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SQL: NOT IN NULL
Got caught out today by a reasonably common issue which can be a bit perplexing.
With a fairly simple query like:
SELECT col_a
FROM table_1
WHERE col_a NOT IN (SELECT col_a FROM table_2)
This can unexpectedly return zero rows if SELECT col_a FROM table_2 returns a NULL, so the obvious way to fix it
is to replace the WHERE condition with:
WHERE col_a NOT IN (SELECT col_a FROM table_2)
or to use the less well know (at least by me) EXISTS operator instead:
SELECT col_a
FROM table_1 t1
WHERE col_a NOT EXISTS (SELECT col_a FROM table_2 t2 WHERE t1.col_a = t2.col_a)
Useful Stack overflow post explaining more about why this issue occurs