无涯教程-Derby - Having语句

HAVING子句使您可以指定条件，以过滤出哪些组输出出现在输出中。

WHERE子句将条件放置在所选列上，而HAVING子句将条件放置在GROUP BY子句创建的组上。

Having - 语法

以下是HAVING子句的语法-

ij> SELECT column1, column2 . . . from table_name GROUP BY column having
condition;

Having - 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13条记录，如下所示-

ID |NAME |SALARY |LOCATION
------------------------------------------------------------------
1 |Amit |30000 |Hyderabad
2 |Rahul |39000 |Lucknow
3 |Kalyan |40000 |Vishakhapatnam
4 |Renuka |50000 |Hyderabad
5 |Learnfk |15000 |Vishakhapatnam
6 |Krishna |40000 |Hyderabad
7 |Trupthi |45000 |Vishakhapatnam
8 |Raghav |12000 |Lucknow
9 |Radha |50000 |Delhi
10 |Anirudh |15000 |Mumbai
11 |Tara |45000 |Kochin
12 |Sucharita |44000 |Kochin
13 |Rizwan |20000 |Lucknow 

以下查询显示该位置至少有3名员工的员工的最高薪水-

ij> SELECT Location, MAX(Salary) from Employees GROUP BY Location having
count(Location)>=3;

这将产生以下输出-

LOCATION |2
------------------------------------------------------------
Hyderabad |50000
Lucknow |39000
Vishakhapatnam |45000
3 rows selected

Having - 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 HavingClauseExample {
   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, MAX(Salary) "
         + "from EmployeesData GROUP BY Location having "
         + "count(Location)>=3";
      ResultSet rs=stmt.executeQuery(query);
      while(rs.next()) {
         System.out.println(rs.getString(1));
         System.out.println(rs.getString(2));
         System.out.println(" ");
      }
   }
}

在执行上述程序时，您将获得以下输出-

Hyderabad
50000

Lucknow
39000

Vishakhapatnam
45000

参考链接

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