目錄
多對一
指的是在多的一端維護關聯關係.
用戶、 組表
創建對象
用戶是多的一段
Group是一的一端
創建接口
public interface UserDao {
User getUserById(int uid);
}
sql 映射文件
<?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="com.iotek.dao.UserDao">
<resultMap type="User" id="userMapper">
<id column="uid" property="uid"/>
<result column="uname" property="uname"/>
<!--association關聯查詢-->
<association property="group" javaType="Group">
<id column="gid" property="gid"/>
<result column="gname" property="gname"/>
</association>
</resultMap>
<select id="getUserById" parameterType="int" resultMap="userMapper">
SELECT gid,gname,uid,uname FROM tb_group g,tb_users u WHERE
g.gid=u.groupid AND u.uid=#{uid}
</select>
</mapper>
測試
@Test
public void getUserById() throws IOException {
SqlSession sqlSession = Utils.getSqlSessionFactory().openSession();
UserDao userDao=sqlSession.getMapper(UserDao.class);
User user=userDao.getUserById(1);
System.out.println(user.getGroup().getGname());
}
結果
方法2：使用單表查詢(分佈查詢)的方式實現多對一
<?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="com.iotek.dao.UserDao">
<!--第二次查詢-->
<select id="aa" parameterType="int" resultType="Group">
SELECT gid,gname FROM tb_group WHERE gid=#{groupid}
</select>
<resultMap type="User" id="userMapper">
<id column="uid" property="uid"/>
<result column="uname" property="uname"/>
<association property="group" javaType="Group"
select="aa" column="groupid">
</association>
</resultMap>
<!--第一次查詢-->
<select id="getUserById" parameterType="int" resultMap="userMapper">
SELECT uid ,uname, groupid FROM tb_users WHERE uid=#{uid}
</select>
</mapper>
一對多
創建po對象
1的一端（在1的一端維護關聯關係）
多的一端
創建接口
public interface GroupDao {
Group getGroupById(int gid);
}
sql語句映射文件
<?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="com.iotek.dao.GroupDao">
<resultMap type="Group" id="aa">
<id column="gid" property="gid"/>
<result column="gname" property="gname"/>
<collection property="users" ofType="User">
<id column="uid" property="uid"/>
<result column="uname" property="uname"/>
</collection>
</resultMap>
<select id="getGroupById" parameterType="int" resultMap="aa">
SELECT gid,gname,uid,uname FROM tb_group g,tb_users u WHERE
g.gid=u.groupid AND g.gid=#{gid}
</select>
</mapper>
測試
@Test
public void getUserById() throws IOException {
SqlSession sqlSession = Utils.getSqlSessionFactory().openSession();
GroupDao gd=sqlSession.getMapper(GroupDao.class);
Group group= gd.getGroupById(1);
System.out.println(group);
}
方法2：使用單表查詢(分佈查詢)的方式實現一對多
<?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="com.iotek.dao.GroupDao">
<!--第二次查詢-->
<select id="aa" parameterType="int" resultType="User">
select uid,uname from tb_users where groupid=#{gid}
</select>
<resultMap type="Group" id="groupMapper">
<id column="gid" property="gid"/>
<result column="gname" property="gname"/>
<collection property="users" ofType="User"
select="aa" column="gid">
</collection>
</resultMap>
<!--第一次查詢-->
<select id="getGroupById" parameterType="int" resultMap="groupMapper">
select gid,gname from tb_group where gid=#{gid}
</select>
</mapper>
多對多
創建po對象
創建接口
public interface StudentDao {
Student getStudentById(int sid);
}
sql語句映射文件
<?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="com.iotek.dao.StudentDao">
<resultMap type="Student" id="studentMapper">
<id column="sid" property="sid"/>
<result column="sname" property="sname"/>
<collection property="courses" ofType="Course">
<id column="cid" property="cid"/>
<result column="cname" property="cname"/>
</collection>
</resultMap>
<select id="getStudentById" parameterType="int" resultMap="studentMapper">
SELECT sid,sname,cid,cname FROM tb_stu s,tb_middle m,tb_course c
WHERE s.sid=m.studentid AND c.cid=m.courseid AND s.sid=#{sid}
</select>
</mapper>
測試
@Test
public void getStudentById() throws IOException {
SqlSession sqlSession = Utils.getSqlSessionFactory().openSession();
StudentDao stuDao=sqlSession.getMapper(StudentDao.class);
Student student=stuDao.getStudentById(1);
System.out.println(student);
Iterator<Course> iter= student.getCourses().iterator();
while(iter.hasNext()){
Course c=iter.next();
System.out.println(c.getCname());
}
}
自關聯
po對象
接口
public interface NewsLabelDao {
/*根據父節點把有子節點查詢出來*/
List<NewsLabel> getChildrenByPid(int pid);
}
sql語句映射文件
<?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="com.iotek.dao.NewsLabelDao">
<select id="aaa" resultMap="newslabelMapper1">
SELECT id,NAME FROM tb_newslabel WHERE pid=#{id}
</select>
<resultMap type="NewsLabel" id="newslabelMapper1">
<id column="id" property="id"/>
<result column="name" property="name"/>
<collection property="children" ofType="NewsLabel"
select="aaa" column="id">
</collection>
</resultMap>
<select id="getChildrenByPid" resultMap="newslabelMapper1">
SELECT id,NAME FROM tb_newslabel WHERE pid=#{pid}
</select>
</mapper>
測試
@Test
public void getStudentById() throws IOException {
SqlSession sqlSession = Utils.getSqlSessionFactory().openSession();
NewsLabelDao newsDao=sqlSession.getMapper(NewsLabelDao.class);
List<NewsLabel> children=newsDao.getChildrenByPid(2);
System.out.println(children);
}