MySQL多表数据查询(DQL)

数据准备：

/* ----------------------------------------创建班级表---------------------------------------- */
CREATE TABLE classes(
    id INT PRIMARY KEY,
    className VARCHAR(10),
    classDesc VARCHAR(10)
);

INSERT INTO classes VALUES(1,'高一(1)班','尖子班');
INSERT INTO classes VALUES(2,'高一(2)班','优秀班');
INSERT INTO classes VALUES(3,'高一(3)班','普通班');
INSERT INTO classes VALUES(4,'高一(4)班','逃课班');


/* ----------------------------------------创建课程表---------------------------------------- */
CREATE TABLE courses(
    cid VARCHAR(10) PRIMARY KEY,
    cname VARCHAR(10),
    teacher VARCHAR(10)
);

INSERT INTO courses VALUES ('ca','政治','Tom');
INSERT INTO courses VALUES ('cb','历史','Smith');
INSERT INTO courses VALUES ('cc','地理','Lida');
INSERT INTO courses VALUES ('cd','物理','David');
INSERT INTO courses VALUES ('ce','化学','Alan');
INSERT INTO courses VALUES ('cf','生物','Jack');


/* ----------------------------------------创建学生表---------------------------------------- */
CREATE TABLE students(
    sid INT PRIMARY KEY,
    sname VARCHAR(10),
    age INT,
    class INT,
    CONSTRAINT students_fk FOREIGN KEY (class) REFERENCES classes(id)
);

INSERT INTO students VALUES (1001,'张三',18,4);
INSERT INTO students VALUES (1002,'李四',20,3);
INSERT INTO students VALUES (1003,'王五',25,1);
INSERT INTO students VALUES (1004,'孙六',21,1);
INSERT INTO students VALUES (1005,'赵七',23,3);


/* ----------------------------------------创建选课表---------------------------------------- */
CREATE TABLE electives(
    id INT PRIMARY KEY AUTO_INCREMENT,
    student INT,
    course VARCHAR(10),
    CONSTRAINT electives_fk1 FOREIGN KEY (student) REFERENCES students(sid),
    CONSTRAINT electives_fk2 FOREIGN KEY (course) REFERENCES courses(cid)
);

INSERT INTO electives(student,course) VALUES (1001,'cc'),(1003,'cb'),(1005,'ca');
INSERT INTO electives(student,course) VALUES (1003,'ce'),(1001,'cf'),(1005,'cd');
INSERT INTO electives(student,course) VALUES (1005,'cc'),(1004,'ce'),(1003,'ca');
INSERT INTO electives(student,course) VALUES (1002,'cd'),(1004,'ca'),(1001,'cb');
INSERT INTO electives(student,course) VALUES (1002,'cb'),(1004,'cd'),(1002,'cf');

架构设计图：

一、内连接查询

1.1 多对一关系查询

Example01：查询全部学生及班级详情

mysql> SELECT s.sname,s.age,c.className,c.classDesc FROM students s,classes c WHERE s.class = c.id; /*隐式内连接查询*/
+--------+------+--------------+-----------+
| sname  | age  | className    | classDesc |
+--------+------+--------------+-----------+
| 张三   |   18 | 高一(4)班    | 逃课班    |
| 李四   |   20 | 高一(3)班    | 普通班    |
| 王五   |   25 | 高一(1)班    | 尖子班    |
| 孙六   |   21 | 高一(1)班    | 尖子班    |
| 赵七   |   23 | 高一(3)班    | 普通班    |
+--------+------+--------------+-----------+
5 rows in set (0.00 sec)

mysql> SELECT s.sname,s.age,c.className,c.classDesc FROM students s INNER JOIN classes c ON s.class = c.id; /*显示内连接查询*/
+--------+------+--------------+-----------+
| sname  | age  | className    | classDesc |
+--------+------+--------------+-----------+
| 张三   |   18 | 高一(4)班    | 逃课班    |
| 李四   |   20 | 高一(3)班    | 普通班    |
| 王五   |   25 | 高一(1)班    | 尖子班    |
| 孙六   |   21 | 高一(1)班    | 尖子班    |
| 赵七   |   23 | 高一(3)班    | 普通班    |
+--------+------+--------------+-----------+
5 rows in set (0.00 sec)

Example02：查询尖子班与普通班的学生平均年龄

mysql> SELECT c.classDesc,AVG(s.age) FROM students s,classes c WHERE s.class = c
.id GROUP BY c.classDesc;
+-----------+------------+
| classDesc | AVG(s.age) |
+-----------+------------+
| 尖子班    |    23.0000 |
| 普通班    |    21.5000 |
| 逃课班    |    18.0000 |
+-----------+------------+
3 rows in set (0.00 sec)

1.2 多对多关系查询

Example03：查询张三的全部选修学科

mysql> SELECT s.sname,c.cname FROM electives e,students s,courses c WHERE e.student = s.sid AND e.course = c.cid AND s.sname = "张三";
+--------+--------+
| sname  | cname  |
+--------+--------+
| 张三   | 地理   |
| 张三   | 生物   |
| 张三   | 历史   |
+--------+--------+
3 rows in set (0.02 sec)

Example04：查询物理老师David的全部学生

mysql> SELECT c.teacher,s.sname FROM electives e,students s,courses c WHERE e.student = s.sid AND e.course = c.cid AND c.teacher = "David";
+---------+--------+
| teacher | sname  |
+---------+--------+
| David   | 赵七   |
| David   | 李四   |
| David   | 孙六   |
+---------+--------+
3 rows in set (0.03 sec)

