MySQL - GROUP BY 子句



MySQL GROUP BY 子句

MySQL 中的GROUP BY 子句用于将表中相同的数据整理成组。

例如,假设您有一个组织的销售数据表,其中包含日期、产品和销售额。要计算特定年份的总销售额,可以使用 GROUP BY 子句对该年份的产品销售额进行分组。类似地,您可以按日期对数据进行分组以计算每天的总销售额,或者按产品和日期的组合对数据进行分组以计算每天每种产品的总销售额。

此 GROUP BY 子句位于 SQL 语句中的 WHERE 子句之后,并且位于 ORDER BY 或 HAVING 子句(如果存在)之前。您可以使用GROUP BY 对来自某列的值进行分组,并且如果需要,可以对该列执行计算。您可以在分组列上使用 COUNT、SUM、AVG 等函数。

语法

以下是使用 GROUP BY 与 SELECT 语句的基本语法:

SELECT column_name(s) FROM table_name GROUP BY [condition | column_name(s)];

示例

此示例演示如何将聚合函数与 GROUP BY 子句一起使用。

首先,使用以下 CREATE TABLE 查询创建一个名为CUSTOMERS的表:

Open Compiler
CREATE TABLE CUSTOMERS ( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR (25), SALARY DECIMAL (18, 2), PRIMARY KEY (ID) );

现在,使用以下 INSERT 语句将以下记录插入 CUSTOMERS 表中:

Open Compiler
INSERT INTO CUSTOMERS VALUES (1, 'Ramesh', 32, 'Ahmedabad', 2000.00), (2, 'Khilan', 25, 'Delhi', 1500.00), (3, 'Kaushik', 23, 'Kota', 2000.00), (4, 'Chaitali', 25, 'Mumbai', 6500.00), (5, 'Hardik', 27, 'Bhopal', 8500.00), (6, 'Komal', 22, 'Hyderabad', 4500.00), (7, 'Muffy', 24, 'Indore', 10000.00);

表创建如下:

ID NAME AGE ADDRESS SALARY
1 Ramesh 32 Ahmedabad 2000.00
2 Khilan 25 Delhi 1500.00
3 Kaushik 23 Kota 2000.00
4 Chaitali 25 Mumbai 6500.00
5 Hardik 27 Bhopal 8500.00
6 Komal 22 Hyderabad 4500.00
7 Muffy 24 Indore 10000.00

现在,使用以下 GROUP BY 查询根据客户的年龄对客户进行分组:

Open Compiler
SELECT AGE, COUNT(Name) FROM CUSTOMERS GROUP BY AGE;

输出

以下是生成的输出结果:

AGE COUNT(Name)
32 1
25 2
23 1
27 1
22 1
24 1

MySQL 单列 GROUP BY

当我们在单个列上使用 GROUP BY 子句时,该列中的所有公共值将加在一起,使其成为一条记录。

示例

在此示例中,让我们按年龄对客户进行分组,并使用以下查询计算每个年龄的平均工资:

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SELECT AGE, AVG(SALARY) AS AVG_SALARY FROM CUSTOMERS GROUP BY AGE;

输出

这将产生以下结果:

AGE AVG_SALARY
32 2000.000000
25 4000.000000
23 2000.000000
27 8500.000000
22 4500.000000
24 10000.000000

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MySQL 多列 GROUP BY

当我们将 GROUP BY 子句与多个列一起使用时,通过组合这些列中的值获得的公共记录将分组到单个记录中。

示例

在此示例中,如果您想知道每个客户的年龄对应的工资总额,则 GROUP BY 查询如下所示:

Open Compiler
SELECT CONCAT(AGE, ' - ', SALARY) AS SALARY_AGEWISE FROM CUSTOMERS GROUP BY AGE, SALARY;

输出

这将产生以下结果:

SALARY_AGEWISE
32 - 2000.00
25 - 1500.00
23 - 2000.00
25 - 6500.00
27 - 8500.00
22 - 4500.00
24 - 10000.00

MySQL GROUP BY 与 ORDER BY 子句

我们可以在 MySQL 中将 ORDER BY 子句与 GROUP BY 一起使用,以根据一个或多个列对结果集进行排序。

语法

以下是 SQL 中使用 ORDER BY 子句与 GROUP BY 子句的语法:

