多表查询

　　查询多张表，每张表取一个或多个字段，返回相关数据。先根据查询的字段创建vo类，再编写sql语句，使用mybatis查询数据库数据

@Data
public class CoursePublishVo {
    private String id;
    private String title;
    private String cover;
    private Integer lessonNum;
    private String subjectLevelOne;
    private String subjectLevelTwo;
    private String teacherName;
    private String price;//只用于显示
}

 

<?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.swimming.serviceedu.mapper.EduCourseMapper">

    <select id="getCoursePublishInfo" resultType="com.swimming.serviceedu.entity.vo.CoursePublishVo">
        SELECT ec.id, ec.title, ec.price, ec.cover, ec.lesson_num as lessonNum,
            et.`name` as teacherName,
            es1.title as subjectLevelOne,
            es2.title as subjectLevelTwo
    from edu_course ec LEFT JOIN edu_teacher et on ec.teacher_id = et.id
                    LEFT JOIN edu_subject es1 on ec.subject_parent_id = es1.id
                    LEFT JOIN edu_subject es2 on ec.subject_id = es2.id
    where ec.id = #{courseId}
    </select>
</mapper>