Convert SQL WHERE IN to JOIN

Question

I have a database storing various information about fictional people. There is a table person with general information, such as name, adress etc and some more specific table

Answer 1

Depends on your SQL engine. Newer SQL systems that have reasonable query optimizers will most likely rewrite both IN and JOIN queries to the same plan. Typically, a sub-query (IN Clause) is rewritten using a join.

In simple SQL engines that may not have great query optimizers, the join should be faster because they may run sub-queries into a temporary in-memory table before running the outer query.

In some SQL engines that have limited memory footprint, however, the sub-query may be faster because it doesn't require joining -- which produces more data.

Answer 2

I think I understand what you're trying to do. There is more than one way to skin a cat, but may I suggest splitting your query into two separate queries, and then replacing the complicated WHERE clause with a couple inner joins? So, something like this:

/* Find connections based on health care */
SELECT p2.p_id as id, p2.fname, p2.lname, p2.image
FROM person p
JOIN health_case hc on hc.patient = p.p_id
JOIN health_case hc2 on hc2.doctor = hc.doctor and hc2.healthcenter = hc.healthcenter and hc.start <= hc2.end and hc.end >= hc2.start and hc2.patient <> hc.patient
JOIN person p2 on p2.p_id = hc2.patient and p2.p_id <> p.p_id
WHERE p.p_id = :id

Then, create a separate query to get connections based on education:

/* Find connections based on education */
SELECT p2.p_id as id, p2.fname, p2.lname, p2.image
FROM person p
JOIN education e on e.pupil = p.p_id
JOIN education e2 on e2.school = e.school and e2.start <= e.end AND e2.end >= e.start and e.pupil <> e2.pupil
JOIN person p2 on p2.p_id = e2.pupil and p2.p_id <> p.p_id
WHERE p.p_id = :id

If you really want the data results to be combined, you can use UNION since both queries return the same columns from the person table.