Tutorial on the principle and application of database system (046) -- MySQL query (VIII): group by

Database system principle and Application Tutorial （046）—— MySQL Inquire about （ 8、 ... and ）： Group query （GROUP BY）

Use GROUP BY Keywords can group query results by one or more columns or expressions , Grouping is based on GROUP BY The following column name or expression .GROUP BY Usually used with aggregate functions .

One 、GROUP BY Usage of

GROUP BY The syntax of the clause is as follows ：

GROUP BY < Name | expression >[,...] [HAVING  Conditional expression ] [WITH ROLLUP]

/*  explain ： （1） When using grouped queries ,select  The following field list can only contain  GROUP BY  The following column names or expressions and aggregate functions , Cannot contain other columns or expressions , Otherwise, an error will be reported . （2） Name | expression ： Group by , Group by column name or expression . （3）HAVING  Conditional expression ： Select groups , The result of the conditional expression will be displayed . （4）WITH ROLLUP： Add a record at the end of all records , This record is the statistical result of all rows . （5） have access to  GROUP_CONCAT()  The function concatenates all the values in a field into a string . */

for example ：

（1） Query the number of students corresponding to each address .

mysql> select addr,count(*) from student group by addr;
+-----------+----------+
| addr      | count(*) |
+-----------+----------+
|  Xinyang City     |        3 |
|  Kaifeng City     |        3 |
|  Xinxiang City     |        2 |
|  Zhengzhou city     |        6 |
+-----------+----------+
4 rows in set (0.00 sec)

--  The columns included in this query  s_name  Neither belong to  group by  Not in the aggregate function , So there was an error .
mysql> select s_name,addr,count(*) from student group by addr;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'mydb.student.s_name' which is not functionally dependent on co
lumns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

（2） Check the list of students corresponding to each address .

mysql> select addr,group_concat(s_name) from student group by addr;
+-----------+----------------------------------------------------------+
| addr      | group_concat(s_name)                                     |
+-----------+----------------------------------------------------------+
|  Xinyang City     |  Zhang Xiaogang , Liu Yan , Obama                                        |
|  Kaifeng City     |  Liu Ruofei , Dong Wenhua , Zhou Jianhua                                      |
|  Xinxiang City     |  Liu Xiaoqing , trump                                             |
|  Zhengzhou city     |  Cao mengde , Liu Yan , Zhou Huajian , Zhang Xueyou , Lee Myung Bak , Vinci                   |
+-----------+----------------------------------------------------------+
4 rows in set (0.00 sec)

Two 、 Use expressions to group

for example ： Group by the first three digits of the mobile phone number , Query the number of students in each group .

mysql> select left(phone,3) phone3,count(*) from student group by 1;
+--------+----------+
| phone3 | count(*) |
+--------+----------+
| 130    |        1 |
| 131    |        1 |
| 132    |        1 |
| 133    |        1 |
| 135    |        1 |
| 136    |        4 |
| 137    |        2 |
| 138    |        3 |
+--------+----------+
8 rows in set (0.00 sec)

3、 ... and 、 Use multiple columns to group

for example ： according to addr and gender grouping , Query the number of records in each group .

mysql> select * from student;
+-------+-----------+--------+---------------------+-------------+-----------+
| s_id  | s_name    | gender | birth               | phone       | addr      |
+-------+-----------+--------+---------------------+-------------+-----------+
| S2011 |  Zhang Xiaogang     |  male      | 1999-12-03 00:00:00 | 13163735775 |  Xinyang City     |
| S2012 |  Liu Xiaoqing     |  Woman      | 1999-10-11 00:00:00 | 13603732255 |  Xinxiang City     |
| S2013 |  Cao mengde     |  male      | 1998-02-13 00:00:00 | 13853735522 |  Zhengzhou city     |
| S2014 |  Liu Yan       |  Woman      | 1998-06-24 00:00:00 | 13623735335 |  Zhengzhou city     |
| S2015 |  Liu Yan       |  Woman      | 1999-07-06 00:00:00 | 13813735225 |  Xinyang City     |
| S2016 |  Liu Ruofei     |  Woman      | 2000-08-31 00:00:00 | 13683735533 |  Kaifeng City     |
| S2021 |  Dong Wenhua     |  Woman      | 2000-07-30 00:00:00 | 13533735564 |  Kaifeng City     |
| S2022 |  Zhou Huajian     |  male      | 1999-05-25 00:00:00 | 13243735578 |  Zhengzhou city     |
| S2023 |  trump     |  male      | 1999-06-21 00:00:00 | 13343735588 |  Xinxiang City     |
| S2024 |  Obama     |  male      | 2000-10-17 00:00:00 | 13843735885 |  Xinyang City     |
| S2025 |  Zhou Jianhua     |  male      | 2000-08-22 00:00:00 | 13788736655 |  Kaifeng City     |
| S2026 |  Zhang Xueyou     |  male      | 1998-07-06 00:00:00 | 13743735566 |  Zhengzhou city     |
| S2031 |  Lee Myung Bak     |  Woman      | 1999-10-26 00:00:00 | 13643732222 |  Zhengzhou city     |
| S2032 |  Vinci     |  male      | 1999-12-31 00:00:00 | 13043731234 |  Zhengzhou city     |
+-------+-----------+--------+---------------------+-------------+-----------+
14 rows in set (0.00 sec)

mysql> select addr,gender,count(*) from student group by 1,2;
+-----------+--------+----------+
| addr      | gender | count(*) |
+-----------+--------+----------+
|  Xinyang City     |  Woman      |        1 |
|  Xinyang City     |  male      |        2 |
|  Kaifeng City     |  Woman      |        2 |
|  Kaifeng City     |  male      |        1 |
|  Xinxiang City     |  Woman      |        1 |
|  Xinxiang City     |  male      |        1 |
|  Zhengzhou city     |  Woman      |        2 |
|  Zhengzhou city     |  male      |        4 |
+-----------+--------+----------+
8 rows in set (0.00 sec)

