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Stop 1 of 5 · Clinic waiting room: the missing patient
Trick to keep in mind+3 XP
NOT IN plus one NULL in the list returns no rows. Reach for NOT EXISTS.
x NOT IN (1, NULL) expands to x <> 1 AND x <> NULL, and the second test is unknown for every x, so the whole condition can never be true. The query returns nothing and raises no error. NOT EXISTS with a correlated subquery compares with = on real rows and is unaffected by NULLs.
Read it once, then a question tests whether the trick stuck.