SQL中 JOIN 的两种连接类型:内连接(自然连接、自连接、交叉连接)、外连接(左外连接、右外连接、全外连接)

SQL中 JOIN 的两种连接类型:内连接(自然连接、自连接、交叉连接)、外连接(左外连接、右外连接、全外连接)

1. 自然连接(natural join)(内连接)

学生表

mysql> select * from student;
+----+--------+----------+
| id | name   | code     |
+----+--------+----------+
|  1 | 张三   | 20181601 |
|  2 | 尔四   | 20181602 |
|  3 | 小红   | 20181603 |
|  4 | 小明   | 20181604 |
|  5 | 小青   | 20181605 |
+----+--------+----------+
5 rows in set (0.00 sec)

CREATE TABLE `student` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(50) COLLATE utf8mb4_general_ci DEFAULT NULL,
  `code` int DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;

INSERT INTO student (name, code) VALUES
    ('张三', 20181601),
    ('尔四', 20181602),
    ('小红', 20181603),
    ('小明', 20181604),
    ('小青', 20181605);

成绩表

mysql> select * from score;
+----+-------+----------+
| id | grade | code     |
+----+-------+----------+
|  1 |    55 | 20181601 |
|  2 |    88 | 20181602 |
|  3 |    99 | 20181605 |
|  4 |    33 | 20181611 |
+----+-------+----------+
4 rows in set (0.00 sec)

CREATE TABLE `score` (
  `id` int NOT NULL AUTO_INCREMENT,
  `grade` int DEFAULT NULL,
  `code` int DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;

INSERT INTO score (id, grade, code) VALUES
(1, 55, 20181601),
(2, 88, 20181602),
(3, 99, 20181605),
(4, 33, 20181611);

自然连接不用指定连接列,也不能使用ON语句,它默认比较两张表里相同的列。

SELECT * FROM student NATURAL JOIN score;

显示结果如下:

mysql> select * from student natural join score;
+----+----------+--------+-------+
| id | code     | name   | grade |
+----+----------+--------+-------+
|  1 | 20181601 | 张三   |    55 |
|  2 | 20181602 | 尔四   |    88 |
+----+----------+--------+-------+
2 rows in set (0.00 sec)

–自然连接 natural join
自动判断连接条件完成连接.
–自然内连接 natural inner join
select *|字段列表 from 左表 natural [inner] join 右表;
自然内连接其实就是内连接,这里的匹配条件是由系统自动指定.

–自然外连接 natural outer join
自然外连接分为自然左外连接和自然右外连接.匹配条件也是由系统自动指定.

–自然左外连接 natural left join
select *|字段列表 from 左表 natural left [outer] join 右表;

–自然右外连接 natural right join
select *|字段列表 from 右表 natural right [outer] join 左表;

2. 内连接(inner join)

和自然连接区别之处在于内连接可以自定义两张表的不同列字段。
内连接有两种形式:显式和隐式。

1)隐式的内连接,没有INNER JOIN,形成的中间表为两个表的笛卡尔积。

SELECT student.name,score.code FROM student,score WHERE score.code=student.code;

2)显示的内连接,一般称为内连接,有INNER JOIN,形成的中间表为两个表经过ON条件过滤后的笛卡尔积。

SELECT student.name,score.code FROM student INNER JOIN score ON score.code=student.code;

例:以下1)、2)语句执行结果相同。

mysql> SELECT student.name,score.code FROM student,score WHERE score.code=student.code;
mysql> SELECT student.name,score.code FROM student INNER JOIN score ON score.code=student.code;
+--------+----------+
| name   | code     |
+--------+----------+
| 张三   | 20181601 |
| 尔四   | 20181602 |
| 小青   | 20181605 |
+--------+----------+
3 rows in set (0.00 sec)



