group by
- group by的含義:將查詢結(jié)果按照1個或多個字段進行分組，字段值相同的為一組
- group by可用于單個字段分組，也可用于多個字段分組
select * from students;
+----+-----------+------+--------+--------+--------+-----------+
| id | name | age | height | gender | cls_id | is_delete |
+----+-----------+------+--------+--------+--------+-----------+
| 1 | 小明 | 18 | 180.00 | 女 | 1 | |
| 2 | 小月月 | 18 | 180.00 | 女 | 2 | ? |
| 3 | 彭于晏 | 29 | 185.00 | 男 | 1 | |
| 4 | 劉德華 | 59 | 175.00 | 男 | 2 | ? |
| 5 | 黃蓉 | 38 | 160.00 | 女 | 1 | |
| 6 | 鳳姐 | 28 | 150.00 | 保密 | 2 | ? |
| 7 | 王祖賢 | 18 | 172.00 | 女 | 1 | ? |
| 8 | 周杰倫 | 36 | NULL | 男 | 1 | |
| 9 | 程坤 | 27 | 181.00 | 男 | 2 | |
| 10 | 劉亦菲 | 25 | 166.00 | 女 | 2 | |
| 11 | 金星 | 33 | 162.00 | 中性 | 3 | ? |
| 12 | 靜香 | 12 | 180.00 | 女 | 4 | |
| 13 | 周杰 | 34 | 176.00 | 女 | 5 | |
| 14 | 郭靖 | 12 | 170.00 | 男 | 4 | |
+----+-----------+------+--------+--------+--------+-----------+
select gender from students group by gender;
+--------+
| gender |
+--------+
| 男 |
| 女 |
| 中性 |
| 保密 |
+--------+
根據(jù)gender字段來分組，gender字段的全部值有4個'男','女','中性','保密'，所以分為了4組 當group by單獨使用時，只顯示出每組的第一條記錄, 所以group by單獨使用時的實際意義不大
group by + group_concat()
- group_concat(字段名)可以作為一個輸出字段來使用，
- 表示分組之后，根據(jù)分組結(jié)果，使用group_concat()來放置每一組的某字段的值的集合
select gender from students group by gender;
+--------+
| gender |
+--------+
| 男 |
| 女 |
| 中性 |
| 保密 |
+--------+
select gender,group_concat(name) from students group by gender;
+--------+-----------------------------------------------------------+
| gender | group_concat(name) |
+--------+-----------------------------------------------------------+
| 男 | 彭于晏,劉德華,周杰倫,程坤,郭靖 |
| 女 | 小明,小月月,黃蓉,王祖賢,劉亦菲,靜香,周杰 |
| 中性 | 金星 |
| 保密 | 鳳姐 |
+--------+-----------------------------------------------------------+
select gender,group_concat(id) from students group by gender;
+--------+------------------+
| gender | group_concat(id) |
+--------+------------------+
| 男 | 3,4,8,9,14 |
| 女 | 1,2,5,7,10,12,13 |
| 中性 | 11 |
| 保密 | 6 |
+--------+------------------+
group by + 集合函數(shù)
- 通過group_concat()的啟發(fā)，我們既然可以統(tǒng)計出每個分組的某字段的值的集合，那么我們也可以通過集合函數(shù)來對這個
值的集合做一些操作
select gender,group_concat(age) from students group by gender;
+--------+----------------------+
| gender | group_concat(age) |
+--------+----------------------+
| 男 | 29,59,36,27,12 |
| 女 | 18,18,38,18,25,12,34 |
| 中性 | 33 |
| 保密 | 28 |
+--------+----------------------+
分別統(tǒng)計性別為男/女的人年齡平均值
select gender,avg(age) from students group by gender;
+--------+----------+
| gender | avg(age) |
+--------+----------+
| 男 | 32.6000 |
| 女 | 23.2857 |
| 中性 | 33.0000 |
| 保密 | 28.0000 |
+--------+----------+
分別統(tǒng)計性別為男/女的人的個數(shù)
select gender,count(*) from students group by gender;
+--------+----------+
| gender | count(*) |
+--------+----------+
| 男 | 5 |
| 女 | 7 |
| 中性 | 1 |
| 保密 | 1 |
+--------+----------+
group by + having
- having 條件表達式：用來分組查詢后指定一些條件來輸出查詢結(jié)果
- having作用和where一樣，但having只能用于group by
select gender,count(*) from students group by gender having count(*)>2;
+--------+----------+
| gender | count(*) |
+--------+----------+
| 男 | 5 |
| 女 | 7 |
+--------+----------+
group by + with rollup
- with rollup的作用是：在最后新增一行，來記錄當前列里所有記錄的總和
select gender,count(*) from students group by gender with rollup;
+--------+----------+
| gender | count(*) |
+--------+----------+
| 男 | 5 |
| 女 | 7 |
| 中性 | 1 |
| 保密 | 1 |
| NULL | 14 |
+--------+----------+
select gender,group_concat(age) from students group by gender with rollup;
+--------+-------------------------------------------+
| gender | group_concat(age) |
+--------+-------------------------------------------+
| 男 | 29,59,36,27,12 |
| 女 | 18,18,38,18,25,12,34 |
| 中性 | 33 |
| 保密 | 28 |
| NULL | 29,59,36,27,12,18,18,38,18,25,12,34,33,28 |
+--------+-------------------------------------------+
示例：
-- 分組
-- group by
-- 按照性別分組，查詢所有性別
-- 這時的count(*)統(tǒng)計的是分組之后的，每一個組中的信息，所以分組字段要有唯一性
select gender,count(*) from student group by gender;
-- 計算男性人數(shù)
select gender,count(*) from student where gender=1 group by gender;
-- 查詢同種性別中的所有姓名
select gender,group_concat(name) from student group by gender;
select gender,group_concat(name,age,id) from student group by gender;
select gender,group_concat(name,"_",age,"_",id) from student group by gender;
-- having
-- having字句可以讓我們篩選成組后的各種數(shù)據(jù)，where字句在聚合前先篩選記錄，也就是說作用在group by和having字句前。而 having子句在聚合后對組記錄進行篩選。
-- 查詢平均年齡超過30歲的性別以及姓名
select gender,group_concat(name) from student group by gender having avg(age)>30;
-- 查詢每個性別中的人數(shù)多于2個的信息
select gender,count(*) from student group by gender having count(*)>2;
