Question

I have the following (abridged schema) – 3 tables – user, job, job_type

TABLE: job_type
COLUMN: job_type_id
1           
2

TABLE: user
COLUMN: user_id
101
102
103

TABLE: job
COLUMNS: job_id | user_id | job_type_id
              4 |     101 |           1
              5 |     101 |           2
              6 |     102 |           1
              7 |     103 |           2

I want to write a query that returns all of the users who have a job in the job table where job_type_id = 1, but if they ALSO have a job with job_type_id = 2, to exclude them from the results. Therefore, the target resultset should return back user 102 only.

Answer 1

You haven’t indicate what DBMS you’re using, but here are some queries that should work in almost any:

SELECT DISTINCT user_id
  FROM job j1
 WHERE job_type_id = 1
   AND NOT EXISTS
        ( SELECT 1
            FROM job j2
           WHERE job_type_id = 2
             AND j2.user_id = j1.user_id
        )
;

SELECT DISTINCT user_id
  FROM job
 WHERE job_type_id = 1
   AND user_id NOT IN
        ( SELECT DISTINCT user_id
            FROM job
           WHERE job_type_id = 2
        )
;

SELECT DISTINCT j1.user_id
  FROM job j1
  LEFT
 OUTER
  JOIN job j2
    ON j1.user_id = j2.user_id
   AND j1.job_type_id = 1
   AND j2.job_type_id = 2
 WHERE j2.user_id IS NULL
;

Some DBMSes also offer a special MINUS or EXCEPT feature that you can use. That would look something like this:

SELECT user_id
  FROM job
 WHERE job_type_id = 1
MINUS                          -- or EXCEPT
SELECT user_id
  FROM job
 WHERE job_type_id = 2
;