2、mybatis实现关联表查询

6、mybatis实现关联表查询

6.1. 一对一关联

1). 提出需求

根据班级id查询班级信息(带老师的信息)

2). 创建表和数据

CREATE TABLE teacher(          t_id INT PRIMARY KEY AUTO_INCREMENT,          t_name VARCHAR(20) ); CREATE TABLE class(          c_id INT PRIMARY KEY AUTO_INCREMENT,          c_name VARCHAR(20),          teacher_id INT ); ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);    INSERT INTO teacher(t_name) VALUES('LS1'); INSERT INTO teacher(t_name) VALUES('LS2');   INSERT INTO class(c_name, teacher_id) VALUES('bj_a', 1); INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2);

3). 定义实体类：

public class Teacher {          private int id;          private String name; } public class Classes {          private int id;          private String name;          private Teacher teacher; }

 

4). 定义sql映射文件ClassMapper.xml

<!-- 方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集          封装联表查询的数据(去除重复的数据) select * from class c, teacher t where c.teacher_id=t.t_id and  c.c_id=1 --> <select id="getClass" parameterType="int" resultMap="ClassResultMap">          select * from class c, teacher t where c.teacher_id=t.t_id and  c.c_id=#{id} </select> <resultMap type="_Classes" id="ClassResultMap">          <id property="id" column="c_id"/>          <result property="name" column="c_name"/>          <association property="teacher" javaType="_Teacher">                    <id property="id" column="t_id"/>                    <result property="name" column="t_name"/>          </association> </resultMap>   <!-- 方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型          SELECT * FROM class WHERE c_id=1;          SELECT * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值 -->    <select id="getClass2" parameterType="int" resultMap="ClassResultMap2">          select * from class where c_id=#{id}  </select>  <resultMap type="_Classes" id="ClassResultMap2">          <id property="id" column="c_id"/>          <result property="name" column="c_name"/>          <association property="teacher" column="teacher_id" select="getTeacher"> </association>  </resultMap>    <select id="getTeacher" parameterType="int" resultType="_Teacher">          SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}  </select>

 

5). 测试

@Test public void testOO() {          SqlSession sqlSession = factory.openSession();          Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass", 1);          System.out.println(c); }   @Test public void testOO2() {          SqlSession sqlSession = factory.openSession();          Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass2", 1);          System.out.println(c); }

一对多关联

1). 提出需求

根据classId查询对应的班级信息,包括学生,老师

2). 创建表和数据：

CREATE TABLE student(          s_id INT PRIMARY KEY AUTO_INCREMENT,          s_name VARCHAR(20),          class_id INT ); INSERT INTO student(s_name, class_id) VALUES('xs_A', 1); INSERT INTO student(s_name, class_id) VALUES('xs_B', 1); INSERT INTO student(s_name, class_id) VALUES('xs_C', 1); INSERT INTO student(s_name, class_id) VALUES('xs_D', 2); INSERT INTO student(s_name, class_id) VALUES('xs_E', 2); INSERT INTO student(s_name, class_id) VALUES('xs_F', 2);

 

3). 定义实体类：

public class Student {          private int id;          private String name; }   public class Classes {          private int id;          private String name;          private Teacher teacher;          private List<Student> students; }

4). 定义sql映射文件ClassMapper.xml

<!-- 方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集 SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND  c.c_id=1  --> <select id="getClass3" parameterType="int" resultMap="ClassResultMap3">          select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and  c.c_id=#{id} </select> <resultMap type="_Classes" id="ClassResultMap3">          <id property="id" column="c_id"/>          <result property="name" column="c_name"/>          <association property="teacher" column="teacher_id" javaType="_Teacher">                    <id property="id" column="t_id"/>                    <result property="name" column="t_name"/>          </association>          <!-- ofType指定students集合中的对象类型 -->          <collection property="students" ofType="_Student">                    <id property="id" column="s_id"/>                    <result property="name" column="s_name"/>          </collection> </resultMap>   <!--          方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型                    SELECT * FROM class WHERE c_id=1;                    SELECT * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值                    SELECT * FROM student WHERE class_id=1  //1是第一个查询得到的c_id字段的值  -->  <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">          select * from class where c_id=#{id}  </select>  <resultMap type="_Classes" id="ClassResultMap4">          <id property="id" column="c_id"/>          <result property="name" column="c_name"/>          <association property="teacher" column="teacher_id" javaType="_Teacher" select="getTeacher2"></association>          <collection property="students" ofType="_Student" column="c_id" select="getStudent"></collection>  </resultMap>    <select id="getTeacher2" parameterType="int" resultType="_Teacher">          SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}  </select>    <select id="getStudent" parameterType="int" resultType="_Student">          SELECT s_id id, s_name name FROM student WHERE class_id=#{id}  </select>

 

5). 测试：

@Test public void testOM() {          SqlSession sqlSession = factory.openSession();          Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass3", 1);          System.out.println(c); }   @Test public void testOM2() {          SqlSession sqlSession = factory.openSession();          Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass4", 1);          System.out.println(c); }