This is a usecase that frequently annoys me:
AS thing1, thing2
SELECT a.thing1, a.thing2
FROM some_other_table a
...this fails when a.thing1 and a.thing2 are anything but STRING, because all output of TRANSFORM is STRING.
In this case, a.thing1 and a.thing2 should be implicitly converted to string.
COALESCE(a.thing1, a.thing2, a.thing3) should similarly do implicit type conversion among the arguments. If two are INT and one is BIGINT, upgrade the INTs, etc.
At the very least, it would be nice to have syntax like
AS thing1 INT, thing2 INT
...which would effectively cast the output column to the specified type. But really, type conversion should work.