Question

我想从两个表中获取四个记录。两个记录中有两个记录，两个记录记录。

SELECT sum( a.user_follower_count ) AS mention_reach, 
       count( a.sno ) AS mention_count, 
       sum( b.user_follower_count ) AS retweet_reach, 
       count( b.username ) AS count_retweet
FROM twitter_mention AS a, twitter_retweet AS b

这里提到了read_reach和retweet_reach，但计数值是否出错？这里有什么不对吗？

我在twitter_retweet中只有6条记录，在twitter_retweet中只有10条记录，但它为两个计数值提供了60条记录。

Answer 1

您的表a和b之间没有加入。

添加JOIN b ON

或者在WHERE部分

中添加显式连接

更新

select *
from 
    (SELECT 
           sum( a.user_follower_count ) AS mention_reach, 
           count( a.sno ) AS mention_count 
    FROM twitter_mention AS a) a,
    (SELECT 
           sum( b.user_follower_count ) AS retweet_reach, 
           count( b.username ) AS count_retweet
    FROM twitter_retweet AS b) b

Answer 2

您的代码正在进行完全加入，即。

对于表a中的每条记录，请给我表b中的所有记录。

 SELECT sum( a.user_follower_count ) AS mention_reach, 
           count( a.sno ) AS mention_count, 
           sum( b.user_follower_count ) AS retweet_reach, 
           count( b.username ) AS count_retweet
    FROM twitter_mention AS a, twitter_retweet AS b

您需要做的是，确定表b中每个记录的表b中的哪条记录，而不是所有记录。这是通过连接

上的ON子句完成的

 SELECT sum( a.user_follower_count ) AS mention_reach, 
           count( a.sno ) AS mention_count, 
           sum( b.user_follower_count ) AS retweet_reach, 
           count( b.username ) AS count_retweet
    FROM twitter_mention AS a
    JOIN twitter_retweet AS b   ON a.<somefield> = B.<somefield>

此外，您应该使用JOIN关键字更新JOIN的语法。您可以看到JOIN语法是我上面的第二个查询。

如果您没有任何公共密钥，我建议使用UNION条款进行两次查询，即

 SELECT 'MENTION' as Type,sum( a.user_follower_count ) AS reach, 
           count( a.sno ) AS count
    FROM twitter_mention AS a
 UNION
 SELECT 'RETWEET' as Type,sum( b.user_follower_count ) , 
           count( b.username ) 
 FROM twitter_retweet AS b

使用count来获取表中的记录数

2 个答案: