Queries within queries: Is there a better way?
- by mririgo
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?
