题解 39 | #21年8月份练题总数#

【场景】：某月的情况 【分类】：聚合函数、日期函数、字符串函数

分析思路

难点：

1.计算总用户数时要对用户去重

2021年8月可以怎么写

• [使用]：month和year
• [使用]：date_format
• [使用]：like
• [使用]：mid
• [使用]：left
• [使用]：substring_index

扩展

前往查看：MySQL 字符串截取 left、right、substring、substring_index

求解代码

方法一： month和year

select
    count(distinct device_id) as did_cnt,
    count(device_id) as question_cnt
from question_practice_detail
where month(date) = 8 and year(date) = 2021

方法二： date_format

select
    count(distinct device_id) as did_cnt,
    count(device_id) as question_cnt
from question_practice_detail
where date_format(date,'%Y%m') = '202108'

方法三： date_format

select
    count(distinct device_id) as did_cnt,
    count(device_id) as question_cnt
from question_practice_detail
where date_format(date,'%y%m') = '2108'  

方法四： like

select
    count(distinct device_id) as did_cnt,
    count(device_id) as question_cnt
from question_practice_detail
where date like '2021-08%'  

方法五： substring

select
    count(distinct device_id) as did_cnt,
    count(device_id) as question_cnt
from question_practice_detail
where substring(date,1,7) = '2021-08'  

方法六： mid

select
    count(distinct device_id) as did_cnt,
    count(device_id) as question_cnt
from question_practice_detail
where mid(date,1,7) = '2021-08' 

方法七： left

select
    count(distinct device_id) as did_cnt,
    count(device_id) as question_cnt
from question_practice_detail
where left(date,7) = '2021-08' 

方法八： substring_index

select
    count(distinct device_id) as did_cnt,
    count(device_id) as question_cnt
from question_practice_detail
where substring_index(date,'-',2) = '2021-08'