SQL Server :: Passing One Input Parameter To The SP?

im passing one input parameter to the SP, based on that parameter i have written the code like below create procedure SP1(@inpName varchar(10))

declare @where varchar(20);
if @inpName = 'ABCD'
@where = 'A';
else if @inpName = '1234'
@where = '1';
else if @inpName = 'AB12'
@where = 'A,B';

SELECT * FROM TABLE1 WHERE COLUMN1 IN(@where);

Now the above query was fine if its 'ABCD' or '1234', but if its 'AB12' then query failed.
How to resolve this issue.?