1。使用左联接

Question

我有两个表talk_comments和talk_comment_votes。

我运行以下代码以选择commentId，numberOfUpvotes，whetherUserUpvoted，numberOfDownvotes，whetherUserDownvoted，并在同一表中使用左联接。

SELECT c.id, COUNT(v1.id) as upvotes, COUNT(v2.id) as userUpvoted, COUNT(v3.id) as downvotes, COUNT(v4.id) as userDownvoted FROM talk_comments c
    LEFT JOIN talk_comment_votes v1 ON v1.comment_id = c.id AND v1.status = 1 
    LEFT JOIN talk_comment_votes v2 ON v2.comment_id = c.id AND v2.status = 1 AND v2.user_id = 1 AND v2.is_anonymous = 0
    LEFT JOIN talk_comment_votes v3 ON c.id = v3.comment_id AND v3.status = 2
    LEFT JOIN talk_comment_votes v4 ON c.id = v4.comment_id AND v4.status = 2 AND v4.user_id = 1 AND v4.is_anonymous = 0
WHERE c.id = 2 GROUP BY c.id

我的talk_comment_votes表中有以下数据

因此，根据查询，它应该分别选择值2,2,0,1,1。当我中断那些JOIN语句并执行查询时，它将返回预期的结果。但是，对于JOIN，它会返回如下内容。

我可以在修复此问题上获得一些帮助吗？

谢谢。

Answer 1

我将使用条件聚合。联接到对tall_comment_votes的单个引用，然后检查表达式中的条件。

SELECT c.id
     , SUM(IF(v.status = 1                                          ,1,0)) AS upvotes
     , SUM(IF(v.status = 1 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userUpvoted
     , SUM(IF(v.status = 2                                          ,1,0)) AS downvotes
     , SUM(IF(v.status = 2 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userDownvoted
  FROM talk_comments c
  LEFT
  JOIN talk_comment_votes v
    ON v.comment_id = c.id
 WHERE c.id = 2
 GROUP
    BY c.id

当v1，v2，v3和v4返回多行时，这避免了部分叉积的问题。

MySQL IF()表达式可以替换为更符合ANSI标准的CASE表达式，例如

    , SUM(CASE WHEN v.status = 1 THEN 1 ELSE 0 END)  AS upvotes

关注

设置测试用例并观察执行计划和性能

填充表格

CREATE TABLE talk_comments (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT);
CREATE TABLE talk_comment_votes (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, comment_id  INT UNSIGNED NOT NULL, user_id  INT UNSIGNED NOT NULL, is_anonymous TINYINT(1) UNSIGNED NOT NULL, STATUS TINYINT UNSIGNED, time_ INT UNSIGNED); 
CREATE INDEX talk_comment_votes_IX1 ON talk_comment_votes (comment_id, STATUS, user_id, is_anonymous) ;
INSERT INTO talk_comments (id) VALUES (1),(2),(3);
INSERT INTO talk_comment_votes (id, comment_id, user_id, is_anonymous, STATUS, time_) VALUES (1,2,2,0,1,0),(2,1,1,0,1,0),(3,2,1,0,2,NULL),(4,7,1,0,2,NULL),(5,1,14,1,1,NULL),(6,2,14,1,1,NULL);

查询执行计划

EXPLAIN
SELECT c.id, COUNT(DISTINCT v1.id) AS upvotes, COUNT(DISTINCT v2.id) AS userUpvoted, COUNT(DISTINCT v3.id) AS downvotes, COUNT(DISTINCT v4.id) AS userDownvoted FROM talk_comments c
  LEFT JOIN talk_comment_votes v1 ON v1.comment_id = c.id AND v1.status = 1 
    LEFT JOIN talk_comment_votes v2 ON v2.comment_id = c.id AND v2.status = 1 AND v2.user_id = 1 AND v2.is_anonymous = 0
    LEFT JOIN talk_comment_votes v3 ON c.id = v3.comment_id AND v3.status = 2
    LEFT JOIN talk_comment_votes v4 ON c.id = v4.comment_id AND v4.status = 2 AND v4.user_id = 1 AND v4.is_anonymous = 0
WHERE c.id = 2 GROUP BY c.id
;

EXPLAIN
SELECT c.id
     , SUM(IF(v.status = 1                                          ,1,0)) AS upvotes
     , SUM(IF(v.status = 1 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userUpvoted
     , SUM(IF(v.status = 2                                          ,1,0)) AS downvotes
     , SUM(IF(v.status = 2 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userDownvoted
    FROM talk_comments c
    LEFT
    JOIN talk_comment_votes v
      ON v.comment_id = c.id
   WHERE c.id = 2
   GROUP BY c.id
;

