use the DEFAULT Constraint to return the data based on a rule.
ALTER TABLE Student_Info
ADD CONSTRAINT Table_constraint
DEFAULT 'Eco' FOR Subject;
MySQL
Syntax:
ALTER TABLE Table_Name
ALTER Column_Name SET DEFAULT value;
Example: Use the DEFAULT Constraint to return the data based on a rule.
ALTER TABLE Student_Info
ALTER Subject SET DEFAULT ‘Eco’;
DROP a DEFAULT Constraint
SQL Server
Syntax:
ALTER TABLE Table_Name
ALTER COLUMN Column_Name DROP DEFAULT;
Example: Use the DEFAULT Constraint to return the data based on a rule.
ALTER TABLE Student_Info
ALTER COLUMN Subject DROP DEFAULT;
MySQL Server
Syntax:
ALTER TABLE Table_Name
ALTER Column_Name DROP DEFAULT;
