Default constraint in sql

https://www.w3schools.com/sql/sql_default.asp

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, the SALARY column is set to 5000.00 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 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 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 5000.00; 

Drop Default Constraint

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

ALTER TABLE CUSTOMERS
ALTER COLUMN SALARY DROP DEFAULT;

https://www.c-sharpcorner.com/UploadFile/f0b2ed/constraints-in-sql-server/
https://www.c-sharpcorner.com/blogs/sql-constraints
https://www.c-sharpcorner.com/article/an-easy-way-to-understand-constraints-in-sql-server/

please refer this url https://www.w3schools.com/sql/sql_default.asp