MySQL学习(四) SQL连接查询

更多情况下,我们查询的数据来源于多张表,所有有必要了解一下MySQL中的连接查询。
SQL中将连接查询分成四类:交叉连接,内连接,外连接和自然连接。
数据准备

student表

-- ----------------------------
-- Table structure for `student`
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(10) DEFAULT NULL,
  `age` tinyint(4) DEFAULT NULL,
  `classId` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', 's1', '20', '1');
INSERT INTO `student` VALUES ('2', 's2', '22', '1');
INSERT INTO `student` VALUES ('3', 's3', '22', '2');
INSERT INTO `student` VALUES ('4', 's4', '22', null);

class表

-- ----------------------------
-- Table structure for `class`
-- ----------------------------
DROP TABLE IF EXISTS `class`;
CREATE TABLE `class` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `cname` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of class
-- ----------------------------
INSERT INTO `class` VALUES ('1', '一班');
INSERT INTO `class` VALUES ('2', '二班');
INSERT INTO `class` VALUES ('3', '');

score表

-- ----------------------------
-- Table structure for `score`
-- ----------------------------
DROP TABLE IF EXISTS `score`;
CREATE TABLE `score` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL,
  `score` decimal(4,1) DEFAULT NULL,
  `studentId` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of score
-- ----------------------------
INSERT INTO `score` VALUES ('1', '语文', '90.0', '1');
INSERT INTO `score` VALUES ('2', '数学', '95.0', '1');
INSERT INTO `score` VALUES ('3', '语文', '92.0', '2');
INSERT INTO `score` VALUES ('4', '数学', '88.0', '2');
INSERT INTO `score` VALUES ('5', '语文', '96.0', '3');
INSERT INTO `score` VALUES ('6', '数学', null, '3');

交叉连接

交叉连接(CROSS JOIN)是用左表中的每一行与右表中的每一行进行连接,不能使用ON关键字。所得到的结果将是这两个表中各行数据的所有组合,即这两个表所有数据的笛卡尔积。如果A表有4条记录,B表有3条,则结果有4*3=12条记录。

mysql> SELECT * FROM student CROSS JOIN class;
+----+------+-----+---------+----+-------+
| id | name | age | classId | id | cname |
+----+------+-----+---------+----+-------+
|  1 | s1   |  20 |       1 |  1 | 一班  |
|  1 | s1   |  20 |       1 |  2 | 二班  |
|  1 | s1   |  20 |       1 |  3 |       |
|  2 | s2   |  22 |       1 |  1 | 一班  |
|  2 | s2   |  22 |       1 |  2 | 二班  |
|  2 | s2   |  22 |       1 |  3 |       |
|  3 | s3   |  22 |       2 |  1 | 一班  |
|  3 | s3   |  22 |       2 |  2 | 二班  |
|  3 | s3   |  22 |       2 |  3 |       |
|  4 | s4   |  22 | NULL    |  1 | 一班  |
|  4 | s4   |  22 | NULL    |  2 | 二班  |
|  4 | s4   |  22 | NULL    |  3 |       |
+----+------+-----+---------+----+-------+
 rows in set

如果给交叉连接加上WHERE关键字,此时将返回符合条件的结果集,这时候与内连接的执行结果一样。

mysql> SELECT * FROM student CROSS JOIN class WHERE student.classId = class.id;
+----+------+-----+---------+----+-------+
| id | name | age | classId | id | cname |
+----+------+-----+---------+----+-------+
|  1 | s1   |  20 |       1 |  1 | 一班  |
|  2 | s2   |  22 |       1 |  1 | 一班  |
|  3 | s3   |  22 |       2 |  2 | 二班  |
+----+------+-----+---------+----+-------+
 rows in set

内连接

内连接(INNER JOIN)是用左表中的每一行与右表中的所有记录进行匹配,查询的结果为两个表经过ON条件过滤后的笛卡尔积

mysql> SELECT * FROM student INNER JOIN class ON student.classId = class.id; -- 推荐写法,INNER可写可不写
+----+------+-----+---------+----+-------+
| id | name | age | classId | id | cname |
+----+------+-----+---------+----+-------+
|  1 | s1   |  20 |       1 |  1 | 一班  |
|  2 | s2   |  22 |       1 |  1 | 一班  |
|  3 | s3   |  22 |       2 |  2 | 二班  |
+----+------+-----+---------+----+-------+
 rows in set

等价于

mysql> SELECT * FROM student, class WHERE student.classId = class.id;
+----+------+-----+---------+----+------+
| id | name | age | classId | id | name |
+----+------+-----+---------+----+------+
|  1 | s1   |  20 |       1 |  1 | 一班 |
|  2 | s2   |  22 |       1 |  1 | 一班 |
|  3 | s3   |  22 |       2 |  2 | 二班 |
+----+------+-----+---------+----+------+
 rows in set

