牛客网mysql刷题记录

牛客网mysql刷题记录

• SQL26 计算25岁以上和以下的用户数量
  if(expression,A,B)

select if(age < 25 or age is null, '25岁以下','25岁及以上') as age_cut,
count(u.device_id) 
from user_profile u group by age_cut;

• SQL27 查看不同年龄段的用户明细

select device_id,gender, 
(case when age is null then '其他'
    when age<20 then '20岁一下' 
    when age<24 then '20-24岁'
    else '25岁及以上' end) as age_cut from user_profile;

- SQL28 计算用户8月每天的练题数量

当有字段名和系统字段名重名时，需要表上加别名。

select substr(q.date,9,2) as day,count(substr(q.date,1,7)) as question_cnt from question_practice_detail q
where q.date like '%-08-%'
group by substr(q.date,9,2)

• SQL29 计算用户的平均次日留存率
  MySQL中 COUNT在对列进行计数时不统计值为 null的条目
  有重复需去重

select 
  count(distinct q2.device_id,q2.date)/count(distinct q1.device_id,q1.date) as avg_ret 
from question_practice_detail q1 left join question_practice_detail q2 
on q1.device_id=q2.device_id and datediff(q2.date,q1.date) = 1;


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
    (SELECT DISTINCT device_id, date FROM question_practice_detail) AS q2
ON q1.device_id = q2.device_id AND q2.date = DATE_ADD(q1.date, interval 1 day)

• SQL30 统计每种性别的人数

从字段中截取值后匹配

SELECT IF(profile LIKE '%female%','female','male') gender,COUNT(*) number
FROM user_submit
GROUP BY gender;

• SQL33 找出每个学校GPA最低的同学

select device_id,university,gpa from user_profile where (university,gpa) in (select university,min(gpa) from user_profile group by university) order by university

SELECT device_id,university,gpa FROM
(SELECT device_id,university,gpa,
RANK() over (PARTITION BY university ORDER BY gpa) rk FROM user_profile) a
WHERE a.rk=1;

• SQL34 统计复旦用户8月练题情况

2. 用到的函数

if(expression, A,B)
case when else end
substr(str,i)
substr(str,i,j)
固定字符串的截取 substring_index(blog_url,‘/’,-1) http:/ur/tiger 提取出tiger