MySQL JOIN
MySQL 中联合查询可以根据多个表中列的关系进行查询，查询效率较高
students 表：
+--------+------+
| stu_no | name |
+--------+------+
| 1 | Tom |
| 2 | Jack |
+--------+------+
class 表：
+---------+--------+
| class | stu_no |
+---------+--------+
| Math | 1 |
| History | 3 |
+---------+--------+
INNER JOIN (内连接)
INNER JOIN 等于 JOIN
当左右表都存在匹配时返回行
SELECT * FROM students INNER JOIN class on students.stu_no = class.stu_no;
SELECT * FROM students JOIN class on students.stu_no = class.stu_no;
执行结果：
+--------+------+-------+--------+
| stu_no | name | class | stu_no |
+--------+------+-------+--------+
| 1 | Tom | Math | 1 |
+--------+------+-------+--------+
LEFT OUTER JOIN (左连接)
LEFT OUTER JOIN 等于 LEFT JOIN
返回左表所有行，如果右表没有匹配时，相应数据为 NULL
SELECT * FROM students LEFT OUTER JOIN class on students.stu_no = class.stu_no;
SELECT * FROM students LEFT JOIN class on students.stu_no = class.stu_no;
执行结果：
+--------+------+-------+--------+
| stu_no | name | class | stu_no |
+--------+------+-------+--------+
| 1 | Tom | Math | 1 |
+--------+------+-------+--------+
| 2 | Jack | NULL | NULL |
+--------+------+-------+--------+
RIGHT OUTER JOIN (右链接)
RIGHT OUTER JOIN 等于 RIGHT JOIN
返回右表所有行，如果左表没有匹配时，相应数据为 NULL
SELECT * FROM students RIGHT OUTER JOIN class on students.stu_no = class.stu_no;
SELECT * FROM students RIGHT JOIN class on students.stu_no = class.stu_no;
执行结果：
+--------+------+---------+--------+
| stu_no | name | class | stu_no |
+--------+------+---------+--------+
| 1 | Tom | Math | 1 |
+--------+------+---------+--------+
| NULL | NULL | History | 3 |
+--------+------+---------+--------+
FULL OUTER JOIN
返回左右表所有行，如果另一表没有匹配时，相应数据为 NULL
注意：MySQL 中没有 FULL OUTER JOIN, 需要使用以下代码代替
SELECT * FROM students LEFT OUTER JOIN class on students.stu_no = class.stu_no
UNION
SELECT * FROM students RIGHT OUTER JOIN class on students.stu_no = class.stu_no;
执行结果：
+--------+------+---------+--------+
| stu_no | name | class | stu_no |
+--------+------+---------+--------+
| 1 | Tom | Math | 1 |
+--------+------+---------+--------+
| 2 | Jack | NULL | NULL |
+--------+------+---------+--------+
| NULL | NULL | History | 3 |
+--------+------+---------+--------+