mybatis关联查询
一、一对一关联
1.1、提出需求
根据班级id查询班级信息(带老师的信息)
1.2、创建表和数据
创建一张教师表和班级表，这里我们假设一个老师只负责教一个班，那么老师和班级之间的关系就是一种一对一的关系。
1 CREATE TABLE teacher(
2 t_id INT PRIMARY KEY AUTO_INCREMENT,
3 t_name VARCHAR(20)
4 );
5 CREATE TABLE class(
6 c_id INT PRIMARY KEY AUTO_INCREMENT,
7 c_name VARCHAR(20),
8 teacher_id INT
9 );
10 ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);
11
12 INSERT INTO teacher(t_name) VALUES('teacher1');
13 INSERT INTO teacher(t_name) VALUES('teacher2');
14
15 INSERT INTO class(c_name, teacher_id) VALUES('class_a', 1);
16 INSERT INTO class(c_name, teacher_id) VALUES('class_b', 2);
表之间的关系如下：
1.3、定义实体类
1、Teacher类，Teacher类是teacher表对应的实体类。
1 package me.gacl.domain;
2
3 /**
4 * @author gacl
5 * 定义teacher表对应的实体类
6 */
7 public class Teacher {
8
9 //定义实体类的属性，与teacher表中的字段对应
10 private int id; //id===>t_id
11 private String name; //name===>t_name
12
13 public int getId() {
14 return id;
15 }
16
17 public void setId(int id) {
18 this.id = id;
19 }
20
21 public String getName() {
22 return name;
23 }
24
25 public void setName(String name) {
26 this.name = name;
27 }
28
29 @Override
30 public String toString() {
31 return "Teacher [id=" + id + ", name=" + name + "]";
32 }
33 }
2、Classes类，Classes类是class表对应的实体类
1 package me.gacl.domain;
2
3 /**
4 * @author gacl
5 * 定义class表对应的实体类
6 */
7 public class Classes {
8
9 //定义实体类的属性，与class表中的字段对应
10 private int id; //id===>c_id
11 private String name; //name===>c_name
12
13 /**
14 * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
15 * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
16 */
17 private Teacher teacher;
18
19 public int getId() {
20 return id;
21 }
22
23 public void setId(int id) {
24 this.id = id;
25 }
26
27 public String getName() {
28 return name;
29 }
30
31 public void setName(String name) {
32 this.name = name;
33 }
34
35 public Teacher getTeacher() {
36 return teacher;
37 }
38
39 public void setTeacher(Teacher teacher) {
40 this.teacher = teacher;
41 }
42
43 @Override
44 public String toString() {
45 return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher+ "]";
46 }
47 }
1.4、定义sql映射文件classMapper.xml
1 <?xml version="1.0" encoding="UTF-8" ?>
2 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
3 <!-- 为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名+sql映射文件名，这样就能够保证namespace的值是唯一的
4 例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后缀)
5 -->
6 <mapper namespace="me.gacl.mapping.classMapper">
7
8 <!--
9 根据班级id查询班级信息(带老师的信息)
10 ##1. 联表查询
11 SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;
12
13 ##2. 执行两次查询
14 SELECT * FROM class WHERE c_id=1; //teacher_id=1
15 SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id
16 -->
17
18 <!--
19 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
20 封装联表查询的数据(去除重复的数据)
21 select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1
22 -->
23 <select id="getClass" parameterType="int" resultMap="ClassResultMap">
24 select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
25 </select>
26 <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
27 <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
28 <id property="id" column="c_id"/>
29 <result property="name" column="c_name"/>
30 <association property="teacher" javaType="me.gacl.domain.Teacher">
31 <id property="id" column="t_id"/>
32 <result property="name" column="t_name"/>
33 </association>
34 </resultMap>
35
36 <!--
37 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
38 SELECT * FROM class WHERE c_id=1;
39 SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值
40 -->
41 <select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
42 select * from class where c_id=#{id}
43 </select>
44 <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
45 <resultMap type="me.gacl.domain.Classes" id="ClassResultMap2">
46 <id property="id" column="c_id"/>
47 <result property="name" column="c_name"/>
48 <association property="teacher" column="teacher_id" select="getTeacher"/>
49 </resultMap>
50
51 <select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">
52 SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
53 </select>
54
55 </mapper>
在conf.xml文件中注册classMapper.xml
<mappers>
<!-- 注册classMapper.xml文件，
classMapper.xml位于me.gacl.mapping这个包下，所以resource写成me/gacl/mapping/classMapper.xml-->
<mapper resource="me/gacl/mapping/classMapper.xml"/>
</mappers>
1.5、编写单元测试代码
