Table event ( how i can get values from another table to calculate value)

i need help to solve this case

i have to table in database
first on SAL
second one Vacations

SAL table have below fields

• basic salary.
• housing allownce.
• Empno

vacations table have below feilds

• sdate
• edate
• ndays
• value

i need to do below
insert sdate,edate,ndays when i click add/save the system will search in SAL table and calculate Value as below
value = basic salary + housing allownce
where vacation.Empno = SAL.Empno

Any body can help me to do this?