MySql 如何结合 LEFT JOIN 查询和 NOT IN 查询

Question

我需要将 LEFT JOIN 查询与 NOT IN 查询结合起来，并且必须从这些查询中获取结果值。我在分别使用这两个查询时得到了正确的结果。

我有两个 table 可能 user 和 answer

user table

user_id 
1
2
3
4
5

answer table

user_id     date 
1           2015-10-15 21:23:14
2           2015-10-15 20:23:14
3           2015-11-11 16:23:14

LEFT JOIN查询：

SELECT user.user_id
FROM user 
LEFT JOIN answer 
  ON user.user_id = answer.user_id 
WHERE DATEDIFF(NOW(),answer.date) > 5

本次查询return结果user_id1,2.

NOT IN查询：

SELECT user.user_id 
FROM user 
WHERE user.user_id NOT IN (SELECT answer.user_id FROM answer)

本次查询return的结果user_ids4, 5.

我需要将这两个查询组合成一个查询，所以我尝试了以下两个查询：

 SELECT user.user_id 
 FROM user 
 LEFT JOIN answer 
   ON user.user_id = answer.user_id 
 WHERE (DATEDIFF(NOW(),answer.date) > 5 
   AND user.user_id NOT IN (SELECT answer.user_id FROM answer))

和

 SELECT user.user_id 
 FROM user 
 LEFT JOIN answer 
   ON user.user_id = answer.user_id 
 WHERE user.user_id NOT IN (SELECT answer.user_id FROM answer) 
   AND DATEDIFF(NOW(),answer.date) > 5

但是这些return是空的user_id。

编辑

预期结果应包含 1,2,4,5

的值

Answer 1

如果您的意思是将 2 个选择组合在一起
尝试 Union All / Union :

SELECT * FROM 
(
SELECT [WITH YOUR LEFT JOIN STATEMENT]
UNION 
SELECT [WITH YOUR NOT IN STATEMENT]
) ResultTABLE

Answer 2

如果您参考 WHERE 子句中的答案 table，answer.date 将被限制为非 NULL 值，使 LEFT JOIN 表现得像定期加入。将条件移动到 ON 条件以检索 all 用户，加上 maybe answers 中的匹配行：

 SELECT user.user_id
 FROM user u
 LEFT JOIN answer a ON u.user_id = a.user_id
                    AND DATEDIFF(NOW(),a.date) < 5
    ;

编辑：在问题编辑之后，OP 似乎想要加入不存在答案的记录或 answer.date old/young:

 SELECT user.user_id
 FROM user u
 LEFT JOIN answer a ON u.user_id = a.user_id
 WHERE a.date IS NULL               -- no answer record
    OR DATEDIFF(NOW(),a.date) > 5   -- too old answer record
    ;

最终版本：由于 OP 想要找到没有（最近）答案的用户，查询可以简化为：

SELECT user.user_id
FROM user u
WHERE NOT EXISTS (
    SELECT * FROM answer a
    WHERE a.user_id = u.user_id
    AND DATEDIFF(NOW(), a.date) <= 5
    );

MySql 如何结合 LEFT JOIN 查询和 NOT IN 查询

MySql how to combine LEFT JOIN query with NOT IN query

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join

left-join

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