可以连接多张表

mysql> SELECT * FROM student
    -> JOIN class ON student.classId = class.id
    -> JOIN score ON student.id = score.studentId;
+----+------+-----+---------+----+-------+----+------+-------+-----------+
| id | name | age | classId | id | cname | id | name | score | studentId |
+----+------+-----+---------+----+-------+----+------+-------+-----------+
|  1 | s1   |  20 |       1 |  1 | 一班  |  1 | 语文 | 90    |         1 |
|  1 | s1   |  20 |       1 |  1 | 一班  |  2 | 数学 | 95    |         1 |
|  2 | s2   |  22 |       1 |  1 | 一班  |  3 | 语文 | 92    |         2 |
|  2 | s2   |  22 |       1 |  1 | 一班  |  4 | 数学 | 88    |         2 |
|  3 | s3   |  22 |       2 |  2 | 二班  |  5 | 语文 | 96    |         3 |
|  3 | s3   |  22 |       2 |  2 | 二班  |  6 | 数学 | NULL  |         3 |
+----+------+-----+---------+----+-------+----+------+-------+-----------+
 rows in set

外连接

左外连接

左外连接包含LEFT JOIN左表所有行,如果左表中某行在右表没有匹配,则结果中对应行右表的部分全部为空(NULL)。

mysql> SELECT * FROM student LEFT JOIN class ON student.classId = class.id; -- 或者LEFT OUTER JOIN
+----+------+-----+---------+------+-------+
| id | name | age | classId | id   | cname |
+----+------+-----+---------+------+-------+
|  1 | s1   |  20 |       1 |    1 | 一班  |
|  2 | s2   |  22 |       1 |    1 | 一班  |
|  3 | s3   |  22 |       2 |    2 | 二班  |
|  4 | s4   |  22 | NULL    | NULL | NULL  |
+----+------+-----+---------+------+-------+
 rows in set

右外连接

右外连接包含RIGHT JOIN左表所有行,如果右表中某行在左表没有匹配,则结果中对应行左表的部分全部为空(NULL)。

mysql> SELECT * FROM student RIGHT JOIN class ON student.classId = class.id; -- 或者RIGHT OUTER JOIN
+------+------+------+---------+----+-------+
| id   | name | age  | classId | id | cname |
+------+------+------+---------+----+-------+
|    1 | s1   |   20 |       1 |  1 | 一班  |
|    2 | s2   |   22 |       1 |  1 | 一班  |
|    3 | s3   |   22 |       2 |  2 | 二班  |
| NULL | NULL | NULL | NULL    |  3 |       |
+------+------+------+---------+----+-------+
 rows in set

全外连接

如果左表中某行在右表没有匹配,则结果中对应行右表的部分全部为空(NULL),如果右表中某行在左表没有匹配,则结果中对应行左表的部分全部为空(NULL)。MySQL不支持FULL JOIN,但是我们可以对左连接和右连接的结果做UNION操作来实现

mysql> SELECT * FROM student LEFT JOIN class ON student.classId = class.id
    -> UNION
    -> SELECT * FROM student RIGHT JOIN class ON student.classId = class.id;
+------+------+------+---------+------+-------+
| id   | name | age  | classId | id   | cname |
+------+------+------+---------+------+-------+
|    1 | s1   |   20 |       1 |    1 | 一班  |
|    2 | s2   |   22 |       1 |    1 | 一班  |
|    3 | s3   |   22 |       2 |    2 | 二班  |
|    4 | s4   |   22 | NULL    | NULL | NULL  |
| NULL | NULL | NULL | NULL    |    3 |       |
+------+------+------+---------+------+-------+
 rows in set

自然连接

自然连接无需指定连接列,SQL会检查两个表中是否有相同名称的列,且相同的列名称只能有一个,自然连接基本不用。

mysql> SELECT * FROM student NATURAL JOIN class;
+----+------+-----+---------+-------+
| id | name | age | classId | cname |
+----+------+-----+---------+-------+
|  1 | s1   |  20 |       1 | 一班  |
|  2 | s2   |  22 |       1 | 二班  |
|  3 | s3   |  22 |       2 |       |
+----+------+-----+---------+-------+
 rows in set

可以看到只有一列id了,因为student与class两张表中相同的id列自动合并了,相当于内连接

SELECT * FROM student INNER JOIN class ON student.id = class.id

如果更改class表的cname字段名称为name会出现什么情况呢?

mysql> ALTER TABLE class CHANGE cname name VARCHAR(10);
Query OK, 3 rows affected
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM student NATURAL JOIN class; -- 因为两张表同名字段有两个,所有结果为空
Empty set

由于现在student与class表有两个同名的字段,所有结果为空