I've set the index options for the .SQL extension to use "Plain Text filter" with "Index Properties and File Contents" selected, and ensure that the directories containing the SQL files are in the "Included Locations".
Unfortunately, Windows desktop search still fails to find anything in the .SQL files. Other file types such as .cs, .docx etc are searched successfully.
I'm running Windows XP SP2 - and it's a work machine where I can't easily install alternative search engines so I'm stuck with Windows Desktop search for indexed searching.
Obviously I can search the .SQL files using other (non-indexed) techniques, but I prefer the speed of Desktop Search - if I could get it to work.
Update: Having copied a couple of .SQL files to .SQL.TXT files in the same directory as suggested by ~quack, added .SQL.TXT as a file extension to the indexing options (using the plain text filter, with "Index Properties and File Contents" selected) and re-indexing, Desktop Search still fails to find strings in either the .SQL or .SQL.TXT files.
Additional info: after re-indexing, I checked the file types in the Desktop Search advanced options and noted that the .SQL.TXT file extension no longer appears in the list of file types.