SQL zero-based introductory learning (16)

SQL zero-based introductory learning (fifteen)

SQL function

SQL MAX() function

The MAX() function returns the maximum value of the specified column.

SQL MAX() Syntax

SELECT MAX(column_name) FROM table_name;

demo database

In this tutorial, we will use the RUNOOB sample database.

Here is the data from the "Websites" table:

+----+--------------+---------------------------+-------+---------+
| 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/    | 5000  | CN      |
| 4  | 微博          | http://weibo.com/         | 20    | CN      |
| 5  | Facebook     | https://www.facebook.com/ | 3     | USA     |
|  6 | 百度         | https://www.baidu.com/    |     4 | CN      |
|  7 | stackoverflow | http://stackoverflow.com/ |     0 | IND     |
+----+---------------+---------------------------+-------+---------+

SQL MAX() instance

The following SQL statement gets the maximum value from the "alexa" column of the "Websites" table:

SELECT MAX(alexa) AS max_alexa FROM Websites;

The result of executing the above SQL is as follows:
insert image description here

MIN() function

The MIN() function returns the minimum value of the specified column.

####SQL MIN() Syntax

SELECT MIN(column_name) FROM table_name;

demo database

In this tutorial, we will use the RUNOOB sample database.

Here is the data from the "Websites" table:

+----+--------------+---------------------------+-------+---------+
| 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     |
|  6 | 百度         | https://www.baidu.com/    |     4 | CN      |
|  7 | stackoverflow | http://stackoverflow.com/ |     0 | IND     |
+----+---------------+---------------------------+-------+---------+

SQL MIN() instance

The following SQL statement gets the minimum value from the "alexa" column of the "Websites" table:

SELECT MIN(alexa) AS min_alexa FROM Websites;

The result of executing the above SQL is as follows:
insert image description here

SUM() function

The SUM() function returns the total number of numeric columns.

SQL SUM() syntax

SELECT SUM(column_name) FROM table_name;

demo database

In this tutorial, we will use the RUNOOB sample database.

Here is the data selected from the "access_log" table:

mysql> SELECT * FROM access_log;
+-----+---------+-------+------------+
| 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 |
+-----+---------+-------+------------+
9 rows in set (0.00 sec)
```
#### SQL SUM() 实例
下面的 SQL 语句查找 "access_log" 表的 "count" 字段的总数：

```
SELECT SUM(count) AS nums FROM access_log;
```
执行以上 SQL 输出结果如下：
![在这里插入图片描述](https://img-blog.csdnimg.cn/2d8860aecb514ff5a01c87ee42e2c536.png)
### 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;
```

#### 演示数据库
在本教程中，我们将使用 RUNOOB 样本数据库。

下面是选自 "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     |
+----+---------------+---------------------------+-------+---------+
```
下面是 "access_log" 网站访问记录表的数据：
```
mysql> SELECT * FROM access_log;
+-----+---------+-------+------------+
| 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 |
+-----+---------+-------+------------+
9 rows in set (0.00 sec)
```
#### GROUP BY 简单应用
统计 access_log 各个 site_id 的访问量：
```
SELECT site_id, SUM(access_log.count) AS nums
FROM access_log GROUP BY site_id;
```
执行以上 SQL 输出结果如下：
![在这里插入图片描述](https://img-blog.csdnimg.cn/197fdafdee134f9fad01ac28246a4637.png)
#### SQL GROUP BY 多表连接
下面的 SQL 语句统计有记录的网站的记录数量：
```
SELECT Websites.name,COUNT(access_log.aid) AS nums FROM access_log
LEFT JOIN Websites
ON access_log.site_id=Websites.id
GROUP BY Websites.name;
```
执行以上 SQL 输出结果如下：
![在这里插入图片描述](https://img-blog.csdnimg.cn/a40a2a53bfe04624a520cda69acff652.png)
### 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 语法
```
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value;
```

#### 演示数据库
在本教程中，我们将使用 RUNOOB 样本数据库。

下面是选自 "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     |
+----+---------------+---------------------------+-------+---------+
```
下面是 "access_log" 网站访问记录表的数据：
```
mysql> SELECT * FROM access_log;
+-----+---------+-------+------------+
| 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 |
+-----+---------+-------+------------+
9 rows in set (0.00 sec)
```

#### SQL HAVING 实例
现在我们想要查找总访问量大于 200 的网站。

我们使用下面的 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：
![在这里插入图片描述](https://img-blog.csdnimg.cn/ccf5ec0cce93401fa209295517cad3b4.png)
现在我们想要查找总访问量大于 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：
![在这里插入图片描述](https://img-blog.csdnimg.cn/fca5baba77fa4f2e86bb129592b714ff.png)