Understanding DEFAULT Constraints in SQL: A Complete Guide

The DEFAULT constraint provides a default value to a column when the INSERT INTO statement does not provide a specific value.

Syntax: Altering an existing column to add a default constraint:

ALTER TABLE { TABLE_NAME }
ADD CONSTRAINT { CONSTRAINT_NAME }
DEFAULT { DEFAULT_VALUE } FOR { EXISTING_COLUMN_NAME }

Syntax: Adding a new column, with default value, to an existing table:

ALTER TABLE { TABLE_NAME } 
ADD { COLUMN_NAME } { DATA_TYPE } { NULL | NOT NULL } 
CONSTRAINT { CONSTRAINT_NAME } DEFAULT { DEFAULT_VALUE }

Example

For example, the following SQL creates a new table called "Customers" and adds five columns. Here, the SALARY column is set to 10000.00 by default, so in case the INSERT INTO statement does not provide a value for this column, then by default this column would be set to 10000.00.

CREATE TABLE Customers(
   ID   INT              NOT NULL,
   NAME VARCHAR (20)     NOT NULL,
   AGE  INT              NOT NULL,
   ADDRESS  CHAR (25) ,
   SALARY   DECIMAL (18, 2) DEFAULT 10000.00,       
   PRIMARY KEY (ID)
);

If the CUSTOMERS table has already been created, then to add a DEFAULT constraint to the SALARY column, you would write a query like the one which is shown in the code block below.

ALTER TABLE Customers

MODIFY SALARY  DECIMAL (18, 2) DEFAULT 10000.00; 

Drop Default Constraint

To drop a DEFAULT constraint, use the following SQL query.

ALTER TABLE Customers
   ALTER COLUMN SALARY DROP DEFAULT;

The DEFAULT constraint is used to provide a default value for a column. The default value will be added to all new records IF no other value is specified.

Example for Default Constraints

Get the TablePerson from this tutorial

Code: The following command will add a default constraint, DF_TablePerson_GenderID.

ALTER TABLE TablePerson
ADD CONSTRAINT DF_TablePerson_GenderID
DEFAULT 1 FOR GenderID

The insert statement below does not provide a value for GenderId column, so the default of 1 will be inserted for this record.

Code:

Insert into TablePerson(ID,Name,Email)
 values(5,'Sam','s@s.com')

On the other hand, the following insert statement will insert NULL, instead of using the default.

Code: SQL Server

Insert into TablePerson(ID,Name,Email,GenderID)
 values (6,'Dan','d@d.com',NULL)

To drop a constraint

Code: SQL Server

ALTER TABLE { TABLE_NAME } 
DROP CONSTRAINT { CONSTRAINT_NAME }