MyBatis 一对多操作
表结构：
public class Teacher { private int id; private String name; // 一个老师多个学生 private Liststudent; }
public class Student { private int id; private String name; private int tid; }
XML文件：
方式一：连表查询
<select id="getTeacher" resultMap="TeacherStudent"> select s.id sid, s.name sname, t.name tname, t.id tid from student s, teacher t where s.tid = t.id and t.id = #{tid}; select> <resultMap id="TeacherStudent" type="Teacher"> <result property="id" column="tid"/> <result property="name" column="tname"/> <collection property="student" ofType="Student"> <result property="id" column="sid"/> <result property="name" column="sname"/> <result property="tid" column="tid"/> collection> resultMap>
方式二：子查询
<select id="getTeacher2" resultMap="TeacherStudent2"> select * from teacher where id = #{tid}; select> <resultMap id="TeacherStudent2" type="Teacher"> <collection property="student" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/> resultMap> <select id="getStudentByTeacherId" resultType="Student"> select * from student where tid = #{tid} select>
测试文件：
public class MyTest { @Test public void getTeacherTest() { SqlSession sqlSession = MybatisUtils.getSqlSession(); TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class); Teacher teacher = mapper.getTeacher2(1); System.out.println(teacher); sqlSession.close(); } }
小结：
- 关联查询 -association 【多对一】
- 集合 -collection 【一对多】
- javaType & ofType
- javaType 用来指定实体类中属性的类型
- ofType 用来指定映射到List或者集合中的pojo类型，泛指中的约束类型！
注意点：
- 保证SQL的可读性，尽量保证通俗易懂
- 注意一对多和多对一中，属性名和字段名的问题！
- 如果问题不好排错，可以使用日志，推荐使用Log4j！
原文：https://www.cnblogs.com/shangwei/p/15267342.html