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。
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下面我们先对第一种进行讲解，
首先创建javaBean
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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
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<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 studentNamefrom course c inner join classroom roomon 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。
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下面看一下嵌套查询
这个只需要改一下配置文件即可
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<!-- 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 areafrom course c inner join classroom roomon c.roomId=room.roomId]]></select>
然后在接口中添加一个方法
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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