想要根据用户分组，以该用户的下单时间为降序，提取所有用户的第二个订单信息。
这属于分组排序，在Oracle有内置函数可以实现，而在mysql就有点麻烦：
CREATE TABLE user_orders (orders_id INT UNSIGNED NOT NULL,
user_id INT UNSIGNED NOT NULL,
add_time INT UNSIGNED NOT NULL,
PRIMARY KEY(orders_id),
KEY(user_id),
KEY(add_time)
)ENGINE=INNODB DEFAULT CHARSET utf8 COMMENT 'mysql实现分组排序测试表';
INSERT INTO user_orders (orders_id, user_id, add_time) VALUES('1','1','1');
INSERT INTO user_orders (orders_id, user_id, add_time) VALUES('2','1','2');
INSERT INTO user_orders (orders_id, user_id, add_time) VALUES('3','1','3');
INSERT INTO user_orders (orders_id, user_id, add_time) VALUES('4','2','1');
INSERT INTO user_orders (orders_id, user_id, add_time) VALUES('5','2','2');
INSERT INTO user_orders (orders_id, user_id, add_time) VALUES('6','2','3');
INSERT INTO user_orders (orders_id, user_id, add_time) VALUES('7','3','1');
INSERT INTO user_orders (orders_id, user_id, add_time) VALUES('8','3','2');
INSERT INTO user_orders (orders_id, user_id, add_time) VALUES('9','3','3');
SELECT orders_id,user_id,add_time,rank FROM (
SELECT @rownum:=@rownum+1 AS rownum,# 行号
IF(@x=uo.user_id,@rank:=@rank+1,@rank:=1) rank,#处理排名，如果@x等于user_id，则表示@x被初始化，将@rank自增1
@x:=uo.user_id, # 初始化@x，@x为中间变量,在rank之后初始化,所以，rank初始化时，@x为null或者是上一个user_id的值
orders_id,user_id,add_time
FROM
user_orders uo,
(SELECT @rownum:=0,@rank:=0) init # 初始化信息表
ORDER BY user_id ASC, add_time DESC
)result
WHERE rank=2
重点：关键在于@x如何赋值。了解@x的赋值之后，立马就能明白rank(名次)的由来。