Example05：查询各个老师的学生个数

mysql> SELECT c.teacher,COUNT(s.sid) FROM electives e,students s,courses c WHERE e.student = s.sid AND e.course = c.cid GROUP BY c.teacher;
+---------+--------------+
| teacher | COUNT(s.sid) |
+---------+--------------+
| Alan    |            2 |
| David   |            3 |
| Jack    |            2 |
| Lida    |            2 |
| Smith   |            3 |
| Tom     |            3 |
+---------+--------------+
6 rows in set (0.02 sec)

二、外连接查询

2.1 左外连接查询

Example06：查询全部学生及班级详情（保留左表的全部记录）

mysql> SELECT s.sname,s.age,c.className,c.classDesc FROM students s LEFT JOIN classes c ON s.class = c.id; 
+--------+------+--------------+-----------+
| sname  | age  | className    | classDesc |
+--------+------+--------------+-----------+
| 王五   |   25 | 高一(1)班    | 尖子班    |
| 孙六   |   21 | 高一(1)班    | 尖子班    |
| 李四   |   20 | 高一(3)班    | 普通班    |
| 赵七   |   23 | 高一(3)班    | 普通班    |
| 张三   |   18 | 高一(4)班    | 逃课班    |
+--------+------+--------------+-----------+
5 rows in set (0.00 sec)

2.2 右外连接查询（保留右表的全部记录）

Example07：查询全部学生及班级详情

mysql> SELECT s.sname,s.age,c.className,c.classDesc FROM students s RIGHT JOIN classes c ON s.class = c.id; 
+--------+------+--------------+-----------+
| sname  | age  | className    | classDesc |
+--------+------+--------------+-----------+
| 张三   |   18 | 高一(4)班    | 逃课班    |
| 李四   |   20 | 高一(3)班    | 普通班    |
| 王五   |   25 | 高一(1)班    | 尖子班    |
| 孙六   |   21 | 高一(1)班    | 尖子班    |
| 赵七   |   23 | 高一(3)班    | 普通班    |
| NULL   | NULL | 高一(2)班    | 优秀班    |
+--------+------+--------------+-----------+
6 rows in set (0.00 sec)

三、子查询

3.1 标量子查询（子查询返回的结果是一行一列的数据）

Example08：查询尖子班的学生详情

mysql> SELECT * from students where class = (SELECT id from classes where classDesc = '尖子班');
+------+--------+------+-------+
| sid  | sname  | age  | class |
+------+--------+------+-------+
| 1003 | 王五   |   25 |     1 |
| 1004 | 孙六   |   21 |     1 |
+------+--------+------+-------+
2 rows in set (0.02 sec)

mysql> select s.* from students s,classes c where s.class = c.id and c.classDesc= '尖子班'; /*内连接查询实现*/
+------+--------+------+-------+
| sid  | sname  | age  | class |
+------+--------+------+-------+
| 1003 | 王五   |   25 |     1 |
| 1004 | 孙六   |   21 |     1 |
+------+--------+------+-------+
2 rows in set (0.00 sec)

3.2 列子查询（子查询返回的结果是一列的数据）

Example09：查询普通班与逃课班的学生详情

mysql> SELECT * FROM students WHERE class IN (SELECT id FROM classes WHERE classDesc IN ('普通班','逃课班'));
+------+--------+------+-------+
| sid  | sname  | age  | class |
+------+--------+------+-------+
| 1001 | 张三   |   18 |     4 |
| 1002 | 李四   |   20 |     3 |
| 1005 | 赵七   |   23 |     3 |
+------+--------+------+-------+
3 rows in set (0.00 sec)

mysql> SELECT s.* FROM students s,classes c WHERE s.class = c.id AND c.classDesc IN ('普通班','逃课班'); /*内连接查询实现*/
+------+--------+------+-------+
| sid  | sname  | age  | class |
+------+--------+------+-------+
| 1001 | 张三   |   18 |     4 |
| 1002 | 李四   |   20 |     3 |
| 1005 | 赵七   |   23 |     3 |
+------+--------+------+-------+
3 rows in set (0.00 sec)

3.3 表子查询（子查询返回的结果是一个表的数据）

Example10：查询年龄大于21的学生的选修学科详情

mysql> SELECT s.sname,s.age,c.cname FROM electives e,(SELECT * FROM students WHERE age > 21) s,courses c WHERE e.student = s.sid AND e.course = c.cid; /*先条件
过滤再内连接*/
+--------+------+--------+
| sname  | age  | cname  |
+--------+------+--------+
| 王五   |   25 | 历史   |
| 王五   |   25 | 化学   |
| 王五   |   25 | 政治   |
| 赵七   |   23 | 政治   |
| 赵七   |   23 | 物理   |
| 赵七   |   23 | 地理   |
+--------+------+--------+
6 rows in set (0.00 sec)

mysql> SELECT s.sname,s.age,c.cname FROM electives e,students s,courses c WHERE e.student = s.sid AND e.course = c.cid  AND s.age > 21; /*先内连接再条件过滤*/
+--------+------+--------+
| sname  | age  | cname  |
+--------+------+--------+
| 王五   |   25 | 历史   |
| 王五   |   25 | 化学   |
| 王五   |   25 | 政治   |
| 赵七   |   23 | 政治   |
| 赵七   |   23 | 物理   |
| 赵七   |   23 | 地理   |
+--------+------+--------+
6 rows in set (0.00 sec)

MySQL多表数据查询(DQL)

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