1.測試數據:html
create table `t_class` ( `id` bigint (11), `name` varchar (150), `age` int (11), `class` int (11) ); insert into `t_class` (`id`, `name`, `age`, `class`) values('1','tom1','22','1'); insert into `t_class` (`id`, `name`, `age`, `class`) values('2','tom2','32','1'); insert into `t_class` (`id`, `name`, `age`, `class`) values('4','jerry2','33','2'); insert into `t_class` (`id`, `name`, `age`, `class`) values('5','hanchao','11','3'); insert into `t_class` (`id`, `name`, `age`, `class`) values('6','hanchao2','30','3'); insert into `t_class` (`id`, `name`, `age`, `class`) values('7','jerry3','32','2'); insert into `t_class` (`id`, `name`, `age`, `class`) values('8','jerry4','23','2'); insert into `t_class` (`id`, `name`, `age`, `class`) values('9','jerry1','33','2');
2.需求:查找每一個班年齡最大的學生的信息。mysql
很明顯,咱們會用到group by,
sql
eg:ide
SELECT id,NAME,MAX(age),class FROM t_class GROUP BY class;【錯誤的寫法!!】
可是,這是錯誤的寫法!!測試
固然,咱們也能夠這麼寫:spa
SELECT * FROM ( SELECT * FROM t_class ORDER BY age DESC) AS b GROUP BY class;
可是,我不建議這麼寫,由於這麼寫有些不可預知的事情發生。咱們先order by 再group by ,結果可能不是取的年齡最大的學生信息,具體緣由,我還在尋找中,固然你知道緣由也能夠告訴我,證據以下:.net
因此,咱們能夠這麼去寫:3d
/**方法1:**/ SELECT * FROM t_class t WHERE t.age = (SELECT MAX(age) FROM t_class WHERE t.class = class) ORDER BY class; /**方法2:**/ SELECT t_class.id,t_class.name,t_class.age,t_class.class FROM t_class INNER JOIN (SELECT class,MAX(age) AS maxAge FROM t_class GROUP BY class)t ON (t.class = t_class.class AND t_class.age = t.maxAge);
至於那個方法的效率高,你能夠本身測試一下!htm
能夠讀一下這篇文字,能夠更好的理解group by:http://www.cnblogs.com/wiseblog/articles/4475936.html blog
http://blog.csdn.net/john_hongming/article/details/42742965
http://www.th7.cn/db/mysql/201502/91713.shtml
http://yueliangdao0608.blog.51cto.com/397025/81278
說明:
(1) 在SQL標準中,做爲from後的子表或者子查詢,其中的order by 無效,會被忽略,這一點在mysql5.7/mariadb 10中獲得修正,以前的版本並不嚴格:
(2) group by對數據分組後,除被group by 的column以外,會選擇任意一條數據返回,也就是說,被group by的其餘列是無序的。
(3) 通過測試,發如今mysql 5.7和mariadb 10.*版本中,對相同的group by column,取值是根據數據寫入的順序來排序,即id順序;
(4) 在group by 以後加order by子句對返回值無效,因只order by只對返回集合排序;
(5) 參考資料
Mariadb 10 :
測試內容: set namesutf8; createtable testorder(id int not null,no int not null,name char(10) not null,primary key(id))engine=innodb; insertinto testorder values (1,1,'Mike'),(2,2,'John'),(3,3,'wyett'),(4,4,'Herry'),(5,5,'Mike'),(6,1,'John'),(7,2,'John'),(8,1,'Mike'),(9,1,'Mike'); select* from testorder;
select* from testorder order by no desc;
select* from (select id,no,name from testorder order by no desc)a group by a.name;
group by的默認排序是asc,所以name列是按照首字母升序排序的,對name相同的值
測試1:
Mariadb10:
結果:.取值是按照id從小到大的順序來的,對相同的no值,按寫入的物理排序進行的;
測試2:Percona5.5
測試3:Percona5.1
測試4:MySQL5.6,關閉sql_mode=ONLY_FULL_GROUP_BY
測試5:MySQL5.1,
(5.1的測試結果)
結果:
1.對name相同的值,按照no從大到小的順序排列,對no相同的值,按物理排序從小到大取值;
測試6:mysql5.7,關閉sql_mode=ONLY_FULL_GROUP_BY
結果:取值是按照id從小到大的順序來的,對相同的no值,按寫入的物理排序進行的;
測試7:mariadb 10,對no加desc排序
alter tabletestorder add index idx_no_name(no desc, name);
select* from (select id,no,name from testorder order by no desc)a group by a.name;
結果:desc是按照寫入順序也就是索引順序排序,不是按照大小排序