生活随笔
收集整理的這篇文章主要介紹了
MySQL之——GROUP BY分组取字段最大值
小編覺得挺不錯的,現(xiàn)在分享給大家,幫大家做個參考.
轉(zhuǎn)載自：http://blog.csdn.net/l1028386804/article/details/54657412
假設(shè)有一個業(yè)務(wù)場景，需要查詢用戶登錄記錄信息，其中表結(jié)構(gòu)如下：
[sql] view plaincopy
CREATE?TABLE?`tb`?(????`id`?int(11)?NOT?NULL?AUTO_INCREMENT,????`uid`?int(11)?NOT?NULL,????`ip`?varchar(16)?NOT?NULL,????`login_time`?datetime,????PRIMARY?KEY?(`id`),????KEY?(`uid`)??);?? 再來點測試數(shù)據(jù)：
[sql] view plaincopy
INSERT?INTO?tb?SELECT?null,?1001,?'192.168.1.1',?'2017-01-21?16:30:47';??INSERT?INTO?tb?SELECT?null,?1003,?'192.168.1.153',?'2017-01-21?19:30:51';??INSERT?INTO?tb?SELECT?null,?1001,?'192.168.1.61',?'2017-01-21?16:50:41';??INSERT?INTO?tb?SELECT?null,?1002,?'192.168.1.31',?'2017-01-21?18:30:21';??INSERT?INTO?tb?SELECT?null,?1002,?'192.168.1.66',?'2017-01-21?19:12:32';??INSERT?INTO?tb?SELECT?null,?1001,?'192.168.1.81',?'2017-01-21?19:53:09';??INSERT?INTO?tb?SELECT?null,?1001,?'192.168.1.231',?'2017-01-21?19:55:34';?? 表數(shù)據(jù)情況：
[plain] view plaincopy
+----+------+---------------+---------------------+??|?id?|?uid??|?ip????????????|?login_time??????????|??+----+------+---------------+---------------------+??|?1??|?1001?|?192.168.1.1???|?2017-01-21?16:30:47?|??|?2??|?1003?|?192.168.1.153?|?2017-01-21?19:30:51?|??|?3??|?1001?|?192.168.1.61??|?2017-01-21?16:50:41?|??|?4??|?1002?|?192.168.1.31??|?2017-01-21?18:30:21?|??|?5??|?1002?|?192.168.1.66??|?2017-01-21?19:12:32?|??|?6??|?1001?|?192.168.1.81??|?2017-01-21?19:53:09?|??|?7??|?1001?|?192.168.1.231?|?2017-01-21?19:55:34?|??+----+------+---------------+---------------------+?? 如果只需要針對用戶查出其最后登錄的時間，可以簡單寫出：
[html] view plaincopy
SELECT?uid,?max(login_time)??FROM?tb??GROUP?BY?uid;?? [plain] view plaincopy
+------+---------------------+??|?uid??|?max(login_time)???????|??+------+---------------------+??|?1001?|?2017-01-21?19:55:34?|??|?1002?|?2017-01-21?19:12:32?|??|?1003?|?2017-01-21?19:30:51?|??+------+---------------------+?? 若還需要查詢用戶最后登錄時的其他信息，就不能用這種sql寫了：
[sql] view plaincopy
SELECT?uid,?ip,?max(login_time)??FROM?tb??GROUP?BY?uid;?? 這樣的語句是非SQL標準的，雖然能夠在MySQL數(shù)據(jù)庫中執(zhí)行成功，但返回的卻是未知的
（如果sql_mode開啟了only_full_group_by，則不會執(zhí)行成功。）
可能ip字段會取uid分組前的第一個row的值，顯然不是所需信息
寫法1
寫一個子查詢：
[sql] view plaincopy
SELECT?a.uid,?a.ip,?a.login_time??FROM?tb?a??WHERE?a.login_time?in?(??SELECT?max(login_time)??FROM?tb??GROUP?BY?uid);?? 寫法2
再或者換一個寫法：
[sql] view plaincopy
SELECT?a.uid,?a.ip,?a.login_time??FROM?tb?a??WHERE?a.login_time?=?(??SELECT?max(login_time)??FROM?tb??WHERE?a.uid?=?uid);?? 順便測了一下
在5.6以前的版本中，寫法②這條sql在大數(shù)據(jù)量的情況下，執(zhí)行計劃不理想，目測性能不佳。
在5.6及以后的版本中，寫法②這條sql會快很多，執(zhí)行計劃也有了改變
5.5.50：
[plain] view plaincopy
+----+--------------------+-------+------+---------------+------+---------+------+------+-------------+??|?id?|?select_type????????|?table?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?Extra???????|??+----+--------------------+-------+------+---------------+------+---------+------+------+-------------+??|?1??|?PRIMARY????????????|?a?????|?ALL??|?NULL?????????????|?NULL??|?NULL??????|?NULL?|?7????|?Using?where?|??|?2??|?DEPENDENT?SUBQUERY?|?tb????|?ALL??|?uid???????????|?NULL??|?NULL??????|?NULL?|?7????|?Using?where?|??+----+--------------------+-------+------+---------------+------+---------+------+------+-------------+?? 5.6.30：
[plain] view plaincopy
+----+--------------------+-------+------+---------------+------+---------+------------+------+-------------+??|?id?|?select_type????????|?table??|?type?|?possible_keys?|?key??|?key_len?|?ref???????|?rows??|?Extra??????|??+----+--------------------+-------+------+---------------+------+---------+------------+------+-------------+??|?1??|?PRIMARY????????????|?a?????|?ALL??|?NULL??????????????|?NULL?|?NULL??????|?NULL????????|?7????|?Using?where?|??|?2??|?DEPENDENT?SUBQUERY?|?tb????|?ref??|?uid???????????|?uid??|?4???????|?test.a.uid?|?1????|?NULL???????????|??+----+--------------------+-------+------+---------------+------+---------+------------+------+-------------+?? 寫法3
直接改成join性能會更加好：
[sql] view plaincopy
SELECT?a.uid,?a.ip,?a.login_time??FROM?(SELECT?uid,?max(login_time)?login_time??FROM?tb??GROUP?BY?uid??)?b?JOIN?tb?a?ON?a.uid?=?b.uid?AND?a.login_time?=?b.login_time;?? 當然，結(jié)果都相同：
[plain] view plaincopy
+------+---------------+---------------------+??|?uid??|?ip????????????|?login_time??????????|??+------+---------------+---------------------+??|?1003?|?192.168.1.153?|?2017-01-21?19:30:51?|??|?1002?|?192.168.1.66??|?2017-01-21?19:12:32?|??|?1001?|?192.168.1.231?|?2017-01-21?19:55:34?|??+------+---------------+---------------------+? ?
轉(zhuǎn)載于:https://www.cnblogs.com/rainydayfmb/p/8371371.html
創(chuàng)作挑戰(zhàn)賽新人創(chuàng)作獎勵來咯，堅持創(chuàng)作打卡瓜分現(xiàn)金大獎
總結(jié)
以上是生活随笔為你收集整理的MySQL之——GROUP BY分组取字段最大值的全部內(nèi)容，希望文章能夠幫你解決所遇到的問題。
如果覺得生活随笔網(wǎng)站內(nèi)容還不錯，歡迎將生活随笔推薦給好友。
