Question

我有两张表，其结构如下

Table name = counter
ref_id INT NOT NULL
count INT NOT NULL

Table name = favs
disqus_id VARCHAR(32) NOT NULL
user_id INT NOT NULL
dormant VAHCHAR(10) NOT NULL

我运行此查询=＆gt; SELECT count, dormant FROM counter AS c LEFT JOIN favs AS f ON c.ref_id = f.disqus_id WHERE ref_id = 'post_5' AND user_id = '1'.

但如果在第二个表favs中找不到匹配项，则不返回任何行。我想要的是，如果找到结果，它应返回两列，否则返回两列，第二列为NULL

我该怎么做？

Answer 1

ON子句

上用户的条件

SELECT c.count, f.dormant 
FROM   counter AS c 
       LEFT JOIN favs AS f 
          ON c.ref_id = f.disqus_id  AND f.user_id = '1'
WHERE  c.ref_id = 'post_5'

或者如果仍然不起作用，请同时移动：

SELECT c.count, f.dormant 
FROM   counter AS c 
       LEFT JOIN favs AS f 
          ON c.ref_id = f.disqus_id  AND 
             f.user_id = '1' AND
             c.ref_id = 'post_5'

即使在第二个表中找不到匹配项，也可以从第一个表中获取结果 - MySQL

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