SQL peculiarities in db2/AS400 (part 1)

SQL's CASE expression. Turns out that it's an expression, not a statement, so… it's supposed to only be allowed in expression contexts, IE the right-hand side of stuff.

WHERE 1 = CASE :student_id WHEN '123' THEN 1 ELSE 0 END

Postgres allows you to do something like the following:

WHERE CASE :student_id WHEN '123' THEN 1 = 1 ELSE 0 = 1 END

that is, the entire CASE can evaluate to a boolean value, not just a single value, which db2 requires.

TODO: document the occasional CAST(? as varchar(100)) block required by db2.