IS Null function in SQL server 2012

The ISNULL( ) function is used to replace NULL value with another value.

For example, if we have the following table,

Table-- Students

Student_Name     Maths        Physics
Student A             300            NULL
Student B             NULL        NULL

The following SQL,

SELECT SUM (ISNULL(Maths,100)) as MathsTotal FROM Student;

returns the following result:   400

This is because NULL has been replaced by 100 via the ISNULL function, so the total becomes 300 + 100 = 400.

2.select ISNULL(physics,200)+ISNULL(maths,100) from students

we will get as

300
300

Let me know if you have any doubts on this.

******what is the purpose of using Isnull( ) ?

Thanks

GSV