Study Mybatis Step By Step 3

在这一篇文章中我们讲解Mybatis的一对多查询

在一对多的查询过程中有两种查询方式，

一种是单次查询，也就是一次查询出所有的结果，这种查询的优点就是速度比较快，但是SQL的可复用性就降低；

另一种是嵌套查询，这种查询就是根据主查询的结果进行多次查询，这种查询速度比较低。

下面我们先对第一种进行讲解，

首先创建javaBean

package com.mybatis.in;

public class Student {

	private String sId;
	private String name;
	
	public String getsId() {
		return sId;
	}
	public void setsId(String sId) {
		this.sId = sId;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
}

 

package com.mybatis.in;

import java.util.List;

public class Course {

	private String courseId;
	private String courseName;
	private String courseType;
	
	private ClassRoom classRoom;
	private List<Student> students;
	
	public String getCourseId() {
		return courseId;
	}
	public void setCourseId(String courseId) {
		this.courseId = courseId;
	}
	public String getCourseName() {
		return courseName;
	}
	public void setCourseName(String courseName) {
		this.courseName = courseName;
	}
	public String getCourseType() {
		return courseType;
	}
	public void setCourseType(String courseType) {
		this.courseType = courseType;
	}
	public void setClassRoom(ClassRoom classRoom) {
		this.classRoom = classRoom;
	}
	public ClassRoom getClassRoom() {
		return classRoom;
	}
	public void setStudents(List<Student> students) {
		this.students = students;
	}
	public List<Student> getStudents() {
		return students;
	}
	
	
}

下面进行配置

<resultMap id="courseClassRoom" type="Course">
		<id property="courseId" column="courseId"/>
		<result property="courseType" column="courseType"/>
		<result property="courseName" column="courseName"/>
		<association property="classRoom" resultMap="classRoom"></association>
		<collection property="students" ofType="Student" javaType="java.util.ArrayList">
			<id property="sId" column="studentId"/>
			<result property="name" column="studentName"/>
		</collection>
	</resultMap>

<select id="getCoursesClassRoom" resultMap="courseClassRoom">
		<![CDATA[select c.courseId courseId,c.courseType courseType,c.courseName courseName,
		room.roomId roomId,room.roomName roomName,room.area area,student.id sId, student.name studentName
		from course c 
		inner join classroom room
			on c.roomId=room.roomId 
		left join student 
			on student.courseId = c.courseId]]>
	</select>

这样相信大家都看的明白，就不过多赘述。

下面看一下嵌套查询

这个只需要改一下配置文件即可

<!-- one to one and one to many nested query -->
	<resultMap id="courseStudentNested" type="Course">
		<id property="courseId" column="courseId"/>
		<result property="courseType" column="courseType"/>
		<result property="courseName" column="courseName"/>
		<association property="classRoom" resultMap="classRoom"></association>
		<collection property="students" ofType="Student" javaType="java.util.ArrayList" 
column="courseId" select="getStudents">
		</collection>
	</resultMap>

 <resultMap type="Student" id="student">
	 	<id property="sId" column="studentId"/>
		<result property="name" column="studentName"/>
	 </resultMap>

<select id="getStudents" parameterType="String" resultMap="student">
		<![CDATA[Select student.id sId, student.name studentName from student where courseId=#{id}]]>
	</select>
	
	<select id="getCoursesStudentNested" resultMap="courseStudentNested">
		<![CDATA[select c.courseId courseId,c.courseType courseType,c.courseName courseName,
		room.roomId roomId,room.roomName roomName,room.area area
		from course c 
		inner join classroom room
			on c.roomId=room.roomId]]>
	</select>

然后在接口中添加一个方法

package com.mybatis.in;

import java.util.List;

public interface CourseMapper {

	public List<Course> getCourses();
	
	public List<Course> getCoursesClassRoom();
	
	public List<Course> getCourseStudents();
	
	public List<Course> getCoursesStudentNested();
	
	public List<Student> getStudents(String courseId);
}

这样就可以进行嵌套查询了，这里有一个问题，如果一个对象里面有一个集合，而集合中包含的对象是他自己的时候该怎么样来查询呢？？仅仅是吧 connection的select属性改成他自身吗？？那么我们下一篇来揭开答案