问SQL Server:选择IF条件性

EN

select case when col1 = 1 then col3
            when col2 = 1 then col5
       end,
       case when col1 = 1 then col4
            when col2 = 1 then col6
       end
from tablename

select col3, col4
from tablename where col1 = 1
union all
select col5, col6
from tablename where col2 = 1

select col1, col2, col3 AS colum_a, col4 AS colum_b
from table1
where col1 = 1
union
select col1, col2, col5 AS colum_a, col6 AS colum_b
from table1
where col2 = 1;