Tutorial on principles and applications of database system (045) -- MySQL query (VII): aggregate function

Database system principle and Application Tutorial （045）—— MySQL Inquire about （ 7、 ... and ）： Aggregate functions

Aggregate function is also called statistical function , You can count and summarize the query results , Calculate the data value of a column in the table and return a single value . Aggregate functions are often associated with GROUP BY Clauses are used together .

Common aggregation functions include SUM、COUNT、AVG、MAX and MIN, Realize the statistics of data in the table （ Sum up 、 Count 、 Average 、 Maximum and minimum values, etc ）.GROUP_CONCAT Functions are similar to aggregate functions , You can connect the data values in a column .

One 、 Usage of aggregate function

The syntax format of the aggregate function is as follows ：

-- 1、 Counting function 
--  Use  count(*)  Do not ignore when  NULL  value , Use  count( Name | expression )  Ignore  NULL  value 
COUNT(distinct *| Name | expression )

-- 2、 Sum function 
--  The type of the column or the type of the return value of the expression must be a numeric type 
SUM( Name | expression )

-- 3、 Find the average function 
--  The type of the column or the type of the return value of the expression must be a numeric type 
AVG( Name | expression )

-- 4、 Find the maximum function 
--  The type of the column or the type of the return value of the expression can be any type 
MAX( Name | expression )

-- 5、 Find the minimum function 
--  The type of the column or the type of the return value of the expression can be any type 
MIN( Name | expression )

-- 6、 Connect data value functions 
--  The type of the column or the type of the return value of the expression can be any type 
--  Use  separator  Specify the separator between data , If omitted  separator, The default separator is comma 
--  Use  distinct  Can remove duplicate data 
GROUP_CONCAT([distinct]  Name  [order by  Sort field  asc|desc] separator ' Separator ')

explain ：

（1） If the aggregate function is used in the query , Without grouping , The query result has only one row （ There is only one statistical result ）. If you use GROUP BY grouping , Then each group has a statistical result .

（2） If the aggregate function is used in the query , Without grouping ,SELECT In addition to the aggregate function , Generally, there should be no column name .

for example ：

mysql> select s_name,count(*) from student;
ERROR 1140 (42000): In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column 'mydb.student.s_name'; this is incompatible with sql_mo
de=only_full_group_by

mysql> select count(*) from student;
+----------+
| count(*) |
+----------+
|       14 |
+----------+
1 row in set (0.00 sec)

Two 、 Counting function ：COUNT

Use COUNT( ) Function is used to count the number of records , Usually with GROUP BY Clause pooling .

The syntax is as follows ：

--  Use  count(*)  Do not ignore when  NULL  value , Use  count( Field | expression )  Ignore  NULL  value 
COUNT(distinct *| Name | expression )

for example ：

（1） Count all the students

mysql> select count(*) from student;
+----------+
| count(*) |
+----------+
|       14 |
+----------+
1 row in set (0.00 sec)

（2） The statistical address is 【 Xinxiang City 】 The number of students

mysql> select count(1) from student where addr = ' Xinxiang City ';
+----------+
| count(1) |
+----------+
|        2 |
+----------+
1 row in set (0.00 sec)

（3） Statistics 2000 The number of students born after

mysql> select count(*) from student where birth >= '2000-1-1';
+----------+
| count(*) |
+----------+
|        4 |
+----------+
1 row in set (0.00 sec)

（4） Count the number of addresses where students are

mysql> select count(distinct addr) from student;
+----------------------+
| count(distinct addr) |
+----------------------+
|                    4 |
+----------------------+
1 row in set (0.01 sec)

（5） Data exists NULL Situation of value

mysql> create table t12(id int primary key,name char(20),salary int);
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t12 
    -> values(1,'Jack',5200),(2,'Tom',4800),(3,'Black',3700),(4,null,null),(5,null,null);
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0

--  Count the number of records 
mysql> select count(*) from t12;
+----------+
| count(*) |
+----------+
|        5 |
+----------+
1 row in set (0.00 sec)

--  Count the number of records （ Ignore  name  by  NULL  The record of ）
mysql> select count(name) from t12;
+-------------+
| count(name) |
+-------------+
|           3 |
+-------------+
1 row in set (0.00 sec)

3、 ... and 、 Sum function ：SUM

Use SUM( ) Function can sum the data of a column in the table , Ignore in Statistics NULL value . If there is no matching line , Then return to NULL value . Often with GROUP BY Clause pooling .

