遞歸查詢樹tree 根據父節點查詢子集

表結構

SELECT
	ID.LEVEL1,
	DATA1.* 
FROM
	(
SELECT
	@ids AS _ids,
	( SELECT @ids := GROUP_CONCAT( pid ) FROM project_grouping WHERE FIND_IN_SET( parent_id, @ids ) ) AS cids,
	@l := @l + 1 AS LEVEL1 
FROM
	project_grouping,
	( SELECT @ids := 1, @l := 0 ) b 
WHERE
	@ids IS NOT NULL 
	) ID,
	project_grouping DATA1 
WHERE
	FIND_IN_SET( DATA1.pid, ID._ids ) 
ORDER BY
	LEVEL1,
	pid

不太理解網上爲什麼用LEVEL當作是別名 明明是關鍵字 

@ids 爲父節點id