Subtracting multi-row queries

Question

I'm trying to write a sql query to figure out by how much have salaries increased in the last year for each department due to new employees.

Table structure is Employees (empno, deptno, msal, hiredate)

I can figure out how to get all the salaries by departments

SELECT sum(msal) FROM employees GROUP BY deptno;

and how to get the salaries from people who were hired in the past year

SELECT sum(msal) FROM employees WHERE hiredate > (DATEADD(year, -1, GETDATE())) GROUP BY deptno;

But whatever way I try to subtract the result of these two queries I only get errors.

Answer 1

Here is what you might do. In this case I'm using a CASE statement to filter new employees:

SELECT SUM(msal) - SUM(CASE WHEN hiredate > ADD_MONTHS(SYSDATE, -12) THEN msal ELSE 0 END)
  FROM employees
 GROUP BY deptno;

FYI, Oracle doesn't have a DATEADD() function nor does it have a GETDATE() function. Note that I used ADD_MONTHS() and SYSDATE (you could also use CURRENT_DATE ) in place of these.

Answer 2

Why not just change the direction of the where ?

SELECT sum(msal)
FROM employees
WHERE hiredate <= DATEADD(year, -1, GETDATE())
GROUP BY deptno;

Also, normally when you aggregate by a field, then the field ( deptno ) is included in the select clause.