想要根據用戶分組,以該用戶的下單時間爲降序,提取全部用戶的第二個訂單信息。
這屬於分組排序,在Oracle有內置函數能夠實現,而在mysql就有點麻煩:mysql
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實現分組排序測試表'; 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,# 行號 IF(@x=uo.user_id,@rank:=@rank+1,@rank:=1) rank,#處理排名,若是@x等於user_id,則表示@x被初始化,將@rank自增1 @x:=uo.user_id, # 初始化@x,@x爲中間變量,在rank以後初始化,因此,rank初始化時,@x爲null或者是上一個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
重點:關鍵在於@x如何賦值。瞭解@x的賦值以後,立馬就能明白rank(名次)的由來。
既然是分組排序,那固然是按組內來編號,每組固然得有一個不變的列,要否則按什麼group by呢?抓住這個特色天然就理解了rank的含義,還有一個點要注意就是group by的時候要有兩個排序的條件,要否則組內不穩定web