[SQL ] 請教SQL達人 (有關GROUP BY)
假設我有一個用來儲存訪客記錄的資料表:
+----+------+------------+
| id | name | visit_date |
+----+------+------------+
| 1 | Tom | 2007-12-01 |
| 2 | Mary | 2007-12-01 |
| 3 | John | 2007-12-10 |
| 4 | Sean | 2007-12-10 |
| 5 | Tim | 2007-12-10 |
| 6 | Ben | 2007-12-20 |
| 7 | Tom | 2008-01-02 |
| 8 | Jack | 2008-01-02 |
+----+------+------------+
若我要統計「每日訪客量」, 我可以用以下 SQL:
SELECT visit_date, COUNT(*) AS visits FROM table GROUP BY visit_date
即可產生如下結果:
+------------+--------+
| visit_date | visits |
+------------+--------+
| 2007-12-01 | 2 |
| 2007-12-10 | 3 |
| 2007-12-20 | 1 |
| 2008-01-02 | 2 |
+------------+--------|
我想請問那「每週」,「每月」, 或「每年」訪客量的 SQL 又該如下呢?
先謝謝了!
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