计算子查询中的行数

Question

我想计算此子查询中的行数：

select t.*
FROM(
select 1 as first, 2 as second
union
select 1 as first, 3 as second
) t

所以预期的结果是：

+-------+--------+-------+
| first | second | count |
+-------+--------+-------+
|     1 |      2 |     2 |
|     1 |      3 |     2 |
+-------+--------+-------+

我已经尝试过将count(*)包含在查询中，但它不起作用。对这个问题有任何想法吗？

Answer 1

您可以使用窗口功能。

窗口功能非常宝贵，值得您花些时间来熟悉它们。

示例

select t.*
      ,count = sum(1) over ()
FROM(
select 1 as first, 2 as second
union
select 1 as first, 3 as second
) t

<强>返回

first   second  count
1       2       2
1       3       2

Answer 2

另一种选择是使用CTE。

with cte as
(
    select 1 as first, 2 as second
    union
    select 1 as first, 3 as second
)
select 
    cte.*,
    (select count(*) from cte) as [count]
from cte

Answer 3

你可以尝试

presto> select (map(array[], array[]) = map()) as is_empty;

 is_empty
----------
 true
(1 row)

或

select t.*, count(*) over (order by first  ) count
FROM(
select 1 as first, 1 as second
union
select 1 as first, 2 as second
union
select 1 as first, 3 as second
union
select 2 as first, 4 as second
) t