试图围绕可怕的SQL查询

Question

有人可以帮我理解这个查询究竟是做什么的吗？

SELECT pp.Sedol
    ,MAX(MAX(Id)) OVER (
        PARTITION BY pp.Sedol
        ,MAX(pp.ValueDate)
        ) PriceId
FROM Prices pp
GROUP BY pp.Sedol

Answer 1

这相当于：

with x as (
  select
    Sedol,
    max(id) max_id,
    Max(ValueDate) max_valuedate
  from
    Prices
  group by
    Sedol
) select
  Sedol,
  max(max_id) over (partition by Sedol, max_valuedate) PriceId
from
  x;

虽然正如喇嘛所说，我看不出任何方式这不会等同于

SELECT Sedol, MAX(Id) PriceId FROM Prices GROUP BY Sedol

SQL Fiddle

Answer 2

我认为Lamak已经解释了sql，我发布了一个理解的例子。 您可以看到两个sqls都给出相同的结果。在sql server中的代码下面复制粘贴并尝试：

declare @Prices table (Id int, ValueDate datetime, Sedol int)

Insert into @Prices values (1,'2014-09-06' ,200),
(2,'2014-09-07' , 100),
(3,'2014-09-08' , 100),
(4,'2014-09-09' , 100),
(5,'2014-09-10' , 300),
(6,'2014-09-11' , 300),
(7,'2014-09-12' , 100),
(8,'2014-09-13' , 200),
(9,'2014-09-14' , 200),
(10,'2014-09-15' , 200)

Select * from @Prices

-- Your SQL
SELECT pp.Sedol
    ,MAX(MAX(Id)) OVER (
        PARTITION BY pp.Sedol
        ,MAX(pp.ValueDate)
        ) PriceId
FROM @Prices pp
GROUP BY pp.Sedol

-- Simple SQL mentioned by Lamak
SELECT Sedol, MAX(Id) PriceId FROM @Prices GROUP BY Sedol

结果集：