Default Constraint in SQL Server – An Overview

Default Constraint in SQL Server – An Overview

Default Constraint is a feature in SQL Server that allows you to specify a default value for a column in a table. This means that if a user does not provide a value for that column while inserting a new record, the default value will be used instead. Default Constraint is a useful feature that can save you a lot of time and effort while working with a large database.

How to Create Default Constraint in SQL Server

In order to create a Default Constraint in SQL Server, you need to use the following syntax:


ALTER TABLE [TableName]
ADD CONSTRAINT [ConstraintName] DEFAULT [DefaultValue] FOR [ColumnName];

Let’s break down the above syntax:

  • ALTER TABLE [TableName] – This specifies the name of the table you want to add the default constraint to.

  • ADD CONSTRAINT [ConstraintName] – This specifies the name you want to give to the constraint.

  • DEFAULT [DefaultValue] – This specifies the default value you want to assign to the column.

  • FOR [ColumnName] – This specifies the name of the column you want to add the constraint to.

Here’s an example:


ALTER TABLE Employees
ADD CONSTRAINT DF_Employees_Department DEFAULT 'Sales' FOR Department;

This will add a Default Constraint to the Department column in the Employees table. If a user does not provide a value for the Department column while inserting a new record, the default value ‘Sales’ will be used instead.

How to Remove Default Constraint in SQL Server

If you want to remove a Default Constraint from a column, you can use the following syntax:


ALTER TABLE [TableName]
DROP CONSTRAINT [ConstraintName];

Let’s break down the above syntax:

  • ALTER TABLE [TableName] – This specifies the name of the table you want to remove the constraint from.

  • DROP CONSTRAINT [ConstraintName] – This specifies the name of the constraint you want to remove.

Here’s an example:


ALTER TABLE Employees
DROP CONSTRAINT DF_Employees_Department;

This will remove the Default Constraint with the name DF_Employees_Department from the Department column in the Employees table.

FAQs

1. Can we have multiple Default Constraints on a single column?

No, you can have only one Default Constraint on a single column.

2. Can we modify the default value of a column?

Yes, you can modify the default value of a column by using the ALTER TABLE statement.

3. What happens if we set a default value for a column that already has data in it?

The default value will not be applied to the existing data in the column. It will only be applied to new data that is added to the table.

4. Can we create a Default Constraint on a computed column?

No, you cannot create a Default Constraint on a computed column because the computed value is already determined by the expression used to compute it.

5. Can we create a Default Constraint on a view?

No, you cannot create a Default Constraint on a view because views are virtual tables that do not actually store data.

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