HiveQL Select-Group By

本章解释 SELECT 语句中 GROUP BY 子句的细节。

GROUP BY 子句用于使用特定的集合列对结果集中的所有记录进行分组。

GROUP BY 子句用于查询一组记录。

语法

GROUP BY子句的语法如下所示：

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];

例

让我们举一个SELECT ... GROUP BY子句的例子。假设雇员表如下所示，其中包括Id，Name，Salary，Designation和Dept字段。生成一个查询来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  |
+------+--------------+-------------+-------------------+--------+

以下查询使用上述方案检索员工详细信息。

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

成功执行查询后，您将看到以下响应：

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

JDBC程序

下面给出的是为给定示例应用Group By子句的JDBC程序。

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();
   }
}

将程序保存在名为HiveQLGroupBy.java的文件中。使用以下命令来编译和执行该程序。

$ javac HiveQLGroupBy.java
$ java HiveQLGroupBy

输出：

Dept     Count(*)
Admin       1
PR          2
TP          3