MySQL Group By 子句

可以使用 GROUP BY 子句對列中的值進(jìn)行分組。如果你愿意，還可以對列實施某種計算?？梢詫Ψ纸M的列使用COUNT、SUM以及AVG等函數(shù)。

為了理解 GROUP BY 子句，考慮表 employee_tbl，它包含如下記錄：

mysql> SELECT * FROM employee_tbl;
+------+------+------------+--------------------+
| id   | name | work_date  | daily_typing_pages |
+------+------+------------+--------------------+
|    1 | John | 2007-01-24 |                250 |
|    2 | Ram  | 2007-05-27 |                220 |
|    3 | Jack | 2007-05-06 |                170 |
|    3 | Jack | 2007-04-06 |                100 |
|    4 | Jill | 2007-04-06 |                220 |
|    5 | Zara | 2007-06-06 |                300 |
|    5 | Zara | 2007-02-06 |                350 |
+------+------+------------+--------------------+
7 rows in set (0.00 sec)

現(xiàn)在，根據(jù)上面這張表，我們來計算一下每位員工的工作天數(shù)。

如果像下面這樣寫 SQL 查詢，那么將得到下列結(jié)果：

mysql> SELECT COUNT(*) FROM employee_tbl;
+---------------------------+
| COUNT(*)                  |
+---------------------------+
| 7                         |
+---------------------------+

但這并不符合預(yù)期，我們希望的是輸出每位員工輸入的頁數(shù)?？梢越Y(jié)合使用聚合函數(shù)與 GROUP BY 子句，如下所示：

mysql> SELECT name, COUNT(*)
    -> FROM   employee_tbl 
    -> GROUP BY name;
+------+----------+
| name | COUNT(*) |
+------+----------+
| Jack |        2 |
| Jill |        1 |
| John |        1 |
| Ram  |        1 |
| Zara |        2 |
+------+----------+
5 rows in set (0.04 sec)

以后，我們還將介紹更多 GROUP BY 與 SUM、AVG 等函數(shù)相結(jié)合的用法。