SELECT column1, column2, ..., aggregate_function(columnX) AS alias FROM table GROUP BY column1, column2, ... ORDER BY column1 [ASC | DESC], column2 [ASC | DESC], ...;

示例

在这里,我们试图找到每个年龄的最高工资,并按从高到低的顺序排序:

Open Compiler
SELECT AGE, MAX(salary) AS MAX_SALARY FROM CUSTOMERS GROUP BY AGE ORDER BY MAX(salary) DESC;

输出

这将产生以下结果:

AGE AVG_SALARY
24 10000.00
27 8500.00
25 6500.00
22 4500.00
32 2000.00
23 2000.00

MySQL GROUP BY 与 HAVING 子句

我们还可以将 GROUP BY 子句与 HAVING 子句一起使用,以根据应用于数据组的条件过滤查询的结果。该条件可以应用于 SELECT 语句中使用的聚合函数或 GROUP BY 子句中的列。

语法

以下是 SQL 中使用 ORDER BY 子句与 HAVING 子句的语法:

SELECT column1, column2, aggregate_function(column) FROM table_name GROUP BY column1, column2 HAVING condition;

示例

在下面的查询中,我们根据客户的年龄对他们进行分组,并计算每个组的平均工资。HAVING 子句用于过滤结果,只显示平均工资大于 8000 的组。

Open Compiler
SELECT AGE, AVG(SALARY) AS AVG_SALARY FROM CUSTOMERS GROUP BY AGE HAVING AVG(salary) > 8000;

输出

这将产生以下结果:

AGE AVG_SALARY
27 8500.000000
24 10000.000000

使用客户端程序的 GROUP BY 子句

除了在 MySQL 服务器中使用 SQL 查询与 GROUP BY 子句一起使用之外,我们还可以使用客户端程序执行 GROUP BY 子句。

语法

以下是各种编程语言中 select 语句中 Group by 子句的语法:

要在 PHP 程序中通过 MySQL 表使用 GROUP BY 子句,我们需要使用 mysqli 连接器提供的名为 query() 的函数执行 SQL 语句。

$sql = "SELECT EXPRESSION1, EXPRESSION2, ... EXPRESSION_N, AGGREGATE_FUNCTION(EXPRESSION) FROM TABLE_NAME [WHERE CONDITION] GROUP BY EXPRESSION1, EXPRESSION2.."; $mysqli->query($sql,$resultmode)

要在 JavaScript 程序中通过 MySQL 表使用 GROUP BY 子句,我们需要使用 mysql2 连接器提供的名为 query() 的函数执行 SQL 语句。

sql= " SELECT column_name(s) FROM table_name GROUP BY [condition | column_name(s)];" Con.query(sql);

要在 Java 程序中通过 MySQL 表使用 GROUP BY 子句,我们需要使用 JDBC type 4 驱动程序提供的名为 executeQuery() 的函数执行 SQL 语句。

String sql = "SELECT column_name(s) FROM table_name GROUP BY [condition | column_name(s)]"; statement.executeQuery(sql);

要在 Java 程序中通过 MySQL 表使用 GROUP BY 子句,我们需要使用 MySQL Connector/Python 提供的名为 execute() 的函数执行 SQL 语句。

group_by_clause_query = "SELECT column_name(s) aggregate_function(column) FROM table_name GROUP BY column_name(s)" cursorObj.execute(group_by_clause_query)

示例

以下是使用各种编程语言实现 GROUP BY 的示例:

$dbhost = 'localhost'; $dbuser = 'root'; $dbpass = 'password'; $dbname = 'TUTORIALS'; $mysqli = new mysqli($dbhost, $dbuser, $dbpass, $dbname); if($mysqli->connect_errno ) { printf("Connect failed: %s", $mysqli->connect_error); exit(); } //printf('Connected successfully.'); $sql = 'SELECT tutorial_title, count(tutorial_id) AS tot_count FROM tutorials_tbl WHERE tutorial_id > 2 GROUP BY tutorial_title'; $result = $mysqli->query($sql); if ($result->num_rows > 0) { printf("Table records: \n"); while($row = $result->fetch_assoc()) { printf("Title: %s, Count: %d", $row["tutorial_title"], $row["tot_count"]); printf("\n"); } } else { printf('No record found.'); } mysqli_free_result($result); $mysqli->close();

