1. 问题描述

一张卡，有一个底表记录这个卡的基本信息，还有一个使用卡的记录表，记录着，这张卡的使用记录，但我们要获取这张卡的最新使用记录，该如何写SQL呢？


解答：

SELECT
	vcr.* 
FROM
	visitor_card_records vcr
	JOIN (
	SELECT no,ic_no, MAX(created_at) AS max_created_at 
	FROM
		visitor_card_records 
	WHERE
		state = '使用中' 
		AND return_at IS NULL 
		AND ((NO = '1892' AND ic_no = '2630469661')) 
	GROUP BY no,ic_no 
	) vcrt ON vcr.NO = vcrt.NO 
	AND vcr.ic_no = vcrt.ic_no 
	AND vcr.created_at = vcrt.max_created_at

结果如下

多张卡查询

SELECT
	vcr.* 
FROM
	visitor_card_records vcr
	JOIN (
	SELECT no,ic_no, MAX(created_at) AS max_created_at 
	FROM
		visitor_card_records 
	WHERE
		state = '使用中' 
		AND return_at IS NULL 
		AND ((NO = '1892' AND ic_no = '2630469661') OR (NO = '1890' AND ic_no = '2630957549')) 
	GROUP BY no,ic_no 
	) vcrt ON vcr.NO = vcrt.NO 
	AND vcr.ic_no = vcrt.ic_no 
	AND vcr.created_at = vcrt.max_created_at

这里使用内连接的方法进行查询

当然如下来写也可以

SELECT id,no,ic_no, visitor_list_id,MAX(created_at) AS max_created_at 
	FROM
		visitor_card_records 
	WHERE
		state = '使用中' 
		AND return_at IS NULL 
		AND ((NO = '1892' AND ic_no = '2630469661') OR (NO = '1890' AND ic_no = '2630957549')) 
	GROUP BY no,ic_no