• #group by
• #group by + group_concat()
• #group by + 集合函數
• #group by + having

#group by

(1) group by的含義:將查詢結果按照1個或多個字段進行分組，字段值相同的為一組
(2) group by可用於單個字段分組，也可用於多個字段分組

select * from employee;
+------+------+--------+------+------+-------------+
| num  | d_id | name   | age  | sex  | homeaddr    |
+------+------+--------+------+------+-------------+
|    1 | 1001 | 張三   |   26 | 男   | beijinghdq  |
|    2 | 1002 | 李四   |   24 | 女   | beijingcpq  |
|    3 | 1003 | 王五   |   25 | 男   | changshaylq |
|    4 | 1004 | Aric   |   15 | 男   | England     |
+------+------+--------+------+------+-------------+

select * from employee group 
 
by d_id,sex;

select * from employee group by sex;
+------+------+--------+------+------+------------+
| num  | d_id | name   | age  | sex  | homeaddr   |
+------+------+--------+------+------+------------+
|    2 | 1002 | 李四   |   24 | 女   | beijingcpq |
|    1 | 1001 | 張三   |   26 | 男   | beijinghdq |
+------+------+--------+------+------+------------+
根據sex字段來分組，sex字段的全部值只有兩個(‘男‘和‘女‘)，所以分為了兩組
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()

(1) group_concat(字段名)可以作為一個輸出字段來使用，
(2) 表示分組之後，根據分組結果，使用group_concat()來放置每一組的某字段的值的集合

select sex from employee group by sex;
+------+
| sex  |
+------+
| 女   |
| 男   |
+------+

select sex,group_concat(name) from employee group by sex;
+------+--------------------+
| sex  | group_concat(name) |
+------+--------------------+
| 女   | 李四               |
| 男   | 張三,王五,Aric     |
+------+--------------------+

select sex,group_concat(d_id)
 
 from employee group by sex;
+------+--------------------+
| sex  | group_concat(d_id) |
+------+--------------------+
| 女   | 1002               |
| 男   | 1001,1003,1004     |
+------+--------------------+

#group by + 集合函數

(1) 通過group_concat()的啟發，我們既然可以統計出每個分組的某字段的值的集合，那麽我們也可以通過集合函數來對這個"值的集合"做一些操作

select sex,group_concat(age) from employee group by sex;
+------+-------------------+
| sex  | group_concat(age) |
+------+-------------------+
| 女   | 24                |
| 男   | 26,25,15          |
+------+-------------------+

分別統計性別為男/女的人年齡平均值
select sex,avg(age) from employee group by sex;
+------+----------+
| sex  | avg(age) |
+------+----------+
| 女   |  24.0000 |
| 男   |  22.0000 |
+------+----------+

分別統計性別為男/女的人的個數
select sex,count(sex) from employee group by sex;
+------+------------+
| sex  | count(sex) |
+------+------------+
| 女   |          1 |
| 男   |          3 |
+------+------------+

#group by + having

(1) having 條件表達式：用來分組查詢後指定一些條件來輸出查詢結果
(2) having作用和where一樣，但having只能用於group by

select sex,count(sex) from employee group by sex having count(sex)>2;
+------+------------+
| sex  | count(sex) |
+------+------------+
| 男   |          3 |
+------+------------+

#group by + with rollup

(1) with rollup的作用是：在最後新增一行，來記錄當前列裏所有記錄的總和

select sex,count(age) from employee group by sex with rollup;
+------+------------+
| sex  | count(age) |
+------+------------+
| 女   |          1 |
| 男   |          3 |
| NULL |          4 |
+------+------------+

select sex,group_concat(age) from employee group by sex with rollup;
+------+-------------------+
| sex  | group_concat(age) |
+------+-------------------+
| 女   | 24                |
| 男   | 26,25,15          |
| NULL | 24,26,25,15       |
+------+-------------------+

[Mysql 查詢語句]——分組查詢group by