获取已合并到多个表中的第一个表计数

Question

Here is a pseudo code what I want. I need to get count of fist union in following statement.

SELECT *  
FROM Table1 
UNION 
SELECT Cats.Id + (SELECT COUNT(*) FROM Fist_Union_Result), 
       Cats.Name 
FROM Table2

Any idea ?

Answer 1

Assuming that the first part is a complex query, you could use the with clause to alias it. That allows you to use it in two places, the top part of the union and the place where you count:

; with  FirstPart as
        (
        select  *
        from    Table1
        )
select  *
from    FirstPart
union all
select  cats.Id - cnt.cnt
,       cats.Name
from    Table2 cats
cross join
        (
        select  count(*) as cnt
        from    FistPart
        ) as cnt

If you just want a unique ID, you could place the union in a subquery and just label the rows 1..N:

select  row_number() over (order by Name) as Id
,       Name
from    (
        select  Name
        from    Table1
        union all
        select  Name
        from    Table2
        ) as SubQueryAlias

Answer 2

Something like this:

with first_union_result as (
  select col1, 
         col2
  from table1
), count_from_first as (
  select count(*) as first_count
  from first_union_result
)
select col1, 
       col2
from first_union_result
union all
select cats.id + cnt.first_count,
       cats.name
from cats
  cross join count_from_first as cnt;

If you are trying to get some unique id or something similar into the overall result, you might be better off using row_number() to generate that for all rows.