说明的输出

--     id  select_type  table   type    possible_keys           key                     key_len  ref                        rows  Extra        
-- ------  -----------  ------  ------  ----------------------  ----------------------  -------  -----------------------  ------  -------------
--      1  SIMPLE       c       const   PRIMARY                 PRIMARY                 4        const                         1  Using index  
--      1  SIMPLE       v1      ref     talk_comment_votes_IX1  talk_comment_votes_IX1  6        const,const                   2  Using index  
--      1  SIMPLE       v2      ref     talk_comment_votes_IX1  talk_comment_votes_IX1  11       const,const,const,const       1  Using index  
--      1  SIMPLE       v3      ref     talk_comment_votes_IX1  talk_comment_votes_IX1  6        const,const                   1  Using index  
--      1  SIMPLE       v4      ref     talk_comment_votes_IX1  talk_comment_votes_IX1  11       const,const,const,const       1  Using index  



--     id  select_type  table   type    possible_keys           key                     key_len  ref       rows  Extra        
-- ------  -----------  ------  ------  ----------------------  ----------------------  -------  ------  ------  -------------
--      1  SIMPLE       c       const   PRIMARY                 PRIMARY                 4        const        1  Using index  
--      1  SIMPLE       v       ref     talk_comment_votes_IX1  talk_comment_votes_IX1  4        const        3  Using index

测得的效果：

100 executions                        round 1      round 2     round 3
------------------------------------  ----------   ----------  ---------
multiple left join, count(distinct    0.123 secs   0.130 secs  0.125 secs
conditional aggregation sum(if        0.113 secs   0.114 secs  0.111 secs

Answer 2

我对基于@ spencer7593和@RaymondNijland的2个答案的查询进行了基准测试。

左联接获胜！

1。使用左联接

SELECT c.id, COUNT(DISTINCT v1.id) as upvotes, COUNT(DISTINCT v2.id) as userUpvoted, COUNT(DISTINCT v3.id) as downvotes, COUNT(DISTINCT v4.id) as userDownvoted FROM talk_comments c
  LEFT JOIN talk_comment_votes v1 ON v1.comment_id = c.id AND v1.status = 1 
    LEFT JOIN talk_comment_votes v2 ON v2.comment_id = c.id AND v2.status = 1 AND v2.user_id = 1 AND v2.is_anonymous = 0
    LEFT JOIN talk_comment_votes v3 ON c.id = v3.comment_id AND v3.status = 2
    LEFT JOIN talk_comment_votes v4 ON c.id = v4.comment_id AND v4.status = 2 AND v4.user_id = 1 AND v4.is_anonymous = 0
WHERE c.id = 2 GROUP BY c.id

1000次查询的时间： 0.55000805854797s

2。使用子查询

SELECT c.id,c.user_id, c.time,c.body, c.reply_to, 
    (SELECT COUNT(v1.id) FROM talk_comment_votes v1 WHERE v1.comment_id = c.id AND v1.status = 1 LIMIT 1) as upvotes, 
    (SELECT COUNT(v2.id) FROM talk_comment_votes v2 WHERE v2.comment_id = c.id AND v2.status = 1 AND v2.user_id = 1 LIMIT 1) as clientUpvoted,
    (SELECT COUNT(v3.id) FROM talk_comment_votes v3 WHERE v3.comment_id = c.id AND v3.status = 2 LIMIT 1) as downvotes, 
    (SELECT COUNT(v4.id) FROM talk_comment_votes v4 WHERE v4.comment_id = c.id AND v4.status = 2 AND v4.user_id = 1 LIMIT 1) as clientDownvoted
      FROM talk_comments c 
           WHERE c.id = 2 GROUP BY c.id

1000次查询时间： 0.95499300956726s

3。使用SUM，IF

SELECT c.id
     , SUM(IF(v.status = 1                                          ,1,0)) AS upvotes
     , SUM(IF(v.status = 1 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userUpvoted
     , SUM(IF(v.status = 2                                          ,1,0)) AS downvotes
     , SUM(IF(v.status = 2 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userDownvoted
    FROM talk_comments c
    LEFT
    JOIN talk_comment_votes v
      ON v.comment_id = c.id
   WHERE c.id = 2
   GROUP BY c.id

1000次查询时间： 1.2266919612885s

谢谢所有答案。

MYSQL LEFT JOIN返回意外结果

2 个答案:

1。使用左联接

2。使用子查询

3。使用SUM，IF