Question

I have the following schema:

user -- user_id | first_name
full_time_job -- ft_job_id | user_id
part_time_job -- pt_job_id | user_id
internship -- internship_id | user_id

I’d like to write a query that returns all users that exist in any of the three job tables. What’s the best way to do this? I’d prefer to use LEFT JOINS over WHERE EXISTS if possible, but will be happy with the best working solution.

I do NOT want to return any users that exist in NONE of the three job tables.

Answer 1

Using LEFT JOINs, you merely need to verify that user_id is NOT NULL in at least one of the tables:

SELECT
  [user].user_id
FROM
  [user]
  /* LEFT JOIN against each of the 3 tables */
  LEFT JOIN full_time_job ft ON [user].user_id = ft.user_id
  LEFT JOIN part_time_job pt ON [user].user_id = pt.user_id
  LEFT JOIN internship intern ON [user].user_id = intern.user_id
WHERE
  /* And check that *any* of them has a non-null user_id */
  ft.user_id IS NOT NULL
  OR pt.user_id IS NOT NULL
  OR intern.user_id IS NOT NULL

Best to fix the schema if possible…

In the long run though, if possible these should all probably be combined into a single table that has a column indicating the job type, rather than 3 similar tables.

Sample: http://sqlfiddle.com/#!2/2cd2c/1

In the above example, users 5,6 don’t exist in any of the 3 tables.