写在前言
one to one - association ; //单对一，使用association
one to many - collection ; //单对多，使用collection
Nested results - column is not necessary ，javaType is necessary ！ //使用嵌套结果，column 不是必需的，但是JavaType是必需的；
nested queries - column is necessary ，javaType is not necessary ！ //使用嵌套查询，column是必需的，JavaType不是必须的，子查询自定义resultType即可！！
表结构：
t_student 表拥有属性 class_id 对应 t_class表 t_id
【1】更改Classes，添加属性
public class Classes {
private int id;
private String name;
private Teacher teacher;
private List<Student> list;
...
}
【2】嵌套结果-获取Classes，Teacher and list
<select id="getClass4" parameterType="int" resultMap="getClass4Map">
select * from t_class c,t_student s ,t_teacher where c.c_id = s.class_id and c.t_id = t_teacher.t_id and c.c_id = #{id}
</select>
<resultMap type="Classes" id="getClass4Map">
<result property="id" column="c_id" javaType="int" jdbcType="INTEGER"/>
<result property="name" column="c_name" javaType="string" jdbcType="VARCHAR"/>
<!-- here column is not necessary !!!-->
<association property="teacher" column="t_id" javaType="Teacher" >
<id property="id" column="t_id" javaType="int" jdbcType="INTEGER"/>
<result property="name" column="t_name" javaType="string" jdbcType="VARCHAR"/>
</association>
<!-- here, no column !!! -->
<collection property="list" ofType="Student" >
<result property="id" column="s_id" javaType="int" jdbcType="INTEGER"/>
<result property="name" column="s_name" javaType="string" jdbcType="VARCHAR"/>
</collection>
</resultMap>
【3】嵌套查询–获取Classes，Teacher and list
<select id="getClass5" parameterType="int" resultMap="getClass5Map">
select * from t_class c where c.c_id = #{id}
</select>
<select id="getTeacher" parameterType="int" resultType="Teacher">
select t_id id,t_name name from t_teacher where t_id = #{id}
</select>
<select id="getStudent" parameterType="int" resultType="Student">
select s_id id,s_name name from t_student t where t.class_id = #{id}
</select>
<resultMap type="Classes" id="getClass5Map">
<result property="id" column="c_id" javaType="int" jdbcType="INTEGER"/>
<result property="name" column="c_name" javaType="string" jdbcType="VARCHAR"/>
<!-- here,column is necessary !!! 有属性select = getTeacher-->
<association property="teacher" column="t_id" javaType="Teacher" select="getTeacher">
</association>
<!-- here,column is necessary !!! 有属性select = getStudent -->
<collection property="list" ofType="Student" select="getStudent" column="c_id">
</collection>
<!-- pay attention to the Query condition of SQL statement t.class_id = #{id} not t.s_id = #{id} !!-->
</resultMap>
【4】Test
获取SqlSessionFactory的工具类：
public static SqlSessionFactory getFactory(){
/* flow the src dir*/
String resource = "mybatis.xml";
/*MybatisUtils.class.getResourceAsStream(resource)----- it's wrong !!!!
* please distinguish the two up and down
* */
InputStream inputStream = MybatisUtils.class.getClassLoader().getResourceAsStream(resource);
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
return factory;
}
测试方法
@Test
public void testSelect4(){
/*set auto commit ,which equals to the above*/
SqlSession session = MybatisUtils.getFactory().openSession(true);
String statement = "com.web.mapper.classMapper.getClass4";
/*return the effect rows*/
Classes classes = session.selectOne(statement, 1);
Teacher teacher = classes.getTeacher();
List<Student> list = classes.getList();
System.out.println("result.."+classes+','+classes.getClass());
System.out.println(teacher);
System.out.println(list);
}
result as follows :
result..Classes [id=1, list=[Student [id=1, name=stu1], Student [id=2, name=stu2], Student [id=3, name=stu3]], name=计算机, teacher=Teacher [id=1, name=李明]],class com.web.model.Classes
Teacher [id=1, name=李明]
[Student [id=1, name=stu1], Student [id=2, name=stu2], Student [id=3, name=stu3]]
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