The syntax is as follows ：

--  The type of the column or the type of the return value of the expression must be a numeric type 
SUM( Field | expression )

for example ：

（1） Statistics students S2011 Total score of elective courses .

mysql> select sum(score) from score where s_id = 'S2011';
+------------+
| sum(score) |
+------------+
|        318 |
+------------+
1 row in set (0.04 sec)

（2） Statistics t12 In the table salary The sum of the columns .

mysql> select * from t12;
+----+-------+--------+
| id | name  | salary |
+----+-------+--------+
|  1 | Jack  |   5200 |
|  2 | Tom   |   4800 |
|  3 | Black |   3700 |
|  4 | NULL  |   NULL |
|  5 | NULL  |   NULL |
+----+-------+--------+
5 rows in set (0.00 sec)

--  Ignore in Statistics  NULL  value 
mysql> select sum(salary) from t12;
+-------------+
| sum(salary) |
+-------------+
|       13700 |
+-------------+
1 row in set (0.00 sec)

（3） Statistics 2000 Number of students born after .

mysql> select sum(1) from student where birth >= '2000-1-1';
+--------+
| sum(1) |
+--------+
|      4 |
+--------+
1 row in set (0.00 sec)

（4） The statistical address is 【 Xinxiang City 】 The number of students .

mysql> select sum(if(addr = ' Xinxiang City ',1,0)) cnt from student;
+------+
| cnt  |
+------+
|    2 |
+------+
1 row in set (0.01 sec)

Four 、 Find the average function ：AVG

Use AVG( ) Function can calculate the average value of a column of data in the table . Ignore in Statistics NULL value . Often with GROUP BY Clause pooling .

The syntax is as follows ：

--  The type of the column or the type of the return value of the expression must be a numeric type 
AVG( Field | expression )

for example ：

（1） Statistics students S2011 Average score of elective courses .

mysql> select avg(score) from score where s_id = 'S2011';
+------------+
| avg(score) |
+------------+
|    79.5000 |
+------------+
1 row in set (0.00 sec)

（2） Statistics t12 In the table salary The average of the columns .

mysql> select * from t12;
+----+-------+--------+
| id | name  | salary |
+----+-------+--------+
|  1 | Jack  |   5200 |
|  2 | Tom   |   4800 |
|  3 | Black |   3700 |
|  4 | NULL  |   NULL |
|  5 | NULL  |   NULL |
+----+-------+--------+
5 rows in set (0.00 sec)

--  Ignore in Statistics  NULL  value 
mysql> select avg(salary) from t12;
+-------------+
| avg(salary) |
+-------------+
|   4566.6667 |
+-------------+
1 row in set (0.00 sec)

（3） Ask the average age of all students .

--  Average value of statistical expression 
mysql> select avg(year(now())-year(birth)) avg_age from student;
+---------+
| avg_age |
+---------+
| 22.9286 |
+---------+
1 row in set (0.00 sec)

5、 ... and 、 Find the maximum function ：MAX

Use MAX( ) Function counts the maximum value of a column of data . Ignore in Statistics NULL value . Often with GROUP BY Clause pooling .

The syntax is as follows ：

--  The type of the column or the type of the return value of the expression can be any type 
MAX( Field | expression )

for example ：

（1） Inquire about C102 The highest score of the course .

mysql> select max(score) from score where c_id = 'C102';
+------------+
| max(score) |
+------------+
|         97 |
+------------+
1 row in set (0.00 sec)

（2） Query the maximum age of all students .

mysql> select max(year(now())-year(birth)) max_age from student;
+---------+
| max_age |
+---------+
|      24 |
+---------+
1 row in set (0.00 sec)

6、 ... and 、 Find the minimum function ：MIN

Use MIN( ) Function counts the minimum value of a column of data . Ignore in Statistics NULL value . Often with GROUP BY Clause pooling .