1 package me.gacl.test;
2
3 import me.gacl.domain.Classes;
4 import me.gacl.util.MyBatisUtil;
5 import org.apache.ibatis.session.SqlSession;
6 import org.junit.Test;
7
8 public class Test3 {
9
10 @Test
11 public void testGetClass(){
12 SqlSession sqlSession = MyBatisUtil.getSqlSession();
13 /**
14 * 映射sql的标识字符串，
15 * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
16 * getClass是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
17 */
18 String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的标识字符串
19 //执行查询操作，将查询结果自动封装成Classes对象返回
20 Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
21 //使用SqlSession执行完SQL之后需要关闭SqlSession
22 sqlSession.close();
23 System.out.println(clazz);//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
24 }
25
26 @Test
27 public void testGetClass2(){
28 SqlSession sqlSession = MyBatisUtil.getSqlSession();
29 /**
30 * 映射sql的标识字符串，
31 * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
32 * getClass2是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
33 */
34 String statement = "me.gacl.mapping.classMapper.getClass2";//映射sql的标识字符串
35 //执行查询操作，将查询结果自动封装成Classes对象返回
36 Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
37 //使用SqlSession执行完SQL之后需要关闭SqlSession
38 sqlSession.close();
39 System.out.println(clazz);//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
40 }
41 }
1.6、MyBatis一对一关联查询总结
MyBatis中使用association标签来解决一对一的关联查询，association标签可用的属性如下：
- property:对象属性的名称
- javaType:对象属性的类型
- column:所对应的外键字段名称
- select:使用另一个查询封装的结果
二、一对多关联
2.1、提出需求
根据classId查询对应的班级信息,包括学生,老师
2.2、创建表和数据
在上面的一对一关联查询演示中，我们已经创建了班级表和教师表，因此这里再创建一张学生表
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('student_A', 1);
INSERT INTO student(s_name, class_id) VALUES('student_B', 1);
INSERT INTO student(s_name, class_id) VALUES('student_C', 1);
INSERT INTO student(s_name, class_id) VALUES('student_D', 2);
INSERT INTO student(s_name, class_id) VALUES('student_E', 2);
INSERT INTO student(s_name, class_id) VALUES('student_F', 2);
2.3、定义实体类
1、Student类
1 package me.gacl.domain;
2
3 /**
4 * @author gacl
5 * 定义student表所对应的实体类
6 */
7 public class Student {
8
9 //定义属性，和student表中的字段对应
10 private int id; //id===>s_id
11 private String name; //name===>s_name
12
13 public int getId() {
14 return id;
15 }
16
17 public void setId(int id) {
18 this.id = id;
19 }
20
21 public String getName() {
22 return name;
23 }
24
25 public void setName(String name) {
26 this.name = name;
27 }
28
29 @Override
30 public String toString() {
31 return "Student [id=" + id + ", name=" + name + "]";
32 }
33 }
2、修改Classes类，添加一个List<Student> students属性，使用一个List<Student>集合属性表示班级拥有的学生，如下：
1 package me.gacl.domain;
2
3 import java.util.List;
4
5 /**
6 * @author gacl
7 * 定义class表对应的实体类
8 */
9 public class Classes {
10
11 //定义实体类的属性，与class表中的字段对应
12 private int id; //id===>c_id
13 private String name; //name===>c_name
14
15 /**
16 * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
17 * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
18 */
19 private Teacher teacher;
20 //使用一个List<Student>集合属性表示班级拥有的学生
21 private List<Student> students;
22
23 public int getId() {
24 return id;
25 }
26
27 public void setId(int id) {
28 this.id = id;
29 }
30
31 public String getName() {
32 return name;
33 }
34
35 public void setName(String name) {
36 this.name = name;
37 }
38
39 public Teacher getTeacher() {
40 return teacher;
41 }
42
43 public void setTeacher(Teacher teacher) {
44 this.teacher = teacher;
45 }
46
47 public List<Student> getStudents() {
48 return students;
49 }
50
51 public void setStudents(List<Student> students) {
52 this.students = students;
53 }
54
55 @Override
56 public String toString() {
57 return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher
58 + ", students=" + students + "]";
59 }
60 }
2.4、修改sql映射文件classMapper.xml
添加如下的SQL映射信息
1 <!--
2 根据classId查询对应的班级信息,包括学生,老师
3 -->
4 <!--
5 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
6 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
7 -->
8 <select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
9 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}
10 </select>
11 <resultMap type="me.gacl.domain.Classes" id="ClassResultMap3">
12 <id property="id" column="c_id"/>
13 <result property="name" column="c_name"/>
14 <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">
15 <id property="id" column="t_id"/>
