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题解 | #牛客直播各科目同时在线人数#

select b.course_id,ct.course_name,max(uv) as max_num from ( select course_id,timestamp, sum(uv) over(partition by course_id order by timestamp) as uv ...

2023-10-03

0 281

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_...

2023-09-30

0 175

题解 | #修改表#

ALTER TABLE user_info ADD column school varchar(15) AFTER LEVEL; ALTER table user_info CHANGE column job profession varchar(10); ALTER TABLE user_info...

2023-09-30

0 239

题解 | #删除记录（三）#

truncate table nowcoder_exam_record; 方法一、truncate情况表中的记录并重置自增id。[使用]：truncate table[注意]：truncate只能作用于表；truncate会清空表中的所有行，但表结构及其约束、索引等保持不变；truncate会重置表...

2023-09-30

0 340

题解 | #删除记录（二）#

delete from exam_record where submit_time is null or timestampdiff(minute, start_time, submit_time) < 5 order by start_time limit 3 在得到满足条件后，再去求得前...

2023-09-30

0 212

题解 | #删除记录（一）#

delete from exam_record where submit_time<date_sub(start_time,interval -5 minute) and score < 60; 在涉及到时间计算时，可以转时间戳，也可以直接使用interval进行计算。

2023-09-30

0 173

题解 | #插入记录（三）#

REPLACE INTO examination_info VALUES(NULL,9003,'SQL','hard',90,'2021-01-01 00:00:00'); 关键字NULL可以用DEFAULT替代。掌握replace into···values的用法replace into 跟 in...

2023-09-29

0 207

题解 | #插入记录（二）#

insert into exam_record_before_2021(uid,exam_id,start_time,submit_time,score) select uid,exam_id,start_time,submit_time,score from exam_record where Y...

2023-09-29

0 201

题解 | #列出供应商及其可供产品的数量#

select Vendors.vend_id,cast(sum(if(Products.prod_id is null,0,1)) AS UNSIGNED) as prod_id from Vendors left join Products on Vendors.vend_id = Product...

2023-09-29

0 211

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)...

2023-09-26

0 153