sql的日期函数

 常用日期函数&＃xff1a;

select
current_date() as now,
date_add(current_date(), interval -1 day) as yesterday,
datediff(current_date(),date_add(current_date(), interval -1 day)) as date_cha,
date_format(current_date(),&＃39;%Y/%m&＃39;) as ym,
str_to_date(&＃39;2020-05-01&＃39;,&＃39;%Y-%m-%d&＃39;) as strdate1,
str_to_date(&＃39;2020.05.01&＃39;,&＃39;%Y.%m.%d&＃39;) as strdate2

例题&＃xff1a; 

计算两个日期之间差的天数<90&＃xff1a;datediff(&＃39;2019-06-30&＃39;,login_date)<&＃61;90

编写一个 SQL 查询&＃xff0c;以查询从今天起最多 90 天内&＃xff0c;每个日期该日期首次登录的用户数。假设今天是 2019-06-30.

select user_id, min(activity_date) login_date
from Traffic
where activity&＃61;&＃39;login&＃39;
group by user_id

select login_date,count(user_id) user_count
from (select user_id, min(activity_date) login_date from Traffic
where activity&＃61;&＃39;login&＃39;
group by user_id) t
where datediff(&＃39;2019-06-30&＃39;,login_date)<&＃61;90
group by login_date;