sql—常用函数

COUNT()函数

COUNT()函数返回匹配指定条件的行数

SQL COUNT(column_name) 语法

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;

示例：select count(brand_name) as num from sec_financial_product_shop_relate where shop_code=000055

SQL COUNT(*) 语法

COUNT(*) 函数返回表中的记录数：

SELECT COUNT(*) FROM table_name;

示例：select count(*) as num from sec_financial_product_shop_relate where shop_code=000055

SQL COUNT(DISTINCT column_name) 语法

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;

示例：select count(distinct brand_name) as num from sec_financial_product_shop_relate where shop_code=000055

注释：COUNT(DISTINCT) 适用于 ORACLE 和 Microsoft SQL Server，但是无法用于 Microsoft Access。

SUM() 函数

SUM() 函数返回数值列的总数。

SQL SUM() 语法

SELECT SUM(column_name) FROM table_name;

示例：SELECT SUM(score) AS nums FROM score_info where subject='语文';

GROUP BY 语句

GROUP BY 语句用于结合聚合函数，根据一个或多个列对结果集进行分组。

SQL GROUP BY 语法

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name;

示例：

统计统计上表各个 user_id 花费的钱

select user_id,sum(price) from `test_table` group by user_id

HAVING 子句

在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与聚合函数一起使用

HAVING 子句可以让我们筛选分组后的各组数据

SQL HAVING 实例

"Websites" 表的数据：

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘宝          | https://www.taobao.com/   | 13    | CN      |
| 3  | 菜鸟教程      | http://www.runoob.com/    | 4689  | CN      |
| 4  | 微博          | http://weibo.com/         | 20    | CN      |
| 5  | Facebook     | https://www.facebook.com/ | 3     | USA     |
| 7  | stackoverflow | http://stackoverflow.com/ |   0 | IND     |
+----+---------------+---------------------------+-------+---------+

"access_log" 网站访问记录表的数据：

+-----+---------+-------+------------+
| 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 |
+-----+---------+-------+------------

示例1：查找总访问量大于 200 的网站。使用下面的 SQL 语句：

SELECT Websites.name, Websites.url, SUM(access_log.count) AS nums FROM (access_log
INNER JOIN Websites
ON access_log.site_id=Websites.id)
GROUP BY Websites.name
HAVING SUM(access_log.count) > 200;

执行以上 SQL 输出结果如下：

示例2：查找总访问量大于 200 的网站，并且 alexa 排名小于 200。

我们在 SQL 语句中增加一个普通的 WHERE 子句：

SELECT Websites.name, SUM(access_log.count) AS nums FROM Websites
INNER JOIN access_log
ON Websites.id=access_log.site_id
WHERE Websites.alexa < 200
GROUP BY Websites.name
HAVING SUM(access_log.count) > 200;

执行以上 SQL 输出结果如下：