如何用 GROUP BY 子句建立一個 MySQL 檢視？

我們可以使用 GROUP BY 對某一列的值分組，而且如果需要的話，我們可以對該列執行計算。你可以在分組的列上使用 COUNT、SUM、AVG 等函式。為了瞭解使用檢視的 GROUP BY 子句，我們要使用具有以下資料的基本表 ‘Student_info’ 建立一個名為 ‘Info’ 的檢視 −

mysql> Select * from Student_info;
+------+---------+------------+------------+
| id   | Name    | Address    | Subject    |
+------+---------+------------+------------+
| 101  | YashPal | Amritsar   | History    |
| 105  | Gaurav  | Chandigarh | Literature |
| 125  | Raman   | Shimla     | Computers  |
| 130  | Ram     | Jhansi     | Computers  |
| 132  | Shyam   | Chandigarh | Economics  |
| 133  | Mohan   | Delhi      | Computers  |
+------+---------+------------+------------+
6 rows in set (0.00 sec)

語法

Create or Replace View view_name AS Select_statements FROM table GROUP BY expression1, expression2, ... expression_n;

示例

mysql> Create or Replace View Info AS select Subject, COUNT(*) FROM Student_info GROUP BY Subject;
Query OK, 0 rows affected (0.10 sec)

mysql> Select * from info;
+------------+----------+
| Subject    | COUNT(*) |
+------------+----------+
| Computers  |    3     |
| Economics  |    1     |
| History    |    1     |
| Literature |    1     |
+------------+----------+
4 rows in set (0.00 sec)

mysql> Create or Replace View Info AS select Subject,Name, COUNT(Subject) FROM Student_info GROUP BY Subject, Name;
Query OK, 0 rows affected (0.05 sec)

上述查詢將在 GROUP BY 子句中有兩列。

mysql> Select * from info;
+------------+---------+----------------+
| Subject    | Name    | COUNT(Subject) |
+------------+---------+----------------+
| Computers  | Mohan   |       1        |
| Computers  | Ram     |       1        |
| Computers  | Raman   |       1        |
| Economics  | Shyam   |       1        |
| History    | YashPal |       1        |
| Literature | Gaurav  |       1        |
+------------+---------+----------------+
6 rows in set (0.00 sec)

上述結果集表明，GROUP BY 子句按照列的值，將一組行分組為一組彙總行。

更新於：30-Jul-2019

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