本章介绍了SELECT语句的GROUP BY子句。GROUP BY子句用于分类所有记录结果的特定集合列。它被用来查询一组记录。

语法

GROUP BY子句的语法如下：

SELECT [ALL | DISTINCT] select_expr, select_expr, ... 
FROM table_reference 
[WHERE where_condition] 
[GROUP BY col_list] 
[HAVING having_condition] 
[ORDER BY col_list]] 
[LIMIT number];

示例

让我们以SELECT... GROUP BY子句为例。假设员工表有如下Id, Name, Salary, Designation, 和 Dept字段。产生一个查询以检索每个部门的员工数量。

+------+--------------+-------------+-------------------+--------+ 
| ID   | Name         | Salary      | Designation       | Dept   |
+------+--------------+-------------+-------------------+--------+ 
|1201  | Gopal        | 45000       | Technical manager | TP     | 
|1202  | Manisha      | 45000       | Proofreader       | PR     | 
|1203  | Masthanvali  | 40000       | Technical writer  | TP     | 
|1204  | Krian        | 45000       | Proofreader       | PR     | 
|1205  | Kranthi      | 30000       | Op Admin          | Admin  |
+------+--------------+-------------+-------------------+--------+

下面使用上述业务情景查询检索员工的详细信息。

hive> SELECT Dept,count(*) FROM employee GROUP BY DEPT;

成功执行查询后，能看到以下回应：

+------+--------------+ 
| Dept | Count(*)     | 
+------+--------------+ 
|Admin |    1         | 
|PR    |    2         | 
|TP    |    3         | 
+------+--------------+

JDBC 程序

下面给出的是JDBC程序应用对给定的GROUP BY子句例子。

import java.sql.SQLException;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.Statement;
import java.sql.DriverManager;

public class HiveQLGroupBy {
   private static String driverName = "org.apache.hadoop.hive.jdbc.HiveDriver";
   
   public static void main(String[] args) throws SQLException {
   
      // Register driver and create driver instance
      Class.forName(driverName);
      
      // get connection
      Connection con = DriverManager.
      getConnection("jdbc:hive://localhost:10000/userdb", "", "");
      
      // create statement
      Statement stmt = con.createStatement();
      
      // execute statement
      Resultset res = stmt.executeQuery(“SELECT Dept,count(*) ” + “FROM employee GROUP BY DEPT; ”);
      System.out.println(" Dept \t count(*)");
      
      while (res.next()) {
         System.out.println(res.getString(1) + " " + res.getInt(2)); 
      }
      con.close();
   }
}

保存程序在一个名为HiveQLGroupBy.java文件。使用下面的命令来编译并执行这个程序。

$ javac HiveQLGroupBy.java
$ java HiveQLGroupBy

输出：

 Dept     Count(*)
 Admin       1
 PR          2
 TP          3

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