【0037】SQL查询--连接查询--内外连接JOIN，LEFT JOIN，RIGHT JOIN，FULL JOIN

1、JOIN（内连接）

inner join可以简写为join，连接分为两种：内连接与外连接。

内连接，即最常见的等值连接，例：

SELECT
student.studentNo,
student.studentName,
student.native,
student.classNo,
class.className
FROM
student
JOIN class ON student.classNo = class.classNo
AND class.institute = '会计学院'

等价于

SELECT
student.studentNo,
student.studentName,
student.native,
student.classNo,
class.className
FROM
student,
class
WHERE
student.classNo = class.classNo
AND class.institute = '会计学院'

SELECT
student.studentNo,
student.studentName
FROM
student
INNER JOIN course ON course.courseName = '计算机原理'
INNER JOIN score ON score.studentNo = student.studentNo AND score.courseNo = course.courseNo

2、LEFT JOIN（左外连接）

左外连接 left outer join 或者 left
join

SELECT
student.studentNo,
student.studentName,
student.native,
student.classNo,
class.className
FROM
student
LEFT JOIN class ON student.classNo = class.classNo
AND class.institute = '会计学院'

注释：LEFT
JOIN 关键字从左表（Student）返回所有的行，即使右表（class）中没有匹配。

3、RIGHT JOIN（右外连接）

右外连接 right outer join 或者 right
join

SELECT
student.studentNo,
student.studentName,
student.native,
student.classNo,
class.className
FROM
student
RIGHT JOIN class ON student.classNo = class.classNo
AND class.institute = '会计学院'

RIGHT JOIN 关键字从右表（Student）返回所有的行，即使左表（class）中没有匹配。

3、FULL JOIN（全外连接）

全外连接 full outer join 或者 full
join

MySQL中不支持
FULL OUTER JOIN