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题解 | #SQL类别高难度试卷得分的截断平均值#

select tag,difficulty,round((sum(score)-max(score)-min(score))/(count(score)-2),1) from examination_info ei join exam_record ...

Mysql

2021-10-25

18 1400

题解 | #删除索引#

方法一： alter table examination_info drop index uniq_idx_exam_id; alter table examination_info drop index&nb...

Mysql

2021-10-25

21 1535

题解 | #创建索引#

create index idx_duration on examination_info(duration); create unique index uniq_idx_exam_id on examination_info(e...

Mysql

2021-10-25

4 685

题解 | #删除表#

drop table if exists exam_record_2011; drop table if exists exam_record_2012; drop table if exists exam_record_2013; drop table if exists exam_record_...

Mysql

2021-10-25

1 481

题解 | #修改表#

alter table user_info add column school varchar(15) after level; alter table user_info change job profession varchar(10) ; alter table user_info modi...

Mysql

2021-10-25

15 1297

题解 | #创建一张新表#

create table user_info_vip( id int(11) primary key auto_increment comment'自增ID', uid int(11) unique not null comment '用户ID', nick_name varchar(64) co...

Mysql

2021-10-25

0 466

题解 | #提取博客URL中的用户名#

select device_id, SUBSTRING(blog_url,11) as username from user_submit substring(str,n,m)表示截取str,从第n位开始，一共截取m个字符，如果没有m,就是表示一直截取到最后

Mysql

2021-10-22

13 1162

题解 | #计算用户的平均次日留存率#

select count(q2.device_id)/count(q1.device_id) as avg_ret from (select distinct device_id,date from question_practice_detail) as q1 left join (selec...

Mysql

2021-10-22

16 764

题解 | #查找山东大学男生的GPA#

select device_id,gender,age,gpa from user_profile where university='山东大学' union all select device_id,gender,age,gpa from user_profile where gender='...

Mysql

2021-10-21

2 491

题解 | #查询结果限制返回行数#

1.select device_id from user_profile limit 0,2 2.select device_id from user_profile limit 2 offset 2...

Mysql

2021-10-18

10 1712