SQL INNER JOIN 关键字

返回两个表中存在匹配关系的记录。

 

1. INNER JOIN 语法

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name=table2.column_name;

或:

SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name=table2.column_name;

注释:INNER JOIN 与 JOIN 是相同的。

SQL INNER JOIN

样本数据库

在本教程中,我们将使用 CodeBaoku 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  | 编程宝库      | http://www.codebaoku.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:

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

 

2. INNER JOIN 范例

下面的 SQL 语句将返回所有网站的访问记录:

SELECT Websites.id, Websites.name, access_log.count, access_log.date
FROM Websites
INNER JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count;

执行结果:
+----+------------+-------+------------+
| id | name       | count |   date     |
+----+------------+-------+------------+
| 2  | 淘宝        |  10   | 2016-05-14 |
| 4  | 微博       |  13   | 2016-05-15 |
| 1  | Google     |  45   | 2016-05-10 |
| 3  | 编程宝库    |  100  | 2016-05-13 |
| 3  | 编程宝库    |  201  | 2016-05-17 |
| 5  | Facebook   |  205  | 2016-05-14 |
| 3  | 编程宝库    |  220  | 2016-05-15 |
| 1  | Google     |  230  | 2016-05-14 |
| 5  | Facebook   |  545  | 2016-05-16 |
+----+------------+-------+-----------+

注释:INNER JOIN 关键字在表中存在至少一个匹配时返回行。如果 "Websites" 表中的行在 "access_log" 中没有匹配,则不会列出这些行。

从左表返回所有的行,不管关联的右表中有没有匹配。如果右表中没有匹配的记录。如果右表中没有匹配的记录,那么右表字段的值均补为NULL。SQL LEFT JOIN 语法:SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name=table2.column_name。