最常用也最重要的连接形式是内连接，有时候也被称作“EQUIJOIN”（等值连接）。
内连接根据连接谓词来组合两个表中的字段，以创建一个新的结果表。SQL 查询会比较逐个比较表 1 和表 2 中的每一条记录，来寻找满足连接谓词的所有记录对。当连接谓词得以满足时，所有满足条件的记录对的字段将会结合在一起构成结果表。
3.1.1 INNER JOIN 基本语法
SELECT table1.column1, table2.column2...
FROM table1
INNER JOIN table2
ON table1.common_field = table2.common_field;
或
SELECT table1.column1, table2.column2...
FROM table1
JOIN table2
ON table1.common_field = table2.common_field;
INNER JOIN 中 INNER 可以省略不写
演示数据库
在本教程中，我们将使用 LintCode 样本数据库。
下面是选自 "teachers" 表的数据：
下面是 "courses" 课程表的数据：
3.1.2 INNER JOIN 实例
请注意，"courses" 表中的 "teacher_id" 列指向 "teachers" 表中的字段 "id"。上面这两个表是通过 "teacher_id" 列联系起来的。
然后，如果我们运行下面的 SQL 语句（包含 INNER JOIN）：
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;
执行输出结果
mysql> 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;
+----+-------------------------+------------------+
| id | course_name | teacher_name |
+----+-------------------------+------------------+
| 1 | Senior Algorithm | Southern Emperor |
| 2 | System Design | Western Venom |
| 3 | Django | Western Venom |
| 4 | Web | Southern Emperor |
| 5 | Big Data | Eastern Heretic |
| 6 | Artificial Intelligence | Western Venom |
| 7 | Java P6+ | Western Venom |
| 8 | Data Analysis | Eastern Heretic |
| 10 | Object Oriented Design | Southern Emperor |
| 12 | Dynamic Programming | Eastern Heretic |
+----+-------------------------+------------------+
10 rows in set (0.05 sec)
练习题：INNER JOIN
查询 “Eastern Heretic” 老师所教的所有课程的课程名和课程id。
代码输入框：
SELECT c.id, c.name AS course_name, t.name AS teacher_name
FROM teachers t
INNER JOIN courses c ON c.teacher_id = t.id AND t.name = 'Eastern Heretic';
执行输出结果
+----+---------------------+-----------------+
| id | course_name | teacher_name |
+----+---------------------+-----------------+
| 5 | Big Data | Eastern Heretic |
| 8 | Data Analysis | Eastern Heretic |
| 12 | Dynamic Programming | Eastern Heretic |
+----+---------------------+-----------------+
