- 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。
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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';
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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;
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
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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;
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;
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:
- AVG() 函数返回数值列的平均值
- MAX() 函数返回指定列的最大值
- MIN() 函数返回指定列的最小值
- LEN() 函数返回文本字段中值的长度NOW() 函数返回当前系统的日期和时间 示例:SELECT name, url, Now() AS date from xxx