Markus Wagner <wagner@imsd.uni-mainz.de> writes:
> We are glad to have found out that we can evaluate such expressions
> simply by calling them with SELECT, e. g. "SELECT 29 - 2 > 18" returns true.
> We think that it would be very nice to benefit from the pg database system
> parser. But can we rely on this functionality in the future? Is this
> standard SQL?
Well, it's not standard SQL: SQL92 requires a FROM clause in SELECT.
But I can assure you it won't disappear from Postgres; it's too darn
useful.
The customary workaround, as seen in systems like Oracle, is to have
a dummy table containing exactly one row; Oracle calls the dummy table
DUAL. (Anyone know why? Seems like SINGLE would be more sensical...)
Then you can do
SELECT 29 - 2 > 18 FROM dual;
The fact that your expression doesn't actually *use* any values from
the one table row doesn't bother the system; you get one evaluation
of the expression, you're happy, and so is the SQL standard.
My advice is to just write "SELECT 29 - 2 > 18", because it's clearer
in intent, simpler, and some tiny fraction faster than specifying
a useless table reference. But you can take comfort in the fact that
there's a simple workaround if you're ever forced to use a DBMS that
won't accept this syntax.
regards, tom lane