16 <result property="name" column="t_name"/>
17 </association>
18 <!-- ofType指定students集合中的对象类型 -->
19 <collection property="students" ofType="me.gacl.domain.Student">
20 <id property="id" column="s_id"/>
21 <result property="name" column="s_name"/>
22 </collection>
23 </resultMap>
24
25 <!--
26 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
27 SELECT * FROM class WHERE c_id=1;
28 SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值
29 SELECT * FROM student WHERE class_id=1 //1是第一个查询得到的c_id字段的值
30 -->
31 <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
32 select * from class where c_id=#{id}
33 </select>
34 <resultMap type="me.gacl.domain.Classes" id="ClassResultMap4">
35 <id property="id" column="c_id"/>
36 <result property="name" column="c_name"/>
37 <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher2"></association>
38 <collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>
39 </resultMap>
40
41 <select id="getTeacher2" parameterType="int" resultType="me.gacl.domain.Teacher">
42 SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
43 </select>
44
45 <select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">
46 SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
47 </select>
2.5、编写单元测试代码
1 package me.gacl.test;
2
3 import me.gacl.domain.Classes;
4 import me.gacl.util.MyBatisUtil;
5 import org.apache.ibatis.session.SqlSession;
6 import org.junit.Test;
7
8 public class Test4 {
9
10 @Test
11 public void testGetClass3(){
12 SqlSession sqlSession = MyBatisUtil.getSqlSession();
13 /**
14 * 映射sql的标识字符串，
15 * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
16 * getClass3是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
17 */
18 String statement = "me.gacl.mapping.classMapper.getClass3";//映射sql的标识字符串
19 //执行查询操作，将查询结果自动封装成Classes对象返回
20 Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
21 //使用SqlSession执行完SQL之后需要关闭SqlSession
22 sqlSession.close();
23 //打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
24 System.out.println(clazz);
25 }
26
27 @Test
28 public void testGetClass4(){
29 SqlSession sqlSession = MyBatisUtil.getSqlSession();
30 /**
31 * 映射sql的标识字符串，
32 * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
33 * getClass4是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
34 */
35 String statement = "me.gacl.mapping.classMapper.getClass4";//映射sql的标识字符串
36 //执行查询操作，将查询结果自动封装成Classes对象返回
37 Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
38 //使用SqlSession执行完SQL之后需要关闭SqlSession
39 sqlSession.close();
40 //打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
41 System.out.println(clazz);
42 }
43 }
2.6、MyBatis一对多关联查询总结
MyBatis中使用collection标签来解决一对多的关联查询，ofType属性指定集合中元素的对象类型。
package com.cy.pj.sys.vo;
import java.util.List;
import lombok.Data;
@Data
public class SysRoleMenuVo {
private Integer id;
private String name;
private String note;
private List<Integer> menuIds;
}
<!--嵌套结果 -->
<resultMap type="com.cy.pj.sys.vo.SysRoleMenuVo"
id="roleMenuMap">
<id column="id" property="id" />
<result column="name" property="name" />
<result column="note" property="note" />
<collection property="menuIds" ofType="java.lang.Integer">
<result property="menuIds" column="menuIds" />
</collection>
</resultMap>
<select id="doFindRoleObjectById" resultMap="roleMenuMap"> select
r.id,r.name,r.note,m.menu_id
menuIds from sys_roles r ,sys_role_menus m
where m.role_id = r.id and r.id=#{id}
</select>
<!-- 嵌套查询 -->
<resultMap type="com.cy.pj.sys.vo.SysRoleMenuVo"
id="roleMenuMap">
<id column="id" property="id" />
<result column="name" property="name" />
<result column="note" property="note" />
<collection property="menuIds" column="id"
ofType="java.lang.Integer"
select="com.cy.pj.sys.dao.SysRoleMenuDao.doFindRoleObjectById"></collection>
</resultMap>
<select id="doFindRoleObjectById" resultMap="roleMenuMap"> select
id,name,note from sys_roles where id = #{id}
</select>
<!--嵌套查询 -->
<resultMap type="java.util.Map" id="roleMenuMap3">
<id column="id" property="id" />
<result column="name" property="name" />
<result column="note" property="note" />
<collection property="menuIds" column="id"
select="com.cy.pj.sys.dao.SysRoleMenuDao.doFindRoleObjectById"></collection>
</resultMap>
<select id="doFindRoleObjectById3" parameterType="int"
resultMap="roleMenuMap3">
select id,name,note from sys_roles where id = #{id}
</select>
