How can I factor out repeated expressions in an SQL Query? Column aliases don't seem to be the ticke
- by Weston C
So, I've got a query that looks something like this:
SELECT id,
DATE_FORMAT(CONVERT_TZ(callTime,'+0:00','-7:00'),'%b %d %Y') as callDate,
DATE_FORMAT(CONVERT_TZ(callTime,'+0:00','-7:00'),'%H:%i') as callTimeOfDay,
SEC_TO_TIME(callLength) as callLength
FROM cs_calldata WHERE
customerCode='999999-abc-blahblahblah' AND
CONVERT_TZ(callTime,'+0:00','-7:00') >= '2010-04-25' AND
CONVERT_TZ(callTime,'+0:00','-7:00') <= '2010-05-25'
If you're like me, you probably start thinking that maybe it would improve readability and possibly the performance of this query if I wasn't asking it to compute CONVERT_TZ(callTime,'+0:00','-7:00') four separate times.
So I try to create a column alias for that expression and replace further occurances with that alias:
SELECT id,
CONVERT_TZ(callTime,'+0:00','-7:00') as callTimeZoned,
DATE_FORMAT(callTimeZoned,'%b %d %Y') as callDate,
DATE_FORMAT(callTimeZoned,'%H:%i') as callTimeOfDay,
SEC_TO_TIME(callLength) as callLength
FROM cs_calldata WHERE
customerCode='5999999-abc-blahblahblah' AND
callTimeZoned >= '2010-04-25' AND
callTimeZoned <= '2010-05-25'
This is when I learned, to quote the MySQL manual:
Standard SQL disallows references to
column aliases in a WHERE clause. This
restriction is imposed because when
the WHERE clause is evaluated, the
column value may not yet have been
determined.
So, that approach would seem to be dead in the water.
How is someone writing queries with recurring expressions like this supposed to deal with it?
