Actually, I got something that works ...

SELECT

T.cat,

T.subcat,

X.code

FROM @test-2 AS T

LEFT JOIN @conversion AS X

ON X.cat = T.cat

AND X.subcat = CASE

WHEN EXISTS (

SELECT subcat FROM @conversion AS X1

WHERE X1.cat...

Thanks for the suggestion, but the results appear to be the same as my first example.

What I'd really like to do is get it all contained in the JOIN clause....