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 matchWHERe block.blocker IS NULL <<-- select only mismatchesAND mail.receiver like 'bob';
