题解 | #每月各旬有多少人练题#

select 
    concat(date_format(event_date,'%Y年%m月'),
    case 
        when day(event_date)<10 then'上旬'
        when day(event_date)<20 then'中旬'
        else '下旬' end) AS dt_range,
    count(distinct device_id) as cnt
FROM question_practice_detail
group by dt_range
order by substr(dt_range,6,2) desc,cnt desc

date_format()

concat()

substr()

case when

使用以上3个函数以case when 语句