mysql> select student.*,score.* from student inner join score;
mysql> select student.*,score.* from student inner join score;
+----+--------+----------+----+-------+----------+
| id | name   | code     | id | grade | code     |
+----+--------+----------+----+-------+----------+
|  1 | 张三   | 20181601 |  4 |    33 | 20181611 |
|  1 | 张三   | 20181601 |  3 |    99 | 20181605 |
|  1 | 张三   | 20181601 |  2 |    88 | 20181602 |
|  1 | 张三   | 20181601 |  1 |    55 | 20181601 |
|  2 | 尔四   | 20181602 |  4 |    33 | 20181611 |
|  2 | 尔四   | 20181602 |  3 |    99 | 20181605 |
|  2 | 尔四   | 20181602 |  2 |    88 | 20181602 |
|  2 | 尔四   | 20181602 |  1 |    55 | 20181601 |
|  3 | 小红   | 20181603 |  4 |    33 | 20181611 |
|  3 | 小红   | 20181603 |  3 |    99 | 20181605 |
|  3 | 小红   | 20181603 |  2 |    88 | 20181602 |
|  3 | 小红   | 20181603 |  1 |    55 | 20181601 |
|  4 | 小明   | 20181604 |  4 |    33 | 20181611 |
|  4 | 小明   | 20181604 |  3 |    99 | 20181605 |
|  4 | 小明   | 20181604 |  2 |    88 | 20181602 |
|  4 | 小明   | 20181604 |  1 |    55 | 20181601 |
|  5 | 小青   | 20181605 |  4 |    33 | 20181611 |
|  5 | 小青   | 20181605 |  3 |    99 | 20181605 |
|  5 | 小青   | 20181605 |  2 |    88 | 20181602 |
|  5 | 小青   | 20181605 |  1 |    55 | 20181601 |
+----+--------+----------+----+-------+----------+
20 rows in set (0.00 sec)

拓展

自连接(内连接)

https://baike.baidu.com/item/%E8%87%AA%E8%BF%9E%E6%8E%A5/2556770
新的学生表

CREATE TABLE `new_student` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL,
  `code` int DEFAULT NULL,
  `grade` int DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;

INSERT INTO `test`.`new_student`(`id`, `name`, `code`, `grade`) VALUES (1, '张三', 20181601, 55);
INSERT INTO `test`.`new_student`(`id`, `name`, `code`, `grade`) VALUES (2, '尔四', 20181602, 88);
INSERT INTO `test`.`new_student`(`id`, `name`, `code`, `grade`) VALUES (3, '小红', 20181603, 77);
INSERT INTO `test`.`new_student`(`id`, `name`, `code`, `grade`) VALUES (4, '小明', 20181604, 66);
INSERT INTO `test`.`new_student`(`id`, `name`, `code`, `grade`) VALUES (5, '小青', 20181605, 99);

问题:查询显示成绩小于小明的学生和成绩?
当表中的某一个字段与这个表中另外字段的相关时,我们可能用到 自连接

mysql> select * from new_student;
+----+--------+----------+-------+
| id | name   | code     | grade |
+----+--------+----------+-------+
|  1 | 张三   | 20181601 |    55 |
|  2 | 尔四   | 20181602 |    88 |
|  3 | 小红   | 20181603 |    77 |
|  4 | 小明   | 20181604 |    66 |
|  5 | 小青   | 20181605 |    99 |
+----+--------+----------+-------+
5 rows in set (0.00 sec)

mysql> select st2.name, st2.grade from new_student st1, new_student st2 where st1.name='小明' and st1.grade < st2.grade;
+--------+-------+
| name   | grade |
+--------+-------+
| 尔四   |    88 |
| 小红   |    77 |
| 小青   |    99 |
+--------+-------+
3 rows in set (0.00 sec)
数据库中自然连接与内连接的区别:

1、自然连接一定是内连接,内连接不一定是自然连接;
2、内连接不把重复的属性除去,自然连接要把重复的属性除去;
3、内连接要求相等的分量,不一定是公共属性,自然连接要求相等的分量必须是公共属性;
4、内连接不把重复的属性除去,自然连接要把重复的属性除去。

3.外连接(outer join)

1)左外连接(left outer join):返回指定左表的全部行+右表对应的行,如果左表中数据在右表中没有与其相匹配的行,则在查询结果集中显示为空值。

例:

SELECT student.name,score.code FROM student LEFT JOIN score ON score.code=student.code; 

查询结果如下:

mysql> select student.name,score.code from student left join score on score.code=student.code;
+--------+----------+
| name   | code     |
+--------+----------+
| 张三   | 20181601 |
| 尔四   | 20181602 |
| 小红   |     NULL |
| 小明   |     NULL |
| 小青   | 20181605 |
+--------+----------+
5 rows in set (0.00 sec)

