group_by syntax

[eluser]liamstask[/eluser]
I've been told now by a couple non-CI SQL people that

Code:

GROUP BY (2 * HOUR(timestamp) + FLOOR( MINUTE(timestamp) / 30 ))

is the appropriate syntax to specify half-hour bins, so I'm feeling reasonably confident that part's ok. Still having trouble getting this to work with the CI active record group_by( ) method, though, which is rendering it as

Code:

GROUP BY (`2` * HOUR(`timestamp`) + FLOOR( MINUTE(`timestamp`) / `30` ))

My working, uninformed theory is that the single quotes should be removed. Any other thoughts, or ideas on how to remove them?

Thanks!