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;
/*
一对一
*/
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;
/*
一对多
*/
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();
/*
一对多
*/
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Devmax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Devmax！