适用于多个SUM的MySQL子查询

Question

如果我需要在相似的WHERE条件下获得5个以上的不同SUM值，那么通过一个包含五个子查询的查询的最佳方法（也许是唯一的方法）吗？ 例如，我的查询是：

SELECT user, 
(SELECT SUM(amount) FROM table t, table2 t2 WHERE t.table2_id = t2.id AND table2.type = 1 AND user = 1 AND date = x) AS one,
(SELECT SUM(amount) FROM table t, table2 t2 WHERE t.table2_id = t2.id AND table2.type = 2 AND user = 1 AND date = x) AS two, 
(SELECT SUM(amount) FROM table t, table2 t2 WHERE t.table2_id = t2.id AND table2.type = 3 AND user = 1 AND date = x) AS three, 
(SELECT SUM(amount) FROM table t, table2 t2 WHERE t.table2_id = t2.id AND table2.type = 4 AND user = 1 AND date = x) AS four, 
(SELECT SUM(amount) FROM table t, table2 t2 WHERE t.table2_id = t2.id AND table2.type = 5 AND user = 1 AND date = x) AS five 
FROM table 
WHERE user = 1

到目前为止，我的查询速度非常快，但是当有更多记录时，我担心五个以上子查询的含义。

出于好奇，与五个子查询相比，具有五个子查询的这样一个查询在性能上是否具有优势/劣势？ 我以为它会更快，因为它只是一次数据库之旅，而不是5次？

编辑：有大约60条记录的表2，用于对表1中的数千条记录进行分类。表2中的每条记录按类型值1-5进行分类（这是我用来对项目进行分组的值）我想要的总和。

Answer 1

您只需要一个查询即可获得每种类型的总和：

SELECT 
  t2.type, 
  SUM(t.amount) 
FROM 
  table t 
INNER JOIN 
  table2 t2 
ON 
  t.table2_id = t2.id 
AND 
  t.user = 1
AND 
  t.date = x 
GROUP BY 
  t2.type

Answer 2

好。 如果这仅适用于一个用户，而您并不需要绝对按此顺序...。您可以这样做。

select user, type, sum(amount)
where user = 1
and date = x
and type between (1,5)
group by user, type

这会给你

1,1,sum
1,2,sum
1,3,sum
1,4,sum
1,5,sum

代替

1,sum1,sum2,sum3,sum4,sum5

对于您怀疑的所有需求，这应该足够好了，而不会造成任何麻烦。

Answer 3

1. 使用ANSI JOINS
2. 您最好像下面的查询一样以PIVOT形式编写查询

改写：

SELECT t.user, 
SUM(CASE table2.type WHEN 1 then amount else 0 end) As one,
SUM(CASE table2.type WHEN 2 then amount else 0 end) As two,
SUM(CASE table2.type WHEN 3 then amount else 0 end) As three,
SUM(CASE table2.type WHEN 4 then amount else 0 end) As four,
SUM(CASE table2.type WHEN 5 then amount else 0 end) As five
FROM table t
INNER JOIN table2 t2 ON t.table2_id = t2.id 
WHERE t.user = 1
  AND table2.type in (1,2,3,4,5)

我了解您的查询是伪查询，但是它有一个悬挂的WHERE user没有FROM子句。 至于性能，上面的内容一口气通过了表格。 5x子查询形式将进行5次传递，但如果有适当的索引可用，则不应慢很多。