Mysql多表查询，取出其中一张表的最大的数据或者去重的字段

需求：1、查询三张表共同存在的数据（本来用的内连接）
2、后来要求再加一张表，把这张表的最大数据查出来合到一起（外键管理订单号）

SELECT 
	@ROW := @ROW + 1 AS DT_RowId,
  tzo.order_id,
  tzo.order_num,
	tzu.user_phone,
	FROM_UNIXTIME(tzv.creat_time , '%Y-%m-%d' ) AS creat_time
FROM
( SELECT @ROW := 0 ) t,
	t_zebra_order tzo,
	t_zebra_user tzu,
	t_zebra_vehiclereport tzv	
	WHERE
	tzo.user_id = tzu.user_id 
	AND tzo.order_num = tzv.order_num	

上面为以前的需求，现在分析后面的需求
（1）先查出最大数据

SELECT max(follow_id) FROM t_zebra_follow GROUP BY follow_order_num

（2）再查出此表所有信息（把第一步作为子查询）

SELECT * FROM t_zebra_follow WHERE follow_id in (SELECT max(follow_id) FROM t_zebra_follow GROUP BY follow_order_num)

最后和以前的需求合并到一起，我用左连接，因为（左边多表查询出来的数据，一个订单号对应新表多条数据或者0条数据）

SELECT
	@ROW := @ROW + 1 AS DT_RowId,
	tzo.order_id,
	tzo.order_num,
	tzu.user_phone,
	FROM_UNIXTIME( tzv.creat_time, '%Y-%m-%d' ) AS creat_time,
	t1.follow_up,
	t1.follow_person 
FROM
	( SELECT @ROW := 0 ) t,
	t_zebra_order tzo,
	t_zebra_user tzu,
	t_zebra_vehiclereport tzv
	LEFT JOIN ( SELECT * FROM t_zebra_follow WHERE follow_id IN ( SELECT max( follow_id ) FROM t_zebra_follow GROUP BY follow_order_num ) ) t1 ON tzv.order_num = t1.follow_order_num 
WHERE
	tzo.user_id = tzu.user_id 
	AND tzo.order_num = tzv.order_num