You need to use dynamic sql, for example sql server:
create procedure MyProc ( @Username varchar(30), @Name varchar(30) ) as begin exec ('SELECT * from tbl_answer where an_del=0 and u_username=isnull(like ''%' +@Username +'%'',u_username) or u_name=isnull(like ''' +@Name +''',u_name)') end
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