How to know when is the last salary update(Incerease) was made?
For Example
If the salary in a given year is 4000 per month and next month he got the raise of 3000,(For two months he got 4000 and rst of the year 7000) now the salary will be 4000 + 3000 per month I,e. 7000 per month ,but in my table when I update the salary field it updates to 7000 and I don't know how to know from when the salary has been incremented for the given Employee.
I have employee tabel
Basisalary field
Date Of Joining Field
Do I have to make another table or I have to add some more fields to know that?
and for Tax returns I need to know when was 4000 salary drawan and later from which perid increased salary drawn.

Typically, a company will have a payroll table which contains a record for each paycheque issued. One of the fields on that record will be the employee's base salary. You could determine the dates of all salary changes by doing a SELECT DISTINCT on that field. There would also (usually) be some kind of employee history table which tracks an employee's status. This would include salary, classification, location, and some sort of job ID (as opposed to employee ID).