SQL - DEFAULT Constraint

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

Example:

For example, the following SQL creates a new table called CUSTOMERS and adds five columns. Here, SALARY column is set to 5000.00 by default, so in case INSERT INTO statement does not provide a value for this column, then by default this column would be set to 5000.00.

CREATE TABLE CUSTOMERS(
       ID   INT              NOT NULL,
       NAME VARCHAR (20)     NOT NULL,
       AGE  INT              NOT NULL,
       ADDRESS  CHAR (25) ,
       SALARY   DECIMAL (18, 2) DEFAULT 5000.00,       
       PRIMARY KEY (ID)
);

If CUSTOMERS table has already been created, then to add a DFAULT constraint to SALARY column, you would write a statement similar to the following:

ALTER TABLE CUSTOMERS
   MODIFY SALARY  DECIMAL (18, 2) DEFAULT 5000.00;

Drop Default Constraint:

To drop a DEFAULT constraint, use the following SQL:

ALTER TABLE CUSTOMERS
   ALTER COLUMN SALARY DROP DEFAULT;