How to select all dates in SQL query - sql

Question

SELECT oi.created_at, count(oi.id_order_item)
FROM order_item oi
The result is the follwoing:
2016-05-05 1562
2016-05-06 3865
2016-05-09 1
...etc
The problem is that I need information for all days even if there were no id_order_item for this date.
Expected result:
Date Quantity
2016-05-05 1562
2016-05-06 3865
2016-05-07 0
2016-05-08 0
2016-05-09 1

Answer 1

You can't count something that is not in the database. So you need to generate the missing dates in order to be able to "count" them.
SELECT d.dt, count(oi.id_order_item)
FROM (
select dt::date
from generate_series(
(select min(created_at) from order_item),
(select max(created_at) from order_item), interval '1' day) as x (dt)
) d
left join order_item oi on oi.created_at = d.dt
group by d.dt
order by d.dt;
The query gets the minimum and maximum date form the existing order items.
If you want the count for a specific date range you can remove the sub-selects:
SELECT d.dt, count(oi.id_order_item)
FROM (
select dt::date
from generate_series(date '2016-05-01', date '2016-05-31', interval '1' day) as x (dt)
) d
left join order_item oi on oi.created_at = d.dt
group by d.dt
order by d.dt;
SQLFiddle: http://sqlfiddle.com/#!15/49024/5

Answer 2

Friend, Postgresql Count function ignores Null values. It literally does not consider null values in the column you are searching. For this reason you need to include oi.created_at in a Group By clause
PostgreSql searches row by row sequentially. Because an integral part of your query is Count, and count basically stops the query for that row, your dates with null id_order_item are being ignored. If you group by oi.created_at this column will trump the count and return 0 values for you.
SELECT oi.created_at, count(oi.id_order_item)
FROM order_item oi
Group by io.created_at
From TechontheNet (my most trusted source of information):
Because you have listed one column in your SELECT statement that is not encapsulated in the count function, you must use a GROUP BY clause. The department field must, therefore, be listed in the GROUP BY section.
Some info on Count in PostgreSql
http://www.postgresqltutorial.com/postgresql-count-function/
http://www.techonthenet.com/postgresql/functions/count.php

Answer 3

Solution #1 You need Date Table where you stored all date data. Then do a left join depending on period.
Solution #2
WITH DateTable AS
(
SELECT DATEADD(dd, 1, CONVERT(DATETIME, GETDATE())) AS CreateDateTime, 1 AS Cnter
UNION ALL
SELECT DATEADD(dd, -1, CreateDateTime), DateTable.Cnter + 1
FROM DateTable
WHERE DateTable.Cnter + 1 <= 5
)
Generate Temporary table based on your input and then do a left Join.