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);

}