SQL数据库---INNER/LEFT/RIGHT JOIN的区

演示数据库

• 表：Websites
  +----+--------------+---------------------------+-------+---------+
  | id | name | url | num | country |
  +----+--------------+---------------------------+-------+---------+
  | 1 | Google | https://www.google.cm/ | 1 | USA |
  | 2 | orange | https://www.orange.com/ | 13 | CN |
  | 3 | vivid | http://www.vivid.com/ | 4689 | CN |
  | 4 | 微信 | http://weixin.com/ | 20 | CN |
  | 5 | Facebook | https://www.facebook.com/ | 3 | USA |
  | 7 | douyin | http://douyin.com/ | 0 | IND |
  +----+---------------+---------------------------+-------+---------+
• 表：work_log
  +-----+---------+-------+------------+
  | aid | site_id | count | date |
  +-----+---------+-------+------------+
  | 1 | 1 | 45 | 2020-05-10 |
  | 2 | 3 | 100 | 2020-05-13 |
  | 3 | 1 | 230 | 2020-05-14 |
  | 4 | 2 | 10 | 2020-05-14 |
  | 6 | 4 | 13 | 2020-05-15 |
  | 7 | 3 | 220 | 2020-05-15 |
  | 8 | 5 | 545 | 2020-05-16 |
  | 9 | 10 | 111 | 2020-05-18 |
  +-----+---------+-------+------------+

一、INNER JOIN

• INNER JOIN 关键字在表中存在至少一个匹配时返回行。
  例如：
  SELECT Websites.name, work_log.count, work_log.date
  FROM Websites
  INNER JOIN work_log
  ON Websites.id=work_log.site_id
  ORDER BY work_log.count;

+-----+---------+-------+------------+
| name | count | date |
+-----+---------+-------+------------+
| orange | 10 | 2020-05-13 |
| 微信 | 13 | 2020-05-15 |
| Google | 45 | 2020-05-10 |
| vivid | 100 | 2020-05-13 |
| vivid | 220 | 2020-05-15 |
| Google | 230 | 2020-05-14 |
| Facebook | 545 | 2020-05-16 |
+-----+---------+-------+------------+

二、LEFT JOIN

• LEFT JOIN 关键字从左表（table1）返回所有的行，即使右表（table2）中没有匹配。如果右表中没有匹配，则结果为 NULL。
  例如：
  SELECT Websites.name, work_log.count, work_log.date
  FROM Websites
  LEFT JOIN work_log
  ON Websites.id=work_log.site_id
  ORDER BY work_log.count DESC;

+-----+---------+-------+------------+
| name | count | date |
+-----+---------+-------+------------+
| Facebook | 545 | 2020-05-16 |
| Google | 230 | 2020-05-14 |
| vivid | 220 | 2020-05-15 |
| vivid | 100 | 2020-05-13 |
| Google | 45 | 2020-05-10 |
| 微信 | 13 | 2020-05-15 |
| orange | 10 | 2020-05-13 |

| douyin | NULL | NULL | （右表没有匹配记录也会显示）

+-----+---------+-------+------------+

三、RIGHT JOIN

• RIGHT JOIN 关键字从右表（table2）返回所有的行，即使左表（table1）中没有匹配。如果左表中没有匹配，则结果为 NULL。
  例如：
  SELECT websites.name, work_log.count, work_log.date
  FROM websites
  RIGHT JOIN work_log
  ON work_log.site_id=websites.id
  ORDER BY work_log.count DESC;

+-----+---------+-------+------------+
| name | count | date |
+-----+---------+-------+------------+
| Facebook | 545 | 2020-05-16 |
| Google | 230 | 2020-05-14 |
| vivid | 220 | 2020-05-15 |

| NULL | 111 | 2020-05-18 | （左表没有匹配记录也会显示）

| vivid | 100 | 2020-05-13 |
| Google | 45 | 2020-05-10 |
| 微信 | 13 | 2020-05-15 |
| orange | 10 | 2020-05-13 |
+-----+---------+-------+------------+