MYSQL - 只有在LEFT JOIN中没有行时才选择

Question

我有2个简单的mysql表。第1名为邮件和有两行：

sender | receiver 
Marley | Bob 
Saget | Bob 

第二个叫块和具有1行：

blocker | blocked 
    Bob | Marley 

我想选择谁送鲍勃电子邮件的第一个表发送器（S）但不会在阻止表中被阻止。所以结果应该是：

sender 
saget 

我尝试下面的查询，但它没有返回结果：

SELECT * FROM mail 
LEFT JOIN block ON (block.blocker = 'Bob') 
WHERE (block.blocked <> mail.sender) 

Answer 1

的左连接会产生null行的不匹配。
这就是你需要过滤的那些行null。

SELECT * FROM mail 
LEFT JOIN block ON (block.blocker = 'Bob') 
WHERE block.blocker IS NULL 

它是一种扼杀要然而在一个固定值的加盟，更常见的连接（给你的表）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'; 

Answer 2

试试这个：

SELECT sender 
FROM mail m 
WHERE NOT EXISTS (SELECT 1 FROM block 
        WHERE blocker = m.receiver 
        AND blocked = m.sender)