Question

我尝试使用外连接和max / count / ... over partition将这些SQL语句（包含许多子查询）重写为更高效的形式。旧陈述：

select a.ID,
     (select max(b.valA) from something b where a.ID = b.ID_T and b.status != 0),
     (select max(b.valB) from something b where a.ID = b.ID_T and b.status != 0),
     (select max(b.valC) from something b where a.ID = b.ID_T and b.status != 0),
     (select max(b.valD) from something b where a.ID = b.ID_T)
from tool a;

这里重要的是 - max（b.valD）有不同的条件。首先，我没有注意到这种差异并写下这样的东西：

select distinct a.ID,
      max(b.valA) over (partition by b.ID_T),
      max(b.valB) over (partition by b.ID_T),
      max(b.valC) over (partition by b.ID_T),
      max(b.valD) over (partition by b.ID_T),
from tool a, 
     (select * from something
     where status != 0) b
where a.ID = b.ID_T(+);

我可以在 b.status！= 0 的 b.status！= 0 中使用 max over partition 的某个地方吗？或者我应该更好地添加第3个表来加入：

select distinct a.ID,
      max(b.valA) over (partition by b.ID_T),
      max(b.valB) over (partition by b.ID_T),
      max(b.valC) over (partition by b.ID_T),
      max(c.valD) over (partition by c.ID_T),
from tool a, 
     (select * from something 
      where status != 0) b, 
     something c
where a.ID = b.ID_T(+)
     and a.ID = c.ID_T(+);

问题在于选择和加入数百万行，我的例子就是简化我的查询。谁能帮助我实现更高效的sql？

Answer 1

您可以尝试使用CASE执行此操作：

select a.ID,
       max(CASE WHEN b.status=0 THEN b.valA END),
       max(CASE WHEN b.status=0 THEN b.valB END),
       max(CASE WHEN b.status=0 THEN b.valC END),
       max(b.valD)
  from tool a
  left join something b ON( b.ID_T = a.ID )
  group by a.ID;

请注意，我使用“new”join-syntax替换了隐式连接，以提高可读性。

Answer 2

另一种方法是使用JOIN和子查询分组：

select a.ID,
     b.MAX_A,
     b.MAX_B,
     b.MAX_C,
     b2.MAX_D 
from tool a
LEFT JOIN
 (
    SELECT ID_T,max(valA) MAX_A, max(valB) MAX_B, max(valC) MAX_C
    FROM something 
    WHERE status != 0
    GROUP BY ID_T     
  ) b
  ON a.ID=b.ID_T
LEFT JOIN
 (
    SELECT ID_T, max(valD) MAX_D
    FROM something 
    GROUP BY ID_T     
  ) b2
  ON a.ID=b2.ID_T

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2 个答案: