sql - 带和不带 ORDER BY 子句的分区分析计数

Question

我不明白为什么在使用 ORDER BY 时会有不同的结果分析中的子句 COUNT功能。

使用一个简单的例子:

with req as
 (select 1 as n, 'A' as cls
    from dual
  union
  select 2 as n, 'A' as cls
    from dual)
select req.*, count(*) over(partition by cls) as cnt from req;

给出以下结果:

N   CLS CNT
2   A   2
1   A   2

而当添加 ORDER BY 时在分析从句中，结果是不同的!

with req as
 (select 1 as n, 'A' as cls
    from dual
  union
  select 2 as n, 'A' as cls
    from dual)
select req.*, count(*) over(partition by cls order by n) as cnt from req;

CNT 列更改:

N   CLS CNT
1   A   1
2   A   2

有人可以解释一下吗？

谢谢

Answer 1

首先，一个link到文档。然而，它有些晦涩。

分析子句由 query_partition_clause 组成, order_by_clause和 windowing_clause .而且，关于 windowing_clause 的一件非常重要的事情是

You cannot specify this clause unless you have specified the order_by_clause. Some window boundaries defined by the RANGE clause let you specify only one expression in the order_by_clause. Refer to "Restrictions on the ORDER BY Clause".

但不仅不能用windowing_clause没有 order_by_clause ，它们被捆绑在一起。

If you omit the windowing_clause entirely, then the default is RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW.

默认窗口子句产生类似于运行总计的东西。 COUNT返回 1对于第一行，因为窗口顶部和当前行之间只有一行，2对于第二行，依此类推。

因此，在您的第一个查询中根本没有窗口，但在第二个查询中有默认窗口。

您可以通过指定完全无界窗口来模拟第一个查询的行为。

with req as
 (select 1 as n, 'A' as cls
    from dual
  union
  select 2 as n, 'A' as cls
    from dual)
select req.*, count(*) over(partition by cls order by n RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) as cnt from req;

是的

N   CLS CNT
1   A   2
2   A   2