Question

I am running into an issue and it is making me a bit crazy. I have the following code:

SELECT COUNT(DISTINCT f.f_id) AS `The Good`,
       COUNT(DISTINCT s.f_id) AS `The Bad`,
       COUNT(DISTINCT x) AS `The Ugly"`
FROM faculty f, student s, (
  SELECT f_id AS x
  FROM faculty
  MINUS (
    SELECT f.f_id
    FROM faculty f, course_section cs
    WHERE f.f_id = cs.f_id

    UNION

    SELECT f_id
    FROM student
  )
);

I am trying to get a count of f_id‘s from faculty table, f_id‘s from student table, and then a count of f_id‘s from faculty table MINUS f_id‘s from the student table and course_section table.

All is well if my last COUNT returns a number that is not 0. However, if it returns a 0 it also changes my first two counts to a 0. The first two COUNTs should be 6 and 4 respectively.

I have been trying to figure out how to fix this for a couple of hours but am having no luck.

Obviously there must be something that I am missing. What is wrong with my code?

Thanks.

Answer 1

You’re using ANSI-ANCIENT notation for the joins TableA, TableB, TableC.

But you don’t have a WHERE clause. That’s joining all the tables with every possible combination of rows (A full cartessian product). That’s a bad idea.

Also, your results don’t actually require any of the tables to be joined together. I’d recommend just using sub-queries in the SELECT clause…

SELECT
  (select count(distinct f_id) from faculty) "The Good",
  (select count(distinct f_id) from student) "The Bad",
  (
    select
      count(distinct f_id)
    from
    (
     select f_id from faculty
     MINUS
     select f.f_id from faculty f INNER JOIN course_section cs ON f.f_id = cs.f_id
     MINUS
     select f_id from student
    )
  ) "The Ugly"
;
-- Add FROM DUAL is this is Oracle.