Question

我有一张表table1，其中有3列column1, column2 and column3。

column1和column2是FOREIGN KEY，其他2个表。但是column3中的数据来自n个表。

例如让我们考虑一下Facebook。要显示活动，可能会维护一个可能包含user1 photoliked photo1或user1 statusliked status1的表格。因此，在这种情况下，column3不能是具有特定表格的FOREIGN KEY。

现在有两种获取真实数据的方法 -

第一路 -

SELECT user_id,
       verb_id,
       CASE WHEN verb_id = photoliked THEN
            (SELECT photo_name FROM photos WHERE photo_id = column3) -- getting the desired data from the third column
         WHEN verb_id = statusliked THEN
            (SELECT status FROM statustable WHERE status_id = column3) 
         ELSE '' END AS performedon
FROM table1
     JOIN table2 ON user_id = user_id  -- joining the first column
     JOIN table3 ON verb_id = verb_id  -- joining the second column

第二路 -

SELECT user_id,
       verb_id,
       CASE WHEN verb_id = photoliked THEN
            p.photo_name
         WHEN verb_id = statusliked THEN
            s.status
         ELSE '' END AS performedon
FROM table1
     JOIN table2 ON user_id = user_id  -- joining the first column
     JOIN table3 ON verb_id = verb_id  -- joining the second column
     LEFT JOIN photos p ON p.photo_id = column3  -- joining the column3 with specific table 
     LEFT JOIN statustable s ON s.status_id = column3

问题

检索数据的两种方法中哪一种更好？哪两个查询更便宜？

Answer 1

我认为JOIN会更快，因为查询会执行一次，我也会尝试过滤verb_id中的JOIN

SELECT user_id,
   verb_id,
   COALESCE(p.photo_name, s.status) AS performedon
FROM table1
    JOIN table2 ON user_id = user_id  -- joining the first column
    JOIN table3 ON verb_id = verb_id  -- joining the second column
    LEFT JOIN photos p ON verb_id = 'photoliked' AND p.photo_id = column3  -- joining the column3 with specific table 
    LEFT JOIN statustable s ON verb_id = 'statusliked' AND s.status_id = column3

Answer 2

第二个会更快，原因是第一个包含所谓的相关子查询。子查询与主查询中的记录具有相关性。因此，子查询需要为主查询中的每个匹配记录运行一次。在您的情况下，它不能运行子查询，直到它确定主查询中的verb_id的值。这是很多要运行的查询。

第一个查询的EXPLAIN应指出此问题。当你在EXPLAIN中看到它时，它通常是一个红旗。

Answer 3

你可以使用这种方法：

SELECT t.user_id,
       t.verb_id,
       p.photo_name  AS performedon
FROM table1 AS t
     JOIN table2 AS t2  ON t2.user_id = t.user_id  
     JOIN table3 AS t3  ON t3.verb_id = t.verb_id 
     JOIN photos AS p  ON  p.photo_id = t.column3  
                       AND t.verb_id = 'photoliked'

UNION ALL

SELECT t.user_id,
       t.verb_id,
       s.status  
FROM table1 AS t
     JOIN table2 AS t2  ON t2.user_id = t.user_id
     JOIN table3 AS t3  ON t3.verb_id = t.verb_id
     JOIN statustable AS s  ON  s.status_id = t.column3
                            AND t.verb_id = 'statusliked' ;

MySQL中的LEFT OUTER JOIN vs SUBSELECT

3 个答案: