Using unique index for cross table constraint

Question

I'm pretty new to Oracle and database design in general but I have a question. I have two basic tables called employee and department. The employee table holds a salary property. Now I need to implement a new business rule: The total of all employee salaries in one department (an employee is part of a department) may not exceed 200k. I know this can be done with a materialized view, but can it also be done with unique indexes? If so, how would one approach this particular situation?

Answer 1

No, you can't use a unique index to enforce this sort of rule.

You could try to enforce this sort of rule via triggers but that tends to get rather difficult in a multiuser environment. You'd need to do things like lock the department row to ensure that only one session can be modifying employee information for a particular department at a time which tends to introduce significant scalability issues. It also tends to involve quite a bit of code to handle all the potential cases.

Answer 2

This can't be done with any indexes. But it is can be done using a trigger.

If you add a column maxDeptSalary to your departments table, then you can create a trigger on Insert and Update in which you

1. query your maxDeptSalary and get the value
2. run Sum on existing records in your employee table for that department
3. compare sum + new.value <= maxDeptSalary
4. Reject your record if criteria is not met

But the reality is, this is should be done on application level, not the database