COUNT(column_name) 函数返回指定列的值的数目(NULL 不计入):
SELECT COUNT(column_name) FROM table_name; |
COUNT(*) 函数返回表中的记录数:
SELECT COUNT(*) FROM table_name; |
COUNT(DISTINCT column_name) 函数返回指定列的不同值的数目:
SELECT COUNT(DISTINCT column_name) FROM table_name; |
注释:COUNT(DISTINCT) 适用于 ORACLE 和 Microsoft SQL Server,但是无法用于 Microsoft Access。
在本教程中,我们将使用 web3.xin 样本数据库。
下面是选自 "access_log" 表的数据:
+-----+---------+-------+------------+ |
| aid | site_id | count | date | |
+-----+---------+-------+------------+ |
| 1 | 1 | 45 | 2016-05-10 | |
| 2 | 3 | 100 | 2016-05-13 | |
| 3 | 1 | 230 | 2016-05-14 | |
| 4 | 2 | 10 | 2016-05-14 | |
| 5 | 5 | 205 | 2016-05-14 | |
| 6 | 4 | 13 | 2016-05-15 | |
| 7 | 3 | 220 | 2016-05-15 | |
| 8 | 5 | 545 | 2016-05-16 | |
| 9 | 3 | 201 | 2016-05-17 | |
+-----+---------+-------+------------+ |
下面的 SQL 语句计算 "access_log" 表中 "site_id"=3 的总访问量:
SELECT COUNT(count) AS nums FROM access_log |
WHERE site_id=3; |
下面的 SQL 语句计算 "access_log" 表中总记录数:
SELECT COUNT(*) AS nums FROM access_log; |
mysql> select count(*) as nums from access_log; |
+------+ |
| nums | |
+------+ |
| 9 | |
+------+ |
1 row in set (0.00 sec) |
下面的 SQL 语句计算 "access_log" 表中不同 site_id 的记录数:
SELECT COUNT(DISTINCT site_id) AS nums FROM access_log; |
mysql> select count(distinct site_id) as nums from access_log; |
+------+ |
| nums | |
+------+ |
| 5 | |
+------+ |
1 row in set (0.00 sec) |