新手村之SQL——函数&多表联结

1.将数值四舍五入——ROUND
ROUND( X )：返回参数 X 四舍五入后的一个整数。
ROUND(X, D)： 返回参数 X 四舍五入且保留 D 位小数后的一个数字。如果 D 为 0，结果将没有小数点或小数部分。

2.判断字段是否为NULL——ISNULL
是否为 NULL 值返回 0 或 1。

SELECT ISNULL(`column_name`)
FROM `table_name`;

IFNULL() 函数也用于判断字段是否为NULL，但是与 ISNULL() 不同的是它接收两个参数，第一个参数 column_name 为列名，第二个参数 value 相当于备用值。

SELECT IFNULL(`column_name`, `value`)
FROM `table_name`;

3.获取当前时间——NOW() 、 CURDATE()、CURTIME()
NOW() 可以用来返回当前日期和时间 格式：YYYY-MM-DD hh:mm:ss
CURDATE() 可以用来返回当前日期 格式：YYYY-MM-DD
CURTIME() 可以用来返回当前时间 格式：hh:mm:ss

在使用 NOW() 和 CURTIME() 时，如果要精确的秒以后的时间的话，可以在（）中加数字，加多少，就表示精确到秒后多少位。
比如 NOW(3) 就是精确到毫秒，表示为： 2021-03-31 15:27:20.645

4.提取日期和时间——DATE()、TIME()

SELECT DATE('2021-03-25 16:16:30') AS `date`,TIME('2021-03-25 16:16:30')  AS `time`;
+------------+----------+
| date       | time     |
+------------+----------+
| 2021-03-25 | 16:16:30 |
+------------+----------+
1 row in set

mysql> SELECT `name`, `created_at`, 
    -> DATE_FORMAT(DATE(`created_at`),"%Y-%m-%d") AS `created_date`,
    -> DATE_FORMAT(TIME(`created_at`),"%H:%i:%s") AS `created_time`
    -> FROM `courses`;
+-------------------------+---------------------+--------------+--------------+
| name                    | created_at          | created_date | created_time |
+-------------------------+---------------------+--------------+--------------+
| Advanced Algorithms     | 2020-06-01 09:10:12 | 2020-06-01   | 09:10:12     |
| System Design           | 2020-07-18 10:11:12 | 2020-07-18   | 10:11:12     |
| Django                  | 2020-02-29 12:10:12 | 2020-02-29   | 12:10:12     |
| Web                     | 2020-04-22 13:01:12 | 2020-04-22   | 13:01:12     |
| Big Data                | 2020-09-11 16:01:12 | 2020-09-11   | 16:01:12     |
| Artificial Intelligence | 2018-05-13 18:12:30 | 2018-05-13   | 18:12:30     |
| Java P6+                | 2019-01-19 13:31:12 | 2019-01-19   | 13:31:12     |
| Data Analysis           | 2019-07-12 13:01:12 | 2019-07-12   | 13:01:12     |
| Object Oriented Design  | 2020-08-08 13:01:12 | 2020-08-08   | 13:01:12     |
| Dynamic Programming     | 2018-08-18 20:01:12 | 2018-08-18   | 20:01:12     |
+-------------------------+---------------------+--------------+--------------+
10 rows in set

5.提取指定的时间信息——EXTRACT
EXTRACT() 函数用于返回日期/时间的单独部分，如 YEAR (年)、MONTH (月)、DAY (日)、HOUR (小时)、MINUTE (分钟)、 SECOND (秒)。

mysql> SELECT `name`, EXTRACT(HOUR FROM `created_at`) AS `created_hour`
FROM `courses`;
+-------------------------+--------------+
| name                    | created_hour |
+-------------------------+--------------+
| Advanced Algorithms     | 9            |
| System Design           | 10           |
| Django                  | 12           |
| Web                     | 13           |
| Big Data                | 16           |
| Artificial Intelligence | 18           |
| Java P6+                | 13           |
| Data Analysis           | 13           |
| Object Oriented Design  | 13           |
| Dynamic Programming     | 20           |
+-------------------------+--------------+
10 row in set

6.格式化输出日期——DATE_FORMAT()

mysql> SELECT DATE_FORMAT(`created_at`, '%Y %m') AS `DATE_FORMAT`
    -> FROM `courses`;
+-------------+
| DATE_FORMAT |
+-------------+
| 2020 06     |
| 2020 07     |
| 2020 02     |
| 2020 04     |
| 2020 09     |
| 2018 05     |
| 2019 01     |
| 2019 07     |
| 2020 08     |
| 2018 08     |
+-------------+
10 rows in set (0.01 sec)

7.增加和减少时间——DATE_ADD&DATE_SUB

select name,date_add(created_at,interval 1 day) as new_created from courses
推迟一天

8.时间差——TIMESTAMPDIFF

select TIMESTAMPDIFF(month,created_at,'2020-04-22') as 'MonthDiff' from courses

8.JOIN 连接子句
INNER JOIN：如果表中有至少一个匹配，则返回行
LEFT JOIN：即使右表中没有匹配，也从左表返回所有的行
RIGHT JOIN：即使左表中没有匹配，也从右表返回所有的行
FULL JOIN：只要其中一个表中存在匹配，则返回行
CROSS JOIN：又称笛卡尔积，两个表数据一一对应，返回结果的行数等于两个表行数的乘积

