Question

As I build bigger, more advanced web applications, I’m finding myself writing extremely long and complex queries. I tend to write queries within queries a lot because I feel making one call to the database from PHP is better than making several and correlating the data.

However, anyone who knows anything about SQL knows about JOINs. Personally, I’ve used a JOIN or two before, but quickly stopped when I discovered using subqueries because it felt easier and quicker for me to write and maintain.

Commonly, I’ll do subqueries that may contain one or more subqueries from relative tables.
Consider this example:

SELECT 
  (SELECT username FROM users WHERE records.user_id = user_id) AS username,
  (SELECT last_name||', '||first_name FROM users WHERE records.user_id = user_id) AS name,
  in_timestamp,
  out_timestamp
FROM records
ORDER BY in_timestamp

Rarely, I’ll do subqueries after the WHERE clause.
Consider this example:

SELECT
  user_id,
  (SELECT name FROM organizations WHERE (SELECT organization FROM locations WHERE records.location = location_id) = organization_id) AS organization_name
FROM records
ORDER BY in_timestamp

In these two cases, would I see any sort of improvement if I decided to rewrite the queries using a JOIN?

As more of a blanket question, what are the advantages/disadvantages of using subqueries or a JOIN? Is one way more correct or accepted than the other?

Answer 1

JOINs are preferable to separate [sub]queries.
If the subselect (AKA subquery) is not correlated to the outer query, it’s very likely the optimizer will scan the table(s) in the subselect once because the value isn’t likely to change. When you have correlation, like in the example provided, the likelihood of single pass optimization becomes very unlikely. In the past, it’s been believed that correlated subqueries execute, RBAR — Row By Agonizing Row. With a JOIN, the same result can be achieved while ensuring a single pass over the table.

This is a proper re-write of the query provided:

   SELECT u.username,
          u.last_name||', '|| u.first_name AS name,
          r.in_timestamp,
          r.out_timestamp
     FROM RECORDS r 
LEFT JOIN USERS u ON u.user_id = r.user_id
 ORDER BY r.in_timestamp

…because the subselect can return NULL if the user_id doesn’t exist in the USERS table. Otherwise, you could use an INNER JOIN:

  SELECT u.username,
         u.last_name ||', '|| u.first_name AS name,
         r.in_timestamp,
         r.out_timestamp
    FROM RECORDS r 
    JOIN USERS u ON u.user_id = r.user_id
ORDER BY r.in_timestamp

Derived tables/inline views are also possible using JOIN syntax.