一、通过映射配置文件实现多表联查
首先，使用Mysql数据库，创建两个表，分别为学生表Student表和班级表Class表，在Student表中添加列classid参照主表的列id的外键约束。
学生表Student表：
班级表Class表 ：
现在去写Dao层和实体类
Student实体类：
package com.ape.bean;
import java.util.Date;
public class Student {
private Integer sid;
private String sname;
private Date birthday;
private String ssex;
private int classid;
private Classs banji;
public Student() {
super();
// TODO Auto-generated constructor stub
}
public Student(Integer sid, String sname, Date birthday, String ssex, int classid, Classs banji) {
this.sid = sid;
this.sname = sname;
this.birthday = birthday;
this.ssex = ssex;
this.classid = classid;
this.banji = banji;
}
public Integer getSid() {
return sid;
}
public void setSid(Integer sid) {
this.sid = sid;
}
public String getSname() {
return sname;
}
public void setSname(String sname) {
this.sname = sname;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public String getSsex() {
return ssex;
}
public void setSsex(String ssex) {
this.ssex = ssex;
}
public int getClassid() {
return classid;
}
public void setClassid(int classid) {
this.classid = classid;
}
public Classs getBanji() {
return banji;
}
public void setBanji(Classs banji) {
this.banji = banji;
}
@Override
public String toString() {
return "Student{" +
"sid=" + sid +
", sname='" + sname + '\'' +
", birthday=" + birthday +
", ssex='" + ssex + '\'' +
", classid=" + classid +
", banji=" + banji +
'}';
}
}Class实体类：
package com.ape.bean;
import java.util.List;
public class Classs {
private int classid;
private String classname;
private List<Student> xuesheng;
public Classs() {
}
public Classs(int classid, String classname, List<Student> xuesheng) {
this.classid = classid;
this.classname = classname;
this.xuesheng = xuesheng;
}
public int getClassid() {
return classid;
}
public void setClassid(int classid) {
this.classid = classid;
}
public String getClassname() {
return classname;
}
public void setClassname(String classname) {
this.classname = classname;
}
public List<Student> getXuesheng() {
return xuesheng;
}
public void setXuesheng(List<Student> xuesheng) {
this.xuesheng = xuesheng;
}
@Override
public String toString() {
return "Classs{" +
"classid=" + classid +
", classname='" + classname + '\'' +
", xuesheng=" + xuesheng +
'}';
}
}Mapper接口：
public interface ClassMapper {
public List<Class> findClass();
}public interface StudentMapper {
public List<Student> findStudent();
public List<Student> findduobiao();
}一对一关系的xml文件配置：
实现一对一的关系查询，即一条student信息对应一条class信息
<resultMap id="stu_class_Map" type="Student">
<result column="sid" property="sid" />
<result column="sname" property="sname"/>
<result column="birthday" property="birthday"/>
<result column="ssex" property="ssex"/>
<result column="classid" property="classid"/>
<!-- 一对一的关系映射，配置封装student的内容 -->
<association property="banji">
<result column="classid" property="classid"/>
<result column="classname" property="classname"/>
</association>
</resultMap>
<select id="findduobiao" resultMap="stu_class_Map">
select * from student inner join class on student.classid = class.classid;
</select>这里主要配置的就是resultMap了，配置javabean类中属性与数据库列名的对应关系，association是用来指定从表方的引用实体属性的。
注意最后写的findstudent方法中，是使用到resultMap作为接收结果值返回，与上文配置的resultMap相对应。
一对多查询xml文件配置：
<resultMap id="class_stu_Map" type="Classs">
<result column="classid" property="classid"/>
<result column="classname" property="classname"/>
<!-- Class中的集合映射 -->
<collection property="xuesheng">
<result column="sid" property="sid" />
<result column="sname" property="sname"/>
<result column="birthday" property="birthday"/>
<result column="ssex" property="ssex"/>
<result column="classid" property="classid"/>
</collection>
</resultMap>
<select id="yiduiduo" resultMap="class_stu_Map">
select * from class left join student on class.classid = student.classid order by class.classid;
</select>虽然知道使用这条sql语句查询后的结果会有重复项，但是不用担心，应为mybatis会自动识别到重复的内容，只保留一个。
二、使用注解的方式
还是同样的我们需要再建实体类，跟上面的一样；其次Mapper接口中的方法需要加注解。
一对一：
public interface AccountDao {
@Select("select * from account")
@Results(id="accountMap",value = {
@Result(id = true,column = "id",property = "id"),
@Result(column = "uid",property = "uid"),
@Result(column = "money",property = "money"),
@Result(property = "user",column = "uid",one =
@One(select="mediacomm.dao.UserDao.findUserById",fetchType= FetchType.DEFAULT))
})
List<Account> findAccountWithUser();public interface UserDao {
@Select("select * from user where id=#{id}")
User findUserById(int id);一对多：
public interface UserDao {
@Select(value = "select * from user")
@Results(id = "userMap",value = {
@Result(id = true,column = "id",property = "id"),
@Result(column = "username",property = "username"),
@Result(column = "password",property = "password"),
@Result(column = "address",property = "address"),
@Result(property = "accounts",column = "id",
many = @Many(select = "mediacomm.dao.AccountDao.findAccountByUid",fetchType = FetchType.DEFAULT))
})
List<User> findAllUser();
public interface AccountDao {
@Select("select * from account where uid=#{uid}")
List<Account> findAccountByUid(int uid);到此这篇关于MyBatis如何实现多表联查的文章就介绍到这了,更多相关MyBatis多表联查内容请搜索编程网以前的文章或继续浏览下面的相关文章希望大家以后多多支持编程网！