输出

获得的输出如下所示:

Table records:
Title: JAVA Tutorial, Count: 1
Title: Learn PHP, Count: 1
Title: Learn MySQL, Count: 2
var mysql = require('mysql2'); var con = mysql.createConnection({ host: "localhost", user: "root", password: "Nr5a0204@123" }); //Connecting to MySQL con.connect(function (err) { if (err) throw err; console.log("Connected!"); console.log("--------------------------"); //Creating a Database sql = "create database TUTORIALS" con.query(sql); //Select database sql = "USE TUTORIALS" con.query(sql); //Creating table sql = "CREATE TABLE IF NOT EXISTS employee_tbl(id INT NOT NULL, name VARCHAR(100) NOT NULL, work_date DATE, daily_typing_pages INT);" con.query(sql); //Inserting Records sql = "INSERT INTO employee_tbl(id, name, work_date, daily_typing_pages) VALUES(1, 'John', '2007-01-24', 250), (2, 'Ram', '2007-05-27', 220), (3, 'Jack', '2007-05-06', 170), (3, 'Jack', '2007-04-06', 100), (4, 'Jill', '2007-04-06', 220),(5, 'Zara', '2007-06-06', 300),(5, 'Zara', '2007-02-06', 350);" con.query(sql); //Using GROUP BY Clause sql = "SELECT name, COUNT(*) FROM employee_tbl GROUP BY name;" con.query(sql, function(err, result){ if (err) throw err console.log(result) }); });

输出

产生的输出如下所示:

Connected!
--------------------------
[
  { name: 'John', 'COUNT(*)': 1 },
  { name: 'Ram', 'COUNT(*)': 1 },
  { name: 'Jack', 'COUNT(*)': 2 },
  { name: 'Jill', 'COUNT(*)': 1 },
  { name: 'Zara', 'COUNT(*)': 2 }
]
import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.Statement; public class GroupByClause { public static void main(String[] args) { String url = "jdbc:mysql://#:3306/TUTORIALS"; String user = "root"; String password = "password"; ResultSet rs; try { Class.forName("com.mysql.cj.jdbc.Driver"); Connection con = DriverManager.getConnection(url, user, password); Statement st = con.createStatement(); //System.out.println("Database connected successfully...!"); String sql = "SELECT AGE, AVG(SALARY) as avg_salary FROM CUSTOMERS GROUP BY age"; rs = st.executeQuery(sql); System.out.println("Table records(gruop by age): "); while(rs.next()){ String age = rs.getString("Age"); String avg_salary = rs.getString("avg_salary"); System.out.println("Age: " + age + ", Salary: " + avg_salary); } }catch(Exception e) { e.printStackTrace(); } } }

输出

获得的输出如下所示:

Table records(gruop by age): 
Age: 32, Salary: 2000.000000
Age: 25, Salary: 4000.000000
Age: 23, Salary: 2000.000000
Age: 27, Salary: 8500.000000
Age: 22, Salary: 4500.000000
Age: 24, Salary: 10000.000000
import mysql.connector #establishing the connection connection = mysql.connector.connect( host='localhost', user='root', password='password', database='tut' ) # Creating a cursor object cursorObj = connection.cursor() group_by_clause_query = """ SELECT ADDRESS, AVG(SALARY) AS average_salary FROM customers GROUP BY ADDRESS """ cursorObj.execute(group_by_clause_query) # Fetching all the grouped rows grouped_rows = cursorObj.fetchall() for row in grouped_rows: address, average_salary = row print(f"Address: {address}, Average Salary: {average_salary}") cursorObj.close() connection.close()

输出

以下是上述代码的输出:

Address: Hyderabad, Average Salary: 7500.000000
Address: Kerala, Average Salary: 8000.000000
Address: Mumbai, Average Salary: 1200.000000
Address: Vishakapatnam, Average Salary: 8500.000000
Address: Delhi, Average Salary: 10000.000000
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