1,实体类：

/**
 * <p>
 * 课程
 * </p>
 *
 * @author testjava
 * @since 2020-03-02
 */
@Data
@EqualsAndHashCode(callSuper = false)
@Accessors(chain = true)
@ApiModel(value="EduCourse对象", description="课程")
public class EduCourse implements Serializable {

    private static final long serialVersionUID = 1L;

    @ApiModelProperty(value = "课程ID")
    @TableId(value = "id", type = IdType.ID_WORKER_STR)
    private String id;

    @ApiModelProperty(value = "课程讲师ID")
    private String teacherId;

    @ApiModelProperty(value = "二级分类ID")
    private String subjectId;

    @ApiModelProperty(value = "一级分类级ID")
    private String subjectParentId;

    @ApiModelProperty(value = "课程标题")
    private String title;

    @ApiModelProperty(value = "课程销售价格，设置为0则可免费观看")
    private BigDecimal price;

    @ApiModelProperty(value = "总课时")
    private Integer lessonNum;

    @ApiModelProperty(value = "课程封面图片路径")
    private String cover;

    @ApiModelProperty(value = "销售数量")
    private Long buyCount;

    @ApiModelProperty(value = "浏览数量")
    private Long viewCount;

    @ApiModelProperty(value = "乐观锁")
    private Long version;

    @ApiModelProperty(value = "课程状态 Draft未发布  Normal已发布")
    private String status;

    @ApiModelProperty(value = "逻辑删除 1（true）已删除， 0（false）未删除")
    private Integer isDeleted;

    @ApiModelProperty(value = "创建时间")
    @TableField(fill = FieldFill.INSERT)
    private Date gmtCreate;

    @ApiModelProperty(value = "更新时间")
    @TableField(fill = FieldFill.INSERT_UPDATE)
    private Date gmtModified;


}

2.然后创建要查询出信息的vo类

@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
}

3.去相对应的Mapper中创建接口

/**
 * <p>
 * 课程 Mapper 接口
 * </p>
 *
 * @author testjava
 * @since 2020-03-02
 */
public interface EduCourseMapper extends BaseMapper<EduCourse> {

    public CoursePublishVo getPublishCourseInfo(String courseId);
}

4.然后再去xml，写sql语句

<?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.atguigu.eduservice.mapper.EduCourseMapper">
    <!--sql语句：根据课程id查询课程确认信息-->
    <select id="getPublishCourseInfo" resultType="com.atguigu.eduservice.entity.vo.CoursePublishVo">
        SELECT ec.id,ec.title,ec.price,ec.lesson_num AS lessonNum,ec.cover,
               et.name AS teacherName,
               es1.title AS subjectLevelOne,
               es2.title AS subjectLevelTwo
        FROM edu_course ec LEFT OUTER JOIN edu_course_description ecd ON ec.id=ecd.id
                           LEFT OUTER JOIN edu_teacher et ON ec.teacher_id=et.id
                           LEFT OUTER JOIN edu_subject es1 ON ec.subject_parent_id=es1.id
                   LEFT OUTER JOIN edu_subject es2 ON ec.subject_id=es2.id
        WHERE ec.id=#{courseId}
    </select>
</mapper>

5.去Controller里填写路径和调用

    //根据课程id查询课程确认信息
    @GetMapping("getPublishCourseInfo/{id}")
    public R getPublishCourseInfo(@PathVariable String id) {
        CoursePublishVo coursePublishVo = courseService.publishCourseInfo(id);
        return R.ok().data("publishCourse",coursePublishVo);
    }

6.Service填写接口

    //根据课程id查询课程确认信息
    CoursePublishVo publishCourseInfo(String id);

7.最后去Servicelmpl里去调用Mapper

    //根据课程id查询课程确认信息
    @Override
    public CoursePublishVo publishCourseInfo(String id) {
        //调用mapper
        CoursePublishVo publishCourseInfo = baseMapper.getPublishCourseInfo(id);
        return publishCourseInfo;
    }