我发现了很多示例,展示了如何从分组集中选择单个最旧/最新的行,但是我无法从数据集中获取最旧的两行。
这是我的示例表:
CREATE TABLE IF NOT EXISTS `orderTable` (
`customer_id` varchar(10) NOT NULL,
`order_id` varchar(4) NOT NULL,
`date_added` date NOT NULL,
PRIMARY KEY (`customer_id`,`order_id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
INSERT INTO `orderTable` (`customer_id`, `order_id`, `date_added`) VALUES
('1234', '5A', '1997-01-22'),
('1234', '88B', '1992-05-09'),
('0487', 'F9', '2002-01-23'),
('5799', 'A12F', '2007-01-23'),
('1234', '3A', '2009-01-22'),
('3333', '7FHS', '2009-01-22'),
('0487', 'Z33', '2004-06-23'),
('3333', 'FF44', '2013-09-11'),
('3333', '44f5', '2013-09-02');
此查询返回多于两行:
SELECT customer_id, order_id, date_added
FROM orderTable T1
WHERE (
select count(*) FROM orderTable T2
where T2.order_id = T1.order_id AND T2.date_added <= T1.date_added
) <= 2;
因为我不是在寻找单行,所以这不是标准的 greatest-n-per-group
类型查询。
我错过了什么,我可以获得每个 customer_id 的前两个订单?
最佳答案
最好(即最高效)的方法是在查询中使用用户定义的变量。
SELECT tmp.customer_id, tmp.date_added
FROM (
SELECT
customer_id, date_added,
IF (@prev <> customer_id, @rownum := 1, @rownum := @rownum+1 ) rank,
@prev := customer_id
FROM orderTable t
JOIN (SELECT @rownum := NULL, @prev := 0) r
ORDER BY t.customer_id
) tmp
WHERE tmp.rank <= 2
ORDER BY customer_id, date_added
结果:
| CUSTOMER_ID | DATE_ADDED |
|-------------|----------------------------------|
| 0487 | January, 23 2002 00:00:00+0000 |
| 0487 | June, 23 2004 00:00:00+0000 |
| 1234 | May, 09 1992 00:00:00+0000 |
| 1234 | January, 22 1997 00:00:00+0000 |
| 3333 | January, 22 2009 00:00:00+0000 |
| 3333 | September, 02 2013 00:00:00+0000 |
| 5799 | January, 23 2007 00:00:00+0000 |
fiddle here .
请注意,连接只是用于初始化变量。
关于mysql - 从组中选择最旧的两条记录,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/18795551/