[英]MYSQL - Select only if row in LEFT JOIN is not present
我有2个简单的mysql表。 第一个叫做邮件,有2行:
sender | receiver
Marley | Bob
Saget | Bob
第二个叫块,有1行:
blocker | blocked
Bob | Marley
我想从第一个表中选择发送Bob电子邮件但未在阻止表中阻止的发件人。 所以结果应该是:
sender
saget
我尝试了以下查询,但它没有返回结果:
SELECT * FROM mail
LEFT JOIN block ON (block.blocker = 'Bob')
WHERE (block.blocked <> mail.sender)
左连接将为不匹配生成null
行。
这是您需要过滤的那些null
行。
SELECT * FROM mail
LEFT JOIN block ON (block.blocker = 'Bob')
WHERE block.blocker IS NULL
加入一个固定值是一种扼杀,然而,一个更常见的连接(给定你的表)将是:
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';
试试这个:
SELECT sender
FROM mail m
WHERE NOT EXISTS (SELECT 1 FROM block
WHERE blocker = m.receiver
AND blocked = m.sender)
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