How to use DEFAULT in SQL

In this SQL article, you will learn how to use DEFAULT on the columns of the table in Structured Query Language.

What is a DEFAULT constraint?

The DEFAULT is a constraint in SQL which allows users to fill a column with the default or fixed value.

If no value is specified to the column at the time of insertion, then the default value will be added automatically to that column.

The following syntax adds the DEFAULT constraint to the column at the time of table creation:

In the SQL DEFAULT syntax, we have to define the value with the DEFAULT constraint. The database users can easily specify the DEFAULT constraint to one or more columns in one SQL table.

The following syntax adds the DEFAULT constraint to the column when the table already exists:

If you want to use the DEFAULT constraint at the time of table creation, you have to follow the steps given below:

1. Create the new database
2. Create a new table and add DEFAULT
3. Insert the records
4. View the table's data

Step 1: Create the Simple new database

Firstly, you have to make a new database in Structured Query Language.

The following query creates the new Industry Database in SQL server:

Step 2: Create the New table and add DEFAULT

The following query creates the Client_Info table in the Industry Database and adds the CHECK constraint to the Client_Age column of the table:

Step 3: Insert the Values

The following INSERT queries insert the records of clients in the Client_Info table:

Step 4: View the Table's Data

The following query shows the data of the Client_Info table.

Add DEFAULT Constraint on Multiple columns

The following CREATE TABLE query specifies the DEFAULT constraint on more than one column to the Doctor_Info table:

The following query inserts the multiple records of doctors in the Doctor_Info table:

The following query shows the details of the Doctor_Info table:

Delete DEFAULT constraint from the table

The ALTER COLUMN keyword with ALTER TABLE statement allows the database users to remove the DEFAULT constraint from the column of the table.

The following ALTER syntax is used to remove the DEFAULT constraint from the SQL table,

The following query deletes the DEFAULT value Russia from the Doctor_Country column of the Doctor_Info table:

To check the result of the above ALTER query, you have to type the following DESC command, which describes the structure of the Doctor_Info table:

Output:

As we can see in the above Doctor_Info table, the value of the DEFAULT column is NULL for the Doctor Country field, which shows that the DEFAULT value is successfully removed from the Doctor_Country column.

Add DEFAULT constraint to Existing table

The database users can easily add DEFAULT value to the column of an existing table by using the ADD keyword in the SQL ALTER TABLE statement.

The following syntax is used in SQL to add the DEFAULT constraint in the existing table:

The following query adds the DEFAULT value as INDIA to the Doctor_Country column of the Doctor_Info table:

To check the result of the above ALTER query, you have to type the following DESC command, which describes the structure of the Doctor_Info table:

Output: