左联接将产生null不匹配的行。 这些null是您需要过滤的行。
SELECT * FROM mail
LEFT JOIN block ON (block.blocker = 'Bob')
WHERE block.blocker IS NULL
以固定值进行联接有点麻烦，但是（根据您的表）更常见的联接是：
SELECT * FROM mail
LEFT JOIN block ON (block.blocker = mail.receiver
and block.blocked = mail.sender)<<-- these should match
WHERE block.blocker IS NULL <<-- select only mismatches
AND mail.receiver like 'bob';
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