mysql查询最近7天的数据，没有数据自动补0

转载自 https://www.cnblogs.com/snake23/p/9562646.html
作者 华丽D转身

问题描述
查询数据库表中最近7天的记录

select count(*),date(create_time) as date from task where datediff(now(),create_time)<=6 group by day(create_time);

但是发现某一天没有数据，结果中没有显示当天(2017-08-28)的数据

这里写图片描述

解决思路
思路一： 可以在自己的程序中做额外的补零处理

思路二： 构建一个最近七天的结果集，然后和查询的结果集合做left join(本文采用第二种方式)

select a.click_date,b.count
from (
SELECT curdate() as click_date
union all
SELECT date_sub(curdate(), interval 1 day) as click_date
union all
SELECT date_sub(curdate(), interval 2 day) as click_date
union all
SELECT date_sub(curdate(), interval 3 day) as click_date
union all
SELECT date_sub(curdate(), interval 4 day) as click_date
union all
SELECT date_sub(curdate(), interval 5 day) as click_date
union all
SELECT date_sub(curdate(), interval 6 day) as click_date
) a left join (
select date(create_time) as datetime, count(*) as count
from arms_task
group by date(create_time)
) b on a.click_date = b.datetime;
当天2017-08-28结果显示为NULL

这里写图片描述

需要把NULL设置为0，利用ifnull函数即可

select a.click_date,ifnull(b.count,0) as count
from (
    SELECT curdate() as click_date
    union all
    SELECT date_sub(curdate(), interval 1 day) as click_date
    union all
    SELECT date_sub(curdate(), interval 2 day) as click_date
    union all
    SELECT date_sub(curdate(), interval 3 day) as click_date
    union all
    SELECT date_sub(curdate(), interval 4 day) as click_date
    union all
    SELECT date_sub(curdate(), interval 5 day) as click_date
    union all
    SELECT date_sub(curdate(), interval 6 day) as click_date
) a left join (
  select date(时间字段) as datetime, count(*) as count
  from 表明
  group by date(时间字段)
) b on a.click_date = b.datetime;