The syntax is as follows ：

--  The type of the column or the type of the return value of the expression can be any type 
MIN( Field | expression )

for example ：

（1） Inquire about C102 Minimum score of the course .

mysql> select min(score) from score where c_id = 'C102';
+------------+
| min(score) |
+------------+
|         52 |
+------------+
1 row in set (0.00 sec)

（2） Query the minimum age of all students .

mysql> select min(year(now())-year(birth)) min_age from student;
+---------+
| min_age |
+---------+
|      22 |
+---------+
1 row in set (0.00 sec)

7、 ... and 、 Connect data value functions ：GROUP_CONCAT

Use GROUP_CONCAT( ) Function can connect the data values of a column into a string , The data values are separated by the specified separator （ Default to comma ）.

The syntax is as follows ：

--  The type of the column or the type of the return value of the expression can be any type 
--  Use  separator  Specify the separator between data , If omitted  separator, The default separator is comma 
--  Use  distinct  Can remove duplicate data 
GROUP_CONCAT([distinct]  Name  [order by  Sort field  asc|desc] separator ' Separator ');

for example ：

（1） The inquiry address is 【 Zhengzhou city 】 List of students , Names are separated by commas .

mysql> select group_concat(s_name) from student where addr = ' Zhengzhou city ';
+----------------------------------------------------------+
| group_concat(s_name)                                     |
+----------------------------------------------------------+
|  Cao mengde , Liu Yan , Zhou Huajian , Zhang Xueyou , Lee Myung Bak , Vinci                   |
+----------------------------------------------------------+
1 row in set (0.00 sec)

（2） The inquiry address is 【 Zhengzhou city 】 List of students , Use 【||】 Separate .

mysql> select group_concat(s_name separator '||') from student where addr = ' Zhengzhou city ';
+---------------------------------------------------------------+
| group_concat(s_name separator '||')                           |
+---------------------------------------------------------------+
|  Cao mengde || Liu Yan || Zhou Huajian || Zhang Xueyou || Lee Myung Bak || Vinci                   |
+---------------------------------------------------------------+
1 row in set (0.00 sec)

（3） The inquiry address is 【 Zhengzhou city 】 List of students , Use 【|】 Separate , And press birth Sort .

mysql> select s_name,birth,addr from student where addr = ' Zhengzhou city ' order by birth;
+-----------+---------------------+-----------+
| s_name    | birth               | addr      |
+-----------+---------------------+-----------+
|  Cao mengde     | 1998-02-13 00:00:00 |  Zhengzhou city     |
|  Liu Yan       | 1998-06-24 00:00:00 |  Zhengzhou city     |
|  Zhang Xueyou     | 1998-07-06 00:00:00 |  Zhengzhou city     |
|  Zhou Huajian     | 1999-05-25 00:00:00 |  Zhengzhou city     |
|  Lee Myung Bak     | 1999-10-26 00:00:00 |  Zhengzhou city     |
|  Vinci     | 1999-12-31 00:00:00 |  Zhengzhou city     |
+-----------+---------------------+-----------+
6 rows in set (0.00 sec)

mysql> select group_concat(s_name order by birth separator '|') from student where addr = ' Zhengzhou city ';
+----------------------------------------------------------+
| group_concat(s_name order by birth separator '|')        |
+----------------------------------------------------------+
|  Cao mengde | Liu Yan | Zhang Xueyou | Zhou Huajian | Lee Myung Bak | Vinci                   |
+----------------------------------------------------------+
1 row in set (0.00 sec)

（4） Look up all the students addr list .

mysql> select group_concat(addr) from student;
+---------------------------------------------------------------------------------------------+
| group_concat(addr)                                                                          |
+---------------------------------------------------------------------------------------------+
|  Xinyang City , Xinxiang City , Zhengzhou city , Zhengzhou city , Xinyang City , Kaifeng City , Kaifeng City , Zhengzhou city , Xinxiang City , Xinyang City , Kaifeng City , Zhengzhou city , Zhengzhou city , Zhengzhou city    |
+---------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

--  Eliminate duplicate addresses 
mysql> select group_concat(distinct addr) from student;
+-----------------------------------------+
| group_concat(distinct addr)             |
+-----------------------------------------+
|  Xinyang City , Kaifeng City , Xinxiang City , Zhengzhou city              |
+-----------------------------------------+
1 row in set (0.00 sec)