无涯教程-Derby - Group By语句

GROUP BY子句与SELECT语句一起使用，如果数据相同，则用于形成子集，通常，此子句后跟ORDER BY子句，并放在WHERE子句之后。

GROUP BY - 语法

以下是GROUP BY子句的语法-

ij>SELECT column1, column2, . . . table_name GROUP BY column1, column2, . . .;

GROUP BY - 命令行示例

假设无涯教程在数据库中有一个名为Employees的表，其中包含以下记录-

ID |NAME |SALARY |LOCATION
------------------------------------------------------------------
1 |Amit |30000 |Hyderabad
2 |Rahul |39000 |Lucknow
3 |Renuka |50000 |Hyderabad
4 |Learnfk |15000 |Vishakhapatnam
5 |Kalyan |40000 |Hyderabad
6 |Trupthi |45000 |Vishakhapatnam
7 |Raghav |12000 |Lucknow
8 |Suchatra |33000 |Vishakhapatnam
9 |Rizwan |20000 |Lucknow 

以下带有GROUP BY子句的SELECT语句根据位置对表进行分组，它显示给某个位置的员工的工资总额。

ij> SELECT Location, SUM(Salary) from Employees GROUP BY Location;

这将生成以下输出-

LOCATION |2
-------------------------------------------------------
Hyderabad |120000
Lucknow |71000
Vishakhapatnam |93000
3 rows selected

以相同的方式，以下查询查找在某个位置作为员工薪水所花在员工身上的平均金额。

ij> SELECT Location, AVG(Salary) from Employees GROUP BY Location;

这将生成以下输出-

LOCATION |2
-----------------------------------------------------
Hyderabad |40000
Lucknow |23666
Vishakhapatnam |31000
3 rows selected

GROUP BY - JDBC示例

以下JDBC示例演示了如何使用 Group By 子句以及如何使用JDBC程序在Apache Derby中的表上执行CURD操作，在这里，无涯教程使用嵌入式驱动程序连接到名为sampleDB的数据库(如果不存在则创建)。

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
public class GroupByClauseExample {
   public static void main(String args[]) throws Exception {
      //注册驱动
      Class.forName("org.apache.derby.jdbc.EmbeddedDriver");

      //创建连接
      String URL="jdbc:derby:sampleDB;create=true";
      Connection conn=DriverManager.getConnection(URL);

      //获取Statement对象
      Statement stmt=conn.createStatement();

      //创建表结构,插入一些测试数据
      stmt.execute("CREATE TABLE EmployeesData( "
         + "Id INT NOT NULL GENERATED ALWAYS AS IDENTITY, "
         + "Name VARCHAR(255), "
         + "Salary INT NOT NULL, "
         + "Location VARCHAR(255), "
         + "PRIMARY KEY (Id))");
      stmt.execute("INSERT INTO EmployeesData(Name, Salary, Location) "
         + "VALUES ('Amit', 30000, 'Hyderabad'), "
         + "('Rahul', 39000, 'Lucknow'), "
         + "('Renuka', 50000, 'Hyderabad'), "
         + "('Learnfk', 15000, 'Vishakhapatnam'), "
         + "('Kalyan', 40000, 'Hyderabad'), "
         + "('Trupthi', 45000, 'Vishakhapatnam'), "
         + "('Raghav', 12000, 'Lucknow'), "
         + "('Suchatra', 33000, 'Vishakhapatnam'), "
         + "('Rizwan', 20000, 'Lucknow')");

      //执行SQL语句
      String query="SELECT Location, SUM(Salary) from EmployeesData GROUP BY Location";
      ResultSet rs=stmt.executeQuery(query);
      while(rs.next()) {
         System.out.println("Location: "+rs.getString(1));
         System.out.println("Sum of salary: "+rs.getString(2));
         System.out.println(" ");
      }
   }
}

在执行上述程序时，您将获得以下输出-

Location: Hyderabad
Sum of salary: 120000

Location: Lucknow
Sum of salary: 71000

Location: Vishakhapatnam
Sum of salary: 93000 

参考链接

https://www.learnfk.com/derby/apache-derby-group-by-clause.html