Query comportamento anomalo

USE [hhh]
GO
/****** Object:  StoredProcedure [dbo].[spwin_xyz]    ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER procedure [dbo].[spwin_GetCiccio]

@a nvarchar(20)=null,
@b int  =null, 
@c int=null,
@d int=null,
@e int=null, 
@f int=null,
@g int=null
as
declare @sql  nvarchar(4000),
    @paramlist nvarchar(4000)



set @sql = ' select * from varie join' 
                   
if((CHARINDEX('WHERE',@sql))>0)
    begin
      if( @a is not null and @a!='')
      begin
         set @sql = @sql + ' AND alike @a+''%'''
          end
      end
else
    begin
      if( @ais not null and @a!='')
            begin
        set @sql = @sql + ' WHERE a like @a+''%'''
            end
    end    

--faccio lo stesso per b, c, d, e, f, g


select @paramlist = '@anvarchar, 
           @b int,
           @c int,
           @d int,
           @e int, 
           @f int,           
           @g int'

exec sp_executesql @sql,@paramlist,@a,@b,@c,@d,@e,@f,@g

if((CHARINDEX('WHERE',@sql))>0)
    begin
      if( @a is not null and @a!='')
      begin
         set @sql = @sql + ' AND alike @a+''%'''
          end
      end
else
    begin
      if( @ais not null and @a!='')
            begin
        set @sql = @sql + ' WHERE a like @a+''%'''
            end
    end   


per b farò
if((CHARINDEX('WHERE',@sql))>0)
    begin
      if( @b is not null and @b !='')
      begin
         set @sql = @sql + ' AND b = @b '
          end
      end
else
    begin
      if( @b is not null and @b !='')
            begin
        set @sql = @sql + ' WHERE b = @b' 
            end
    end