問題：mysql按日期查詢乙個月內的資料，查詢返回時結果集中只顯示有資料的結果
需求：查詢結果中假如當天沒有資料，自動填零並輸出
事件描述：
sql語句：
select date_format(date_added,"%m.%d") as day,count(product_id) as total from ht_product where date_format(date_added,"%m")=7 group by day;
返回結果：
day total
07.23 1
07.25 4
07.26 2
07.27 1
期望結果：
day total
07.01 0
07.02 0
07.22 0
07.23 1
07.24 0
07.25 4
07.26 2
07.27 1
07.28 0
07.31 0
解決方案
create table ht_product( date_added datetime, product_id int);
insert ht_product values
("2016-7-4", 11), ("2016-7-4", 33),
("2016-7-7", 11), ("2016-7-10", 33);
select _dt.day, coalesce(data.total, 0) as total
from(
select date_format(_id.date,"%m.%d") as day from(
select date_add("2016-07-01", interval @i day) as date, @i:=@i+1 as id
from information_schema.columns a, (select @i:=0) _a
order by id limit 31
) _id
where date_format(_id.date,"%m")=7
) _dt
left join(
select date_format(date_added,"%m.%d") as day,count(product_id) as total
from ht_product
where date_format(date_added,"%m")=7
group by day
) data on data.day = _dt.day
order by day
drop table ht_product;
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