individual SQL select queries vs single big SQLselect query: what is better & why? [closed]

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Closed 8 years ago.

I have these three separate queries, here is the pseudo version of them:

SELECT packageid from tblhosting where userid=1234 AND id=4351
SELECT id FROM tblcustomfields WHERE relid =tblhosting.packageid AND fieldname ='foo'
SELECT value FROM  `tblcustomfieldsvalues` WHERE  `fieldid` =tblcustomfields.id AND relid=tblhosting.id

currently, these are separate queries & they appear as SUBqueries of each. does it make sense to combine them into single query? if yes, then why & how should one combine this?

Answer 1

Sorry to be a bit dense, but your question doesn't appear to use subqueries.

If the real code is using subqueries, along the lines of

SELECT value 
FROM  `tblcustomfieldsvalues` 
WHERE  `fieldid` = 
   (SELECT id 
   FROM tblcustomfields 
   WHERE relid =(
     SELECT packageid 
     from tblhosting 
     where userid=1234 
     AND id=4351)
 AND fieldname ='foo')
AND relid=tblhosting.id

then you only have to compare it with @Chopin's version to see that a join is far nicer from a readability point of view.

Joins are also the idiomatic way to do this - most SQL developers looking at the subquery-based approach would scratch their head; this makes it less maintainable and extensible.

In terms of performance, I'm guessing the query optimizer will recognize they're equivalent; there may not be any improvement from the "nicer" version.

Answer 2

The query could be something like this:

SELECT value FROM tblcustomfieldsvalues
INNER JOIN tblcustomfields ON tblcustomfields.id = tblcustomfieldsvalues.fieldid
INNER JOIN tblhosting ON tblhosting.packageid = tblcustomfields.relid
WHERE tblcustomfields.fieldname = 'foo' AND tblhosting.userid = 1234 AND tblhosting.id = 4351

Here you're joining tblcustomfieldsvalues with tblcustomfields, and the last with tblhosting. Look at the ON conditions to be sure they're correctly joined.

Finally, apply the WHERE clause on the attributes you want to constraint.

Hope this helps!