mybatis-联表查询+查询输入的参数是集合（利用参数是集合进行查询，实现级联查询）

一、数据库表的介绍
1、班级表：
CREATE TABLE

classes

(

c_id

int(3) NOT NULL,

c_name

varchar(20) DEFAULT NULL,

teacher_id

int(3) DEFAULT NULL,
PRIMARY KEY (

c_id

),
KEY

teacher_id

(

teacher_id

),
CONSTRAINT

classes_ibfk_1

FOREIGN KEY (

teacher_id

) REFERENCES

teacher

(

t_id

)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
2、学生表：
CREATE TABLE

student

(

s_id

int(3) NOT NULL,

s_name

varchar(20) DEFAULT NULL,

classes_id

int(3) DEFAULT NULL,
PRIMARY KEY (

s_id

),
KEY

classes_id

(

classes_id

),
CONSTRAINT

student_ibfk_1

FOREIGN KEY (

classes_id

) REFERENCES

classes

(

c_id

)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
3、老师表：
CREATE TABLE

teacher

(

t_id

int(3) NOT NULL,

t_name

varchar(20) DEFAULT NULL,
PRIMARY KEY (

t_id

)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
4、数据库概念模型：
二、数据库连表查询：
1、查询要求：通过班级编号，查询学生信息及对应的老师和班级的信息
2、mapper映射文件的编写：

<!--resultMap属性与后面取得resultMapper的id名相同，注意这里是三张表的连表查询-->
<select id="selStuClasTeacher" parameterType="int" resultMap="stuClasTeacher">
SELECT s.*,t.t_name,c.c_name
from student s,teacher t,classes c
where s.classes_id=c.c_id=t.t_id
HAVING s.classes_id=#{c_id}
</select>

<resultMap type="Classes" id="stuClasTeacher">
<id column="c_id" property="c_id"/>
<result column="c_name" property="c_name"/>
<!--   association    是实体类里面的类属性-->
<association property="teacher" javaType="Teacher" column="t_id">
<id column="t_id" property="t_id"/>
<result column="t_name" property="t_name"/>
</association>
<association property="student" javaType="Student" column="s_id">
<id column="s_id" property="s_id"/>
<result column="s_name" property="s_name"/>
</association>
</resultMap>

三、参数是集合的查询（类似于级联查询）
1、查询条件：根据学生的多个id查询学生信息，传入的参数是集合，就是做SQL查询时，需要循环进行查询时，利用参数是集合的方式，在SQL语句中就实现重复查询的作用
2、查询mapper映射文件的编写

<!--注意这里的SQL语句用的是in关键字，这里的的级联可以输入数组-->
<select id="selStuById" resultMap="Students">
SELECT * from student WHERE s_id in
<foreach collection="list" open="(" separator="," close=")" item="a">
#{a}
</foreach>
</select>

四、实体接口：

五、实体类：
1、班级实体类

public class Classes {
private int c_id;
private String c_name;
private Teacher teacher;
private Student student;
public int getC_id() {
return c_id;
}
public void setC_id(int c_id) {
this.c_id = c_id;
}
public String getC_name() {
return c_name;
}
public void setC_name(String c_name) {
this.c_name = c_name;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
public Student getStudent() {
return student;
}
public void setStudent(Student student) {
this.student = student;
}
public Classes() {
super();
}
@Override
public String toString() {
return "Classes [c_id=" + c_id + ", c_name=" + c_name + ", teacher="
+ teacher + ", student=" + student + "]";
}

}
2、老师实体类

public class Teacher {
private int t_id;
private String t_name;
private Classes classes;
public int getT_id() {
return t_id;
}
public void setT_id(int t_id) {
this.t_id = t_id;
}
public String getT_name() {
return t_name;
}
public void setT_name(String t_name) {
this.t_name = t_name;
}
public Classes getClasses() {
return classes;
}
public void setClasses(Classes classes) {
this.classes = classes;
}
@Override
public String toString() {
return "Teacher [t_id=" + t_id + ", t_name=" + t_name + ", classes="
+ classes + "]";
}
public Teacher(int t_id, String t_name, Classes classes) {
super();
this.t_id = t_id;
this.t_name = t_name;
this.classes = classes;
}
public Teacher() {
super();
}

}

3、学生实体类：

public class Student {
private int s_id;
private String s_name;
private Classes classes;
public int getS_id() {
return s_id;
}
public void setS_id(int s_id) {
this.s_id = s_id;
}
public String getS_name() {
return s_name;
}
public void setS_name(String s_name) {
this.s_name = s_name;
}
public Classes getClasses() {
return classes;
}
public void setClasses(Classes classes) {
this.classes = classes;
}
public Student(int s_id, String s_name, Classes classes) {
super();
this.s_id = s_id;
this.s_name = s_name;
this.classes = classes;
}
public Student() {
super();
}
@Override
public String toString() {
return "Student [s_id=" + s_id + ", s_name=" + s_name + ", classes="
+ classes + "]";
}

}