Mybatis连表查询

Mybatis 连表查询

准备

创建数据表

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for student
-- ----------------------------
CREATE TABLE `student`  (
  `id` int(255) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
  `teacher_id` int(11) NOT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 4 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES (1, '小王', 1);
INSERT INTO `student` VALUES (2, '小李', 1);
INSERT INTO `student` VALUES (3, '小孙', 1);

SET FOREIGN_KEY_CHECKS = 1;


-- ----------------------------
-- Table structure for teacher
-- ----------------------------
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 2 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of teacher
-- ----------------------------
INSERT INTO `teacher` VALUES (1, '张老师');

SET FOREIGN_KEY_CHECKS = 1;

创建实体类

public class Student {
    private int id;
    private String name;
    private Teacher teacher;

    public Student(int id, String name, Teacher teacher) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
    }

    public Student() {
    }

    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 "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", teacher=" + teacher +
                '}';
    }
}

public class Teacher {
    private int id;
    private String name;

    public Teacher(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public 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;
    }

    @Override
    public String toString() {
        return "Teacher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

多对一

创建接口类

public interface StudentDao {
    List<Student> getStudentList();
}

配置xml

<?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.example.dao.StudentDao">


    <select id="getStudentList" resultMap="studentResultMap">
        select
        s.id, s.name as s_name, t.name as t_name
        from
        student as s, teacher as t
        where
        s.teacher_id = t.id;
    </select>

    <resultMap id="studentResultMap" type="com.example.pojo.Student">
        <result property="id" column="id"/>
        <result property="name" column="s_name"/>
        <association property="teacher" javaType="com.example.pojo.Teacher">
            <result property="name" column="t_name"/>
        </association>
    </resultMap>

</mapper>

在mybatis里面注册

 <mappers>
        <mapper resource="studentMapper.xml"/>
    </mappers>

一对多

实体类

import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;
    private int teacherId;
}

import lombok.Data;

import java.util.List;

@Data
public class Teacher {
    private int id;
    private String name;
    private List<Student> students;

}

接口类

import com.example.pojo.Teacher;
import org.apache.ibatis.annotations.Param;


public interface TeacherDao {

    Teacher getTeacherById(@Param("tid") int id);
}

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.example.dao.TeacherDao">


    <select id="getTeacherById" resultMap="teacherResultMap">
        select
            s.id as s_id, s.name as s_name, s.teacher_id as st_id, t.name as t_name
        from
            student as s, teacher as t
        where
            s.teacher_id = t.id and t.id = #{tid};
    </select>

    <resultMap id="teacherResultMap" type="com.example.pojo.Teacher">
        <result property="id" column="id"/>
        <result property="name" column="t_name"/>
        <collection property="students" ofType="com.example.pojo.Student">
            <result property="id" column="s_id"/>
            <result property="name" column="s_name"/>
            <result property="teacherId" column="st_id"/>
        </collection>
    </resultMap>

</mapper>

最后将xml配置到mybatis的配置里面

<mappers>
        <mapper resource="teacherMapper.xml"/>
    </mappers>