Swapped EXCEPT operands, same data

Question

Accepted Answer

EXCEPT returns rows from the first set that are absent from the second set. Here the first set is opted_out, so it keeps rows in opted_out that are not in registered. bob@x.com is in both, so it is removed. dave@x.com is only in opted_out, so it survives. Flipping the order - `registered EXCEPT opted_out` - would give alice@x.com and carol@x.com, a completely different answer. Always think about which set is the 'source' and which is the 'filter' when writing EXCEPT.