Question

Few months ago I was programming a simple application with som other guy in PHP. There we needed to preform a SELECT from multiple tables based on a userid and another value that you needed to get from the row that was selected by userid.

My first idea was to create multiple SELECTs and parse all the output in the PHP script (with all that mysql_num_rows() and similar functions for checking), but then the guy told me he’ll do that. "Okay no problem!" I thought, just much more less for me to write. Well, what a surprise when i found out he did it with just one SQL statement:

SELECT
  d.uid AS uid, p.pasmo_cas AS pasmo, d.pasmo AS id_pasmo ...
FROM
  table_values AS d, sectors AS p
WHERE
  d.userid='$userid' and p.pasmo_id=d.pasmo
ORDER BY
  datum DESC, p.pasmo_id DESC

(shortened piece of the statement (…))

Mostly I need to know the differences between this method (is it the right way to do this?) and JOIN – when should I use which one?

Also any references to explanations and examples of these two would come in pretty handy (not from the MySQL ref though – I’m really a novice in this kind of stuff and it’s written pretty roughly there.)

Answer 1

, notation was replaced in ANSI-92 standard, and so is in one sense now 20 years out of date.

Also, when doing OUTER JOINs and other more complex queries, the JOIN notation is much more explicit, readable, and (in my opinion) debuggable.

As a general principle, avoid , and use JOIN.

In terms of precedence, a JOIN’s ON clause happens before the WHERE clause. This allows things like a LEFT JOIN b ON a.id = b.id WHERE b.id IS NULL to check for cases where there is NOT a matching row in b.

Using , notation is similar to processing the WHERE and ON conditions at the same time.