mysql5.7 先排序 后分组

mysql5.7之前可以直接先排序后分组 （分组后获取排序第一的数据）

SELECT * FROM 
	( SELECT * FROM table t
		ORDER BY t.pay_date desc
	) x	
GROUP BY x.payment_id

5.7版本的mysql在执行这条sql语句的时候缺少了一个derived的操作，
mysql5.7对子查询进行了优化，认为子查询中的order by可以进行忽略，
只要Derived table里不包含如下条件就可以进行优化：

①、UNION clause

②、GROUP BY

③、DISTINCT

④、Aggregation

⑤、LIMIT or OFFSET

SELECT * FROM 
	( SELECT * FROM table pp
		ORDER BY pp.pay_date desc	
		LIMIT 10000
	) x	
GROUP BY x.payment_id

SELECT * FROM 
	( SELECT * FROM table pp
		GROUP BY pp.id
		ORDER BY pp.pay_date desc
	) x	
GROUP BY x.payment_id


SELECT * FROM 
	( SELECT DISTINCT * FROM table pp
		ORDER BY pp.pay_date desc
	) x	
GROUP BY x.payment_id;

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