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。
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CREATE TABLE teacher( t_id INT PRIMARY KEY AUTO_INCREMENT, t_name VARCHAR() ); CREATE TABLE class ( c_id INT PRIMARY KEY AUTO_INCREMENT, c_name VARCHAR(), 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( 'teacher' ); INSERT INTO teacher(t_name) VALUES( 'teacher' ); INSERT INTO class (c_name, teacher_id) VALUES( 'class_a' , ); 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。
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package me.gacl.domain; /** * @author gacl * 定义teacher表对应的实体类 */ public class Teacher { //定义实体类的属性,与teacher表中的字段对应 private int id; //id===>t_id private String name; //name===>t_name public int getId() { return id; } public void setId( int id) { this .id = id; } public String getName() { return name; } public void setName(String name) { this .name = name; } @Override public String toString() { return "Teacher [id=" + id + ", name=" + name + "]" ; } } |
2、Classes类,Classes类是class表对应的实体类
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package me.gacl.domain; /** * @author gacl * 定义class表对应的实体类 */ public class Classes { //定义实体类的属性,与class表中的字段对应 private int id; //id===>c_id private String name; //name===>c_name /** * class表中有一个teacher_id字段,所以在Classes类中定义一个teacher属性, * 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 */ private Teacher teacher; public int getId() { return id; } public void setId( int id) { this .id = id; } public String getName() { return name; } public void setName(String name) { this .name = name; } public Teacher getTeacher() { return teacher; } public void setTeacher(Teacher teacher) { this .teacher = teacher; } @Override public String toString() { return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher+ "]" ; } } |
1.4、定义sql映射文件classMapper.xml
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<?xml version= "." encoding= "UTF-" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper .//EN" "http://mybatis.org/dtd/mybatis--mapper.dtd" > <!-- 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 例如namespace= "me.gacl.mapping.classMapper" 就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后缀) --> <mapper namespace= "me.gacl.mapping.classMapper" > <!-- 根据班级id查询班级信息(带老师的信息) ##. 联表查询 SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=; ##. 执行两次查询 SELECT * FROM class WHERE c_id=; //teacher_id= SELECT * FROM teacher WHERE t_id=; //使用上面得到的teacher_id --> <!-- 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= --> <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> <!-- 使用resultMap映射实体类和字段之间的一一对应关系 --> <resultMap type= "me.gacl.domain.Classes" id= "ClassResultMap" > <id property= "id" column= "c_id" /> <result property= "name" column= "c_name" /> <association property= "teacher" javaType= "me.gacl.domain.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=; SELECT * FROM teacher WHERE t_id= // 是上一个查询得到的teacher_id的值 --> <select id= "getClass" parameterType= "int" resultMap= "ClassResultMap" > select * from class where c_id=#{id} </select> <!-- 使用resultMap映射实体类和字段之间的一一对应关系 --> <resultMap type= "me.gacl.domain.Classes" id= "ClassResultMap" > <id property= "id" column= "c_id" /> <result property= "name" column= "c_name" /> <association property= "teacher" column= "teacher_id" select= "getTeacher" /> </resultMap> <select id= "getTeacher" parameterType= "int" resultType= "me.gacl.domain.Teacher" > SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id} </select> </mapper> |
在conf.xml文件中注册classMapper.xml
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<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
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package me.gacl.test; import me.gacl.domain.Classes; import me.gacl.util.MyBatisUtil; import org.apache.ibatis.session.SqlSession; import org.junit.Test; public class Test { @Test public void testGetClass(){ SqlSession sqlSession = MyBatisUtil.getSqlSession(); /** * 映射sql的标识字符串, * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值, * 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 */ String statement = "me.gacl.mapping.classMapper.getClass" ; //映射sql的标识字符串 //执行查询操作,将查询结果自动封装成Classes对象返回 Classes clazz = sqlSession.selectOne(statement,); //查询class表中id为的记录 //使用SqlSession执行完SQL之后需要关闭SqlSession sqlSession.close(); System.out.println(clazz); //打印结果:Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher]] } @Test public void testGetClass(){ SqlSession sqlSession = MyBatisUtil.getSqlSession(); /** * 映射sql的标识字符串, * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值, * 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 */ String statement = "me.gacl.mapping.classMapper.getClass" ; //映射sql的标识字符串 //执行查询操作,将查询结果自动封装成Classes对象返回 Classes clazz = sqlSession.selectOne(statement,); //查询class表中id为的记录 //使用SqlSession执行完SQL之后需要关闭SqlSession sqlSession.close(); System.out.println(clazz); //打印结果:Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher]] } } |
