mydate is a DATE field, 'ccyy-mm' is the year and month part of a date
in ISO string representation. I'm surprised that this WHERE clause
works, and of course it's much shorter than

WHERE EXTRACT(YEAR FROM mydate) = thisyear AND EXTRACT(MONTH FROM
mydate) = thismonth

with integers thisyear and thismonth. At first glance it seems to give
the same results.

Is this legal SQL or what did I overlook? Speed is no issue here, the
database is very small and will grow very slowly.

Using Firebird 2.1 on Windows XP (but I suppose the OS isn't relevant
here).

Thanks for comments on this!