2)右外连接(right outer join):与左外连接类似,是左外连接的反向连接。

SELECT student.name,score.codeFROM student RIGHT JOIN score ON score.code=student.code;
mysql> select student.name,score.code from student right join score on score.code=student.code;
+--------+----------+
| name   | code     |
+--------+----------+
| 张三   | 20181601 |
| 尔四   | 20181602 |
| 小青   | 20181605 |
| NULL   | 20181611 |
+--------+----------+
4 rows in set (0.00 sec)

3)全外连接(full outer join):把左右两表进行自然连接,左表在右表没有的显示NULL,右表在左表没有的显示NULL。(MYSQL不支持全外连接,适用于Oracle和DB2。)

在MySQL中,可通过求左外连接与右外连接的合集来实现全外连接。
例:

SELECT student.name,score.code FROM student LEFT JOIN score ON
score.code=student.code UNION SELECT student.name,score.code
FROM student RIGHT JOIN score ON score.code=student.code;
mysql> select student.name,score.code from student left join score on score.code=student.code union select student.name,score.code from student right join score on score.code=student.code;
+--------+----------+
| name   | code     |
+--------+----------+
| 张三   | 20181601 |
| 尔四   | 20181602 |
| 小红   |     NULL |
| 小明   |     NULL |
| 小青   | 20181605 |
| NULL   | 20181611 |
+--------+----------+
6 rows in set (0.00 sec)

4.交叉连接(cross join):相当与笛卡尔积,左表和右表组合。 (内连接)

SELECT student.name,score.code FROM student CROSS JOIN score ON score.code=student.code; 
mysql> select student.name,score.code from student cross join score on score.code=student.code;
+--------+----------+
| name   | code     |
+--------+----------+
| 张三   | 20181601 |
| 尔四   | 20181602 |
| 小青   | 20181605 |
+--------+----------+
3 rows in set (0.00 sec)

mysql> select student.*,score.* from student cross join score on score.code=student.code;
+----+--------+----------+----+-------+----------+
| id | name   | code     | id | grade | code     |
+----+--------+----------+----+-------+----------+
|  1 | 张三   | 20181601 |  1 |    55 | 20181601 |
|  2 | 尔四   | 20181602 |  2 |    88 | 20181602 |
|  5 | 小青   | 20181605 |  3 |    99 | 20181605 |
+----+--------+----------+----+-------+----------+
3 rows in set (0.00 sec)

mysql> select student.*,score.* from student cross join score;
+----+--------+----------+----+-------+----------+
| id | name   | code     | id | grade | code     |
+----+--------+----------+----+-------+----------+
|  1 | 张三   | 20181601 |  4 |    33 | 20181611 |
|  1 | 张三   | 20181601 |  3 |    99 | 20181605 |
|  1 | 张三   | 20181601 |  2 |    88 | 20181602 |
|  1 | 张三   | 20181601 |  1 |    55 | 20181601 |
|  2 | 尔四   | 20181602 |  4 |    33 | 20181611 |
|  2 | 尔四   | 20181602 |  3 |    99 | 20181605 |
|  2 | 尔四   | 20181602 |  2 |    88 | 20181602 |
|  2 | 尔四   | 20181602 |  1 |    55 | 20181601 |
|  3 | 小红   | 20181603 |  4 |    33 | 20181611 |
|  3 | 小红   | 20181603 |  3 |    99 | 20181605 |
|  3 | 小红   | 20181603 |  2 |    88 | 20181602 |
|  3 | 小红   | 20181603 |  1 |    55 | 20181601 |
|  4 | 小明   | 20181604 |  4 |    33 | 20181611 |
|  4 | 小明   | 20181604 |  3 |    99 | 20181605 |
|  4 | 小明   | 20181604 |  2 |    88 | 20181602 |
|  4 | 小明   | 20181604 |  1 |    55 | 20181601 |
|  5 | 小青   | 20181605 |  4 |    33 | 20181611 |
|  5 | 小青   | 20181605 |  3 |    99 | 20181605 |
|  5 | 小青   | 20181605 |  2 |    88 | 20181602 |
|  5 | 小青   | 20181605 |  1 |    55 | 20181601 |
+----+--------+----------+----+-------+----------+
20 rows in set (0.00 sec)

参考链接:

自然连接、内连接、外连接(左外连接、右外连接、全外连接)、交叉连接
百科自连接
数据库中自然连接与内连接的区别
MySQL数据库的46种基本语法
MySQL 自连接讲解

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