HiveQL Select Group By

本章介紹了SELECT語句的GROUP BY子句。GROUP BY子句用於分類所有記錄結果的特定集合列。它被用來查詢一組記錄。

語法

GROUP BY子句的語法如下：

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

讓我們以SELECT... GROUP BY子句為例。假設員工表有如下Id, Name, Salary, Designation, 和 Dept字段。產生一個查詢以檢索每個部門的員工數量。

+------+--------------+-------------+-------------------+--------+
| 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  |
+------+--------------+-------------+-------------------+--------+

下麵使用上述業務情景查詢檢索員工的詳細資訊。

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

成功執行查詢後，能看到以下回應：

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

JDBC 程式

下麵給出的是JDBC程式應用對給定的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();
   }
}

保存程式在一個名為HiveQLGroupBy.java檔。使用下麵的命令來編譯並執行這個程式。

$ javac HiveQLGroupBy.java
$ java HiveQLGroupBy

輸出：

 Dept     Count(*)
 Admin       1
 PR          2
 TP          3