Four 、 When grouping, use WHERE Clause

If WHERE Clause , First use WHERE Filter the data in the table , Then group and count .

for example ： Query the address corresponding to each address in the student table 【 schoolboy 】 Number .

mysql> select addr,count(*) from student where gender = ' male ' group by addr;
+-----------+----------+
| addr      | count(*) |
+-----------+----------+
|  Xinyang City     |        2 |
|  Kaifeng City     |        1 |
|  Xinxiang City     |        1 |
|  Zhengzhou city     |        4 |
+-----------+----------+
4 rows in set (0.00 sec)

5、 ... and 、 Use HAVING Select groups

Use HAVING Clause allows you to select groups . When HAVING Clause and WHER When clauses are used at the same time , The order of query execution is ： First use WHERE Filter the records in the table , Then group and count the records that meet the conditions , Reuse HAVING Clause to select groups .

for example ：

（1） according to addr grouping , Query the number of people corresponding to each address .

mysql> select addr,count(*) from student group by addr;
+-----------+----------+
| addr      | count(*) |
+-----------+----------+
|  Xinyang City     |        3 |
|  Kaifeng City     |        3 |
|  Xinxiang City     |        2 |
|  Zhengzhou city     |        6 |
+-----------+----------+
4 rows in set (0.00 sec)

（2） according to addr grouping , The number of people in each address exceeds 3 Group of people .

mysql> select addr,count(*) from student group by addr having count(*)>3;
+-----------+----------+
| addr      | count(*) |
+-----------+----------+
|  Zhengzhou city     |        6 |
+-----------+----------+
1 row in set (0.00 sec)

（3） according to addr grouping , The number of boys in each address exceeds 3 Group of people .

mysql> select addr,count(*) from student where gender = ' male ' 
    -> group by addr having count(*)>3;
+-----------+----------+
| addr      | count(*) |
+-----------+----------+
|  Zhengzhou city     |        4 |
+-----------+----------+
1 row in set (0.00 sec)

--  The execution process of this query is as follows ：
-- 1、 The screening gender is 【 male 】 Student records of 
mysql> select * from student where gender = ' male ';
+-------+-----------+--------+---------------------+-------------+-----------+
| s_id  | s_name    | gender | birth               | phone       | addr      |
+-------+-----------+--------+---------------------+-------------+-----------+
| S2011 |  Zhang Xiaogang     |  male      | 1999-12-03 00:00:00 | 13163735775 |  Xinyang City     |
| S2013 |  Cao mengde     |  male      | 1998-02-13 00:00:00 | 13853735522 |  Zhengzhou city     |
| S2022 |  Zhou Huajian     |  male      | 1999-05-25 00:00:00 | 13243735578 |  Zhengzhou city     |
| S2023 |  trump     |  male      | 1999-06-21 00:00:00 | 13343735588 |  Xinxiang City     |
| S2024 |  Obama     |  male      | 2000-10-17 00:00:00 | 13843735885 |  Xinyang City     |
| S2025 |  Zhou Jianhua     |  male      | 2000-08-22 00:00:00 | 13788736655 |  Kaifeng City     |
| S2026 |  Zhang Xueyou     |  male      | 1998-07-06 00:00:00 | 13743735566 |  Zhengzhou city     |
| S2032 |  Vinci     |  male      | 1999-12-31 00:00:00 | 13043731234 |  Zhengzhou city     |
+-------+-----------+--------+---------------------+-------------+-----------+
8 rows in set (0.00 sec)

-- 2、 in the light of  addr  grouping 
mysql> select addr,count(*) from student where gender = ' male ' group by addr;
+-----------+----------+
| addr      | count(*) |
+-----------+----------+
|  Xinyang City     |        2 |
|  Kaifeng City     |        1 |
|  Xinxiang City     |        1 |
|  Zhengzhou city     |        4 |
+-----------+----------+
4 rows in set (0.00 sec)

-- 3、 Filter the groups 
mysql> select addr,count(*) from student 
       where gender = ' male ' group by addr having count(*)>3;
+-----------+----------+
| addr      | count(*) |
+-----------+----------+
|  Zhengzhou city     |        4 |
+-----------+----------+
1 row in set (0.00 sec)

6、 ... and 、WITH ROLLUP Parameters

Add a record at the end of all records , This record is the statistical result of all rows .

for example ： according to addr grouping , Query the number of people corresponding to each address .

mysql> select addr,count(*) from student group by addr with rollup;
+-----------+----------+
| addr      | count(*) |
+-----------+----------+
|  Xinyang City     |        3 |
|  Kaifeng City     |        3 |
|  Xinxiang City     |        2 |
|  Zhengzhou city     |        6 |
| NULL      |       14 |
+-----------+----------+
5 rows in set (0.00 sec)