MySQL correlated subquery in JOIN syntax

Question

I would like to provide a WHERE condition on an inner query by specifying innertable.id = outertable.id. However, MySQL (5.0.45) reports \"Unknown column \'outertable.id\' i

Answer 1

You're using the Entity-Attribute-Value design, and there's ultimately no way to make this scalable if you try to generate conventional result sets. Don't try to do this in one query.

Instead, query your normalized tables first:

SELECT t.ticketid, u.userid, t.fullname, u.loginapi_userid, t.email, 
  tp.subject, tp.contents
FROM swtickets t
 INNER JOIN swticketposts tp ON (t.ticketid = tp.ticketid)
 INNER JOIN swusers u ON (t.userid = u.userid)
WHERE t.ticketid = 2458;

Then query your custom fields, with the result on multiple rows of the result set:

SELECT cfv.customfieldid, cfv.fieldvalue
FROM swcustomfieldvalues cfv
WHERE cfv.typeid = 2458;

You'll get multiple rows in the result set, one row for each custom field:

+---------------+--------------+
| customfieldid | fieldvalue   |
+---------------+--------------+
|             1 | 415-555-1234 |
|             3 | Third office |
|             5 | 123          |
|             8 | Support      |
|             9 | Engineering  |
+---------------+--------------+

You then need to write application code to map the result-set fields to the application object fields, in a loop.

Using an Entity-Attribute-Value table in this way is more scalable both in terms of performance and code maintenance.