MyBatis关联查询

在实际的应用开发中，对单表进行查询是很少的。即使是通过视图这种方式，也是对表进行关联而来的。在ORM中，通常需要在一个实体中关联另一个实体。在MyBatis中一对多关系使用collection标签，一对一关系使用association标签。在这里我们考虑这样一张场景，学生，班级。学生和班级是多对一的关系。现在我们要在查询班级的时候得到学生信息，在查学生时要得到班级信息。
准备工作。
建表：

表 student
create table t_student
(
id varchar2(10) primary key,
name varchar2(20),
sex varchar(2),
class_id varchar2(10)
)

表 class
create table t_class
(
id varchar2(10) primary key,
class_name varchar(10),
teacher varchar2(10)
)

Student
package sy.model;

public class TStudent {
private String id;
private String name;
private String sex;
private String classId;
private TClass tClass;

public void setId(String id){
this.id = id;
}

public String getId(){
return this.id;
}
public void setName(String name){
this.name = name;
}

public String getName(){
return this.name;
}

public String getSex() {
return sex;
}

public void setSex(String sex) {
this.sex = sex;
}

public void setClassId(String classId){
this.classId = classId;
}
public String getClassId(){
return this.classId;
}
public void setTClass(TClass tClass){
this.tClass= tClass;
}
public TClass getTClass (){
return this.tClass;
}

}

实体 TClass
package sy.model;

import java.util.List;

public class TClass {
private String id;
private String className;
private String teacher;
private List<TStudent> studentList;

public List<TStudent> getStudentList() {
return studentList;
}
public void setStudentList(List<TStudent> studentList) {
this.studentList = studentList;
}
public void setId(String id){
this.id = id;
}
public String getId(){
return this.id;
}
public void setClassName(String className){
this.className= className;
}
public String getClassName(){
return this.className;
}
public void setTeacher(String teacher){
this.teacher= teacher;
}
public String getTeacher(){
return this.teacher;
}

}

TStudent 映射
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="sy.dao.TStudentMapper" >
<resultMap id="studentResultMap" type="sy.model.TStudent">
<id property="id" column="id"  />
<result property="name" column="name"  />
<result property="sex" column="sex"  />
<result property="classId" column="class_id"  />
</resultMap>

<resultMap id="classResultMap" type="sy.model.TClass">
<id property="id" column="id"  />
<result property="className" column="class_name"  />
<result property="teacher" column="teacher"  />
</resultMap>

<resultMap id="studentWithClassMap" type="sy.model.TStudent" extends="studentResultMap">
<association property="tClass" javaType="sy.model.TClass" resultMap="classResultMap"/>
</resultMap>

<select id="selectStudentWithClass" resultMap="studentWithClassMap">
select  s.id,
s.name,
s.sex,
s.class_id,
t.id,
t.class_name,
t.teacher
from t_student s,t_class t
where s.class_id = t.id
</select>

<resultMap id="studentWithClassMap2" type="sy.model.TStudent" extends="studentResultMap">
<!-- association 中的column表示的是关联的字段-->
<association property="tClass" javaType="sy.model.TClass" select="selectClass" column="class_id"/>
</resultMap>

<select id="selectClass" parameterType="string" resultMap="classResultMap">
select * from t_clsss t
where t.id=#{class_id}
</select>
<select id="selectStudentWithClass2" resultMap="studentWithClassMap">
select *
from t_student s
</select>
</mapper>

TClass映射
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="sy.dao.TClassMapper" >
<resultMap id="classResultMap" type="sy.model.TClass">
<id property="id" column="id"  />
<result property="className" column="name"  />
<result property="teacher" column="teacher"  />
</resultMap>

<resultMap id="classWithStudentMap" type="sy.model.TClass" extends="classResultMap">
<collection property="studentList" javaType="list" ofType="sy.model.TStudent">
<!-- property对应实体中的属性，column对应的是查询出的**字段或者是别名**。查询出来的字段中有重名，就必须取别名，否则会映射不到-->
<id property="id" column="student_id" />
<result property="name" column="name" />
<result property="sex" column="sex" />
<result property="classId" column="class_id" />
</collection>
</resultMap>

<select id="selectClassWithStudent" resultMap="classWithStudentMap">
select  t.id,
t.class_name,
t.teacher,
s.id student_id,
s.name,
s.sex,
s.class_id
from t_student s,t_class t
where s.class_id = t.id
</select>
</mapper>