平平无奇的灌木丛

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题解 | #插入记录（三）#

当表中已经存在某条数据现在想要插入一条新的覆盖他可以 update 修改它可以 delete 删除再添加可以 replace 替代它 # 方法一 # delete from examination_info where exam_id = 9003; # insert into examin...

Mysql

2022-01-12

0 288

题解 | #插入记录（二）#

复制一张表的数据到另外一个结构完全一样的表中自增id列用null insert into exam_record_before_2021 select null, uid, exam_id, start_time, submit_time, score from exam_record wher...

Mysql

2022-01-12

0 249

题解 | #浙大不同难度题目的正确率#

(sum(case when qpd.result = 'right' then 1 else 0 end) / count(qpd.result)) as correct_rated select qd.difficult_level, (sum(case when qpd.result = ...

Mysql

2022-01-11

0 443

题解 | #21年8月份练题总数#

select count(distinct device_id) as did_cnt, count(question_id) as question_cnt from question_practice_detail where year(date) = 2021 and month(date)...

Mysql

2022-01-11

0 317

题解 | #统计复旦用户8月练题情况#

sum(if(qpd.result = 'right', 1, 0) as right_question_cnt) select up.device_id, up.university, count(question_id) as question_cnt, sum(if(qpd.r...

Mysql

2022-01-11

0 330

题解 | #找出每个学校GPA最低的同学#

方法一：left join select p.device_id, u.university, u.gpa from (select university, min(gpa) as gpa from user_profile group by university) u left join user...

Mysql

2022-01-11

0 319

题解 | #截取出年龄#

select SUBSTRING_INDEX(SUBSTRING_INDEX(PROFILE, ',', -2), ',', 1) as age, count(*) as number from user_submit group by age; substring_index() 受到上...

Mysql

2022-01-11

0 323

题解 | #统计每种性别的人数#

SELECT substring_index(profile, ',', -1) as gender, COUNT(*) AS number FROM user_submit group by gender;

Mysql

2022-01-11

0 287

题解 | #计算用户的平均次日留存率#

select count(b.device_id) / count(a.device_id) from ( select distinct device_id, date from question_practice_detail ) as a left join ( sel...

Mysql

2022-01-11

0 277

题解 | #计算用户8月每天的练题数量#

select day(date) as day, count(question_id) as question_cnt from question_practice_detail where month(date) = 8 group by date day month year 从日期中分别取出...

Mysql

2022-01-11

0 267