Filter

C1111111111111' UNION SELECT CHAR(45,120,49,45,81,45),CHAR(45,120,50,45,81,45),CHAR(45,120,51,45,81,45),CHAR(45,120,52,45,81,45),CHAR(45,120,53,45,81,45) -- /* order by 'as

The used SELECT statements have a different number of columns
select distinct(books_id) from display_cat where main_cat = 'C1111111111111' UNION SELECT CHAR(45,120,49,45,81,45),CHAR(45,120,50,45,81,45),CHAR(45,120,51,45,81,45),CHAR(45,120,52,45,81,45),CHAR(45,120,53,45,81,45) -- /* order by 'as' order by id desc