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
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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
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package me.gacl.domain; /** * @author gacl * 定义student表所对应的实体类 */ public class Student { //定义属性,和student表中的字段对应 private int id; //id===>s_id private String name; //name===>s_name public int getId() { return id; } public void setId( int id) { this .id = id; } public String getName() { return name; } public void setName(String name) { this .name = name; } @Override public String toString() { return "Student [id=" + id + ", name=" + name + "]" ; } } |
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:
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package me.gacl.domain; import java.util.List; /** * @author gacl * 定义class表对应的实体类 */ public class Classes { //定义实体类的属性,与class表中的字段对应 private int id; //id===>c_id private String name; //name===>c_name /** * class表中有一个teacher_id字段,所以在Classes类中定义一个teacher属性, * 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 */ private Teacher teacher; //使用一个List<Student>集合属性表示班级拥有的学生 private List<Student> students; public int getId() { return id; } public void setId( int id) { this .id = id; } public String getName() { return name; } public void setName(String name) { this .name = name; } public Teacher getTeacher() { return teacher; } public void setTeacher(Teacher teacher) { this .teacher = teacher; } public List<Student> getStudents() { return students; } public void setStudents(List<Student> students) { this .students = students; } @Override public String toString() { return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher + ", students=" + students + "]" ; } } |
2.4、修改sql映射文件classMapper.xml
添加如下的SQL映射信息
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<!-- 根据classId查询对应的班级信息,包括学生,老师 --> <!-- 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= --> <select id= "getClass" parameterType= "int" resultMap= "ClassResultMap" > 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= "me.gacl.domain.Classes" id= "ClassResultMap" > <id property= "id" column= "c_id" /> <result property= "name" column= "c_name" /> <association property= "teacher" column= "teacher_id" javaType= "me.gacl.domain.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= "me.gacl.domain.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=; SELECT * FROM teacher WHERE t_id= // 是上一个查询得到的teacher_id的值 SELECT * FROM student WHERE class_id= //是第一个查询得到的c_id字段的值 --> <select id= "getClass" parameterType= "int" resultMap= "ClassResultMap" > select * from class where c_id=#{id} </select> <resultMap type= "me.gacl.domain.Classes" id= "ClassResultMap" > <id property= "id" column= "c_id" /> <result property= "name" column= "c_name" /> <association property= "teacher" column= "teacher_id" javaType= "me.gacl.domain.Teacher" select= "getTeacher" ></association> <collection property= "students" ofType= "me.gacl.domain.Student" column= "c_id" select= "getStudent" ></collection> </resultMap> <select id= "getTeacher" parameterType= "int" resultType= "me.gacl.domain.Teacher" > SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id} </select> <select id= "getStudent" parameterType= "int" resultType= "me.gacl.domain.Student" > SELECT s_id id, s_name name FROM student WHERE class_id=#{id} </select> |
2.5、编写单元测试代码
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package me.gacl.test; import me.gacl.domain.Classes; import me.gacl.util.MyBatisUtil; import org.apache.ibatis.session.SqlSession; import org.junit.Test; public class Test { @Test public void testGetClass(){ SqlSession sqlSession = MyBatisUtil.getSqlSession(); /** * 映射sql的标识字符串, * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值, * 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 */ String statement = "me.gacl.mapping.classMapper.getClass" ; //映射sql的标识字符串 //执行查询操作,将查询结果自动封装成Classes对象返回 Classes clazz = sqlSession.selectOne(statement,); //查询class表中id为的记录 //使用SqlSession执行完SQL之后需要关闭SqlSession sqlSession.close(); //打印结果:Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher], students=[Student [id=, name=student_A], Student [id=, name=student_B], Student [id=, name=student_C]]] System.out.println(clazz); } @Test public void testGetClass(){ SqlSession sqlSession = MyBatisUtil.getSqlSession(); /** * 映射sql的标识字符串, * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值, * 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 */ String statement = "me.gacl.mapping.classMapper.getClass" ; //映射sql的标识字符串 //执行查询操作,将查询结果自动封装成Classes对象返回 Classes clazz = sqlSession.selectOne(statement,); //查询class表中id为的记录 //使用SqlSession执行完SQL之后需要关闭SqlSession sqlSession.close(); //打印结果:Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher], students=[Student [id=, name=student_A], Student [id=, name=student_B], Student [id=, name=student_C]]] System.out.println(clazz); } } |
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。
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