INNER JOIN：
标准语法：

SELECT `table1`.`column1`, `table2`.`column2`...
FROM `table1`
INNER JOIN `table2`
ON `table1`.`common_field` = `table2`.`common_field`;

示例：

SELECT `c`.`id`, `c`.`name` AS `course_name`, `t`.`name` AS `teacher_name`
FROM `courses` `c`
INNER JOIN `teachers` `t` ON `c`.`teacher_id` = `t`.`id`;

LEFT JOIN：
以关键字 LEFT JOIN 左边的表为参考表。左外连接的结果包括 LEFT OUTER 子句中指定的左表的所有行，而不仅仅是连接列所匹配的行，这就意味着，左连接会返回左表中的所有记录，加上右表中匹配到的记录。如果左表的某行在右表中没有匹配行，那么在相关联的结果行中，右表的所有选择列表均为空值。

mysql-> SELECT c.name AS course_name, t.name AS teacher_name
     -> FROM teachers t
          LEFT JOIN courses c ON c.teacher_id = t.id;
+------------------------+--------------------+
|  course_name           | teacher_name       |
+------------------------+--------------------+
| Big Data	         |Eastern Heretic     |
| Data Analysis          |Eastern Heretic     |
| Dynamic Programming	 |Eastern Heretic     | 
| NULL	                 |Northern Beggar     |
| System Design          |Western Venom       |
| Django	         |Western Venom       |
| Artificial Intelligence|Western Venom       |
| Java P6+	         |Western Venom       |
| Senior Algorithm	 |Southern Emperor    |
| Web	                 |Southern Emperor    |
| Object Oriented Design |Southern Emperor    |
| NULL	                 |Linghu Chong        |
+------------------------+--------------------+
12 rows in set (0.01 sec)

RIGHT JOIN:
以关键字 RIGHT JOIN 右边的表为参考表，如果右表的某行在左表中没有匹配行，左表就返回空值。

mysql-> SELECT c.name AS course_name, t.name AS teacher_name, t.email AS teacher_email
     -> FROM courses c
	     RIGHT JOIN teachers t ON c.teacher_id = t.id;
+------------------------+----------------------+---------------------------+
|  course_name           | teacher_name         | teacher_email             |
+------------------------+----------------------+---------------------------+
| Dynamic Programming	 | Eastern Heretic	| eastern.heretic@gmail.com |
| Data Analysis   	 | Eastern Heretic	| eastern.heretic@gmail.com |
| Big Data   	         | Eastern Heretic	| eastern.heretic@gmail.com |
| Dynamic Programming    | Northern Beggar	| northern.beggar@qq.com    |
| Java P6+	         | Western Venom	| western.venom@163.com     |
| Artificial Intelligence| Western Venom	| western.venom@163.com     |
| Django	         | Western Venom	| western.venom@163.com     |
| System Design	         | Western Venom	| western.venom@163.com     |
| Object Oriented Design | Southern Emperor	| southern.emperor@qq.com   |
| Web	                 | Southern Emperor	| southern.emperor@qq.com   |
| Advanced Algorithms	 | Southern Emperor	| southern.emperor@qq.com   |
| NULL	                 | Linghu Chong	        | NULL                      |
+------------------------+----------------------+---------------------------+
12 rows in set (0.01 sec)

FULL (OUTER) JOIN:
只要左表（table1）和右表（table2）其中一个表中存在匹配，则返回行。FULL OUTER JOIN 关键字结合了 LEFT JOIN 和 RIGHT JOIN 的结果。

MySQL 数据库不支持全连接，想要实现全连接可以使用 UNION ALL 来将左连接和右连接结果组合在一起实现全连接。

mysql-> SELECT c.name AS course_name, t.age AS teacher_age
     -> FROM courses c
	    LEFT JOIN teachers t ON c.teacher_id = t.id
     -> UNION
     -> SELECT c.name AS course_name, t.age AS teacher_age
     -> FROM courses c
	    RIGHT JOIN teachers t ON c.teacher_id = t.id;
+------------------------+---------------+
|  course_name           | teacher_age   | 
+------------------------+---------------+
| Advanced Algorithms	 | 21            | 
| System Design	         | 28            | 
| Django                 | 28            | 
| Web Southern	         | 21            |  
| Big Data	         | 20            |  
| Artificial Intelligence| 28            | 
| Java P6+	         | 28            |  
| Data Analysis Eastern	 | 20            |  
| Object Oriented Design | 21            |  
| Dynamic Programming	 | 20            |  
| Linghu Chong 	         | 18            | 
| NULL	                 | 21            |  
| NULL	                 | 18            |  
+------------------------+----------------------+---------------------------+
13 rows in set (0.01 sec)