MySql – Conditional display order

This is the select statement in MySql to get records in order with parent and child view.

 

SELECT *, CONCAT( if(parent_id > 0,(select display_order from `table_a` C where C.id = P.parent_id),''), if(parent_id > 0,'-',''), display_order ) AS new_col FROM `table_a` P order by new_col