COUNT函數(shù)是 SQL 中最簡單的函數(shù)了,對于統(tǒng)計由 SELECT 語句返回的記錄非常有用。
要理解 COUNT 函數(shù),請考慮 employee_tbl 表,表中的記錄如下所示:
SQL> 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,假設(shè)你想要統(tǒng)計上表中記錄的總數(shù),那么可以依如下所示步驟達(dá)到目的:
SQL>SELECT COUNT(*) FROM employee_tbl ;
+----------+
| COUNT(*) |
+----------+
| 7 |
+----------+
1 row in set (0.01 sec)
類似地,如果你想要統(tǒng)計 Zara 的數(shù)目,就可以像下面這樣:
SQL>SELECT COUNT(*) FROM employee_tbl
-> WHERE name="Zara";
+----------+
| COUNT(*) |
+----------+
| 2 |
+----------+
1 row in set (0.04 sec)
注意:所有的 SQL 查詢都是不區(qū)分大小寫的,因此在 WHERE 子句的條件中,ZARA 和 Zara 是沒有任何區(qū)別的。