有四张表
用户表:student 列名:stu_id,stu_name
学生分数表:student_score 列名:stu_id,score
班级表:class 列名:class_id,class_name
班级用户表:class_student 列名:class_id,stu_id
使用SQL语句实现:
1、实现学生的分数>80,并且<90的stu_id,stu_name,score
2、实现获取班级名称为'实验班'的班级的平均分
3、获取班级人数>40的班级,输出班级id,班级名称,班级人数
解:
1、
mysql> select student.stu_id,student.stu_name,student_score.score from student -> left join student_score -> on student.stu_id = student_score.stu_id -> where student_score.score between 80 and 90;
解析:
用到二表左连,生成临时表格后输出 stu_id,stu_name,score三列,其中where子句使用between and语法限定条件>80<90的值
2、
mysql> select class.class_name,avg(score) -> from class inner join class_student -> on class.class_id = class_student.class_id -> join student_score -> on class_student.stu_id = student_score.stu_id -> where class.class_name = '实验班';
解析:
用到三表内连,生成临时表格后输出class_name,avg(score)两列,其中where子句限定class_name='实验班'
3、
mysql> select class.class_id,class.class_name,count(class.class_name) -> from class inner join class_student -> on class.class_id = class_student.class_id -> group by class.class_name -> having count(class.class_name) > 40;
解析:
用到二表内连,生成临时表格后输出class_id,class_name,class_name的数量,其中having语句实现条件人数>40。注意,生成临时表格后,需要使用group by语句才能实现表格内容的分组,否则不能正确输出having后跟的count函数条件
mysql初学,欢迎评论指正拍砖,谢谢!