sql sub category

寫了一條sql 去拎sub category
有3層category, 用個parent id 去拎

寫左我好耐... ><

但sub select一定會慢... 要諗下有冇再好d寫先 

#get all child cat by parent id

SELECT ID FROM CATEGORY WHERE ID IN 

(SELECT ID FROM CATEGORY WHERE ID = [parent_id] OR PARENT_ID = [parent_id]) OR PARENT_ID IN

(SELECT ID FROM CATEGORY WHERE ID = [parent_id] OR PARENT_ID = [parent_id])