case语句中具有多列的子查询

Question

select case when integer=1 then (select col1,col2 from table1) 
            when integer=2 then (select col11,col22 from table2)
       end

Can the subquery return multiple values,if not which approach is better option than this?

Answer 1

CASE is for scalars not rows. For your problem I would use this:

IF integer=1
  SELECT col1 AS colA, col2 AS colB FROM table1
ELSE IF integer=2
  SELECT col11 AS colA, col22 AS colB FROM table2

Answer 2

Maybe it's not particularly clean, but this should work:

SELECT          CASE my_table.integer
                  WHEN 1 THEN table1.col1
                  ELSE        table2.col11
                END AS col_1,
                CASE my_table.integer
                  WHEN 1 THEN table1.col2
                  ELSE        table2.col2
                END AS col_2
FROM            my_table
LEFT OUTER JOIN table1
             ON ....
LEFT OUTER JOIN table2
             ON ....
...

Answer 3

Did you using MS Sql Server if yes you can try this

declare @script nvarchar(400)
select @script = case SomeField -- conditions field
when 1 then 'select * from [TableA]'
when 2 then 'select * from [TableB]'
end
from [TableC] where ...
exec sp_executesql @script

but be aware to concat any input from user to script.

Answer 4

You can do this with a single query using union all :

select col1, col2
from table1
where integer = 1
union all
select col1, col2
from table2
where integer = 2;