This is not a recommended usage since the optimizer will probably not
generate a good plan. It would be better to use a IF statement and two
select statements. In addition, I find the logic more than a bit strange.
In any event, below is an example

use Northwind
go
--select * from invoices

declare @a int

/*set @a = 14
select * from invoices
where ProductID = @a

set @a = 10249
select * from invoices
where OrderID = @a*/

set @a = 10249
set @a = 14
select * from invoices
where case when @a > 1000 then OrderID else @a end = @a
and case when @a <= 1000 then ProductID else @a end = @a




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Hi!
how can i use if condition in where clause of select statement
I want the work like this
but this is not working correct
select * from Table1
where
if (@a>1)
field1=@a
else
field2=@a

select * from Table1 where @a>1 and field1=@a
Union
select * from Table1 where @a<1 and field2=@a


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