Mybatis入门（四）------联表查询

Mybatis联表查询

一、1对1查询

1、数据库建表

假设一个老师带一个学生

CREATE TABLE teacher(
    t_id INT PRIMARY KEY,
    t_name VARCHAR(30)
);
CREATE TABLE student(
    s_id INT PRIMARY KEY,
    s_name VARCHAR(30),
    t_id INT ,
    FOREIGN KEY(t_id) REFERENCES teacher(t_id)
);
INSERT INTO teacher VALUES(1, "wanglaoshi");
INSERT INTO student VALUES(1, "zhangsan", 1);

2、TeacherMapper.xml配置

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mybatis.mapper.TeacherMapper">
    <!-- 关联查询中你需要告诉 MyBatis 如何加载关联。MyBatis 在这方面会有两种不同的方式:  -->
    <!-- 1、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 -->
    <resultMap type="com.mybatis.model.Student" id="Student1">
        <id column="s_id" property="id"></id>
        <result column="s_name" property="name"></result>
        <association property="teacher" javaType="com.mybatis.model.Teacher">
            <id column="t_id" property="id"></id>
            <result column="t_name" property="name"></result>
        </association>
    </resultMap>

    <select id="selectStudent1" resultMap="Student1">
        select * from student s, teacher t where t.t_id = s.t_id and s.s_id = #{id}
    </select>
    <!-- 2、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 -->
    <resultMap type="com.mybatis.model.Student" id="Student2">
        <id column="s_id" property="id"></id>
        <result column="s_name" property="name"></result>
        <association column="t_id" property="teacher" select="selectTeacher"></association>
    </resultMap>

    <select id="selectStudent2" resultMap="Student2">
        select * from student s, teacher t where t.t_id = s.t_id and s.s_id = #{id}
    </select>

    <select id="selectTeacher" resultType="com.mybatis.model.Teacher">
        select t_id id, t_name name from teacher t where t.t_id = #{id}
    </select>
</mapper>

3、在Configuration.xml中注册TeacherMapper.xml

 <mappers>
    <!-- 注册UserMapper.xml文件 -->
    <mapper resource="com/mybatis/mapper/UserMapper.xml"/>
    <!-- 注册TeacherMapper.xml文件 -->
    <mapper resource="com/mybatis/mapper/TeacherMapper.xml"/>
    <!-- 注册UserMapperI接口 -->
    <mapper class="com.mybatis.mapper.UserMapperI"/>
  </mappers>

4、编写JavaBean

public class Teacher {
    //主键
    private String id;
    //姓名
    private String name;
    //学生
    private Student student;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Student getStudent() {
        return student;
    }
    public void setStudent(Student student) {
        this.student = student;
    }
}

public class Student {
    //主键
    private String id;
    //姓名
    private String name;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

5、单元测试代码

public class DbTest4 {

    private SqlSessionFactory sqlSessionFactory;

    @Before
    public void init() throws IOException {
        //通过mybatis提供的资源加载类加载配置文件
        Reader reader = Resources.getResourceAsReader("Configuration.xml");
        //获取SqlSessionFactoryBuilder
        SqlSessionFactoryBuilder sqlSessionFactroyBuilder = new SqlSessionFactoryBuilder();
        //根据XML配置文件构建SqlSessionFactory
        SqlSessionFactory sqlSessionFactory = sqlSessionFactroyBuilder.build(reader);
        this.sqlSessionFactory = sqlSessionFactory;
    }

    @Test
    public void queryStudent1() {
        SqlSession session = sqlSessionFactory.openSession(true);
        try {
            Teacher teacher = session.selectOne("com.mybatis.mapper.TeacherMapper.selectTeacher1", 1);
            System.out.println(JsonUtils.objectToJsonString(teacher));
        } finally {
            session.close();
        }
    }

    @Test
    public void queryStudent2() {
        SqlSession session = sqlSessionFactory.openSession(true);
        try {
            Teacher teacher = session.selectOne("com.mybatis.mapper.TeacherMapper.selectTeacher2", 1);
            System.out.println(JsonUtils.objectToJsonString(teacher));
        } finally {
            session.close();
        }
    }
}

6、输出结果

嵌套结果：

嵌套查询

二、1对多查询

1、数据库建表

CREATE TABLE task(
    task_id INT PRIMARY KEY,
    description VARCHAR(50),
    score DOUBLE,
    s_id INT,
    FOREIGN KEY(s_id) REFERENCES student(s_id)
);
INSERT INTO task VALUES(1, "数学作业", "100", 1);
INSERT INTO task VALUES(2, "英语作业", "98", 1);

2、TeacherMapper.xml配置

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mybatis.mapper.TeacherMapper">
    <resultMap type="com.mybatis.model.Teacher" id="Teacher">
        <id column="t_id" property="id"></id>
        <result column="t_name" property="name"></result>
        <association property="student" javaType="com.mybatis.model.Student">
            <id column="s_id" property="id"></id>
            <result column="s_name" property="name"></result>
            <collection property="tasks" ofType="com.mybatis.model.Task">
                <id column="task_id" property="id"></id>
                <result column="description" property="description"></result>
                <result column="score" property="score"></result>
            </collection>
        </association>
    </resultMap>

    <select id="selectTeacher" resultMap="Teacher">
        select * from student s, teacher t, task e where t.t_id = s.t_id and e.s_id = s.s_id and t.t_id = #{id}
    </select>
</mapper>

3、编写JavaBean

public class Teacher {
    //主键
    private String id;
    //姓名
    private String name;
    //学生
    private Student student;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Student getStudent() {
        return student;
    }
    public void setStudent(Student student) {
        this.student = student;
    }
}

public class Student {
    //主键
    private String id;
    //姓名
    private String name;
    //作业集合
    private List<Task> tasks;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public List<Task> getTasks() {
        return tasks;
    }
    public void setTasks(List<Task> tasks) {
        this.tasks = tasks;
    }
}

public class Task {
    //主键
    private String id;
    //描述
    private String description;
    //分数
    private double score;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    public double getScore() {
        return score;
    }
    public void setScore(double score) {
        this.score = score;
    }
}

4、单元测试代码

public class DbTest5 {

    private SqlSessionFactory sqlSessionFactory;

    @Before
    public void init() throws IOException {
        //通过mybatis提供的资源加载类加载配置文件
        Reader reader = Resources.getResourceAsReader("Configuration.xml");
        //获取SqlSessionFactoryBuilder
        SqlSessionFactoryBuilder sqlSessionFactroyBuilder = new SqlSessionFactoryBuilder();
        //根据XML配置文件构建SqlSessionFactory
        SqlSessionFactory sqlSessionFactory = sqlSessionFactroyBuilder.build(reader);
        this.sqlSessionFactory = sqlSessionFactory;
    }

    @Test
    public void queryStudent() {
        SqlSession session = sqlSessionFactory.openSession(true);
        try {
            Teacher teacher = session.selectOne("com.mybatis.mapper.TeacherMapper.selectTeacher", 1);
            System.out.println(JsonUtils.objectToJsonString(teacher));
        } finally {
            session.close();
        }
    }
}

5、输出结果