Select list of dates from given range in Firebird - sql

Question

I'm designing a report that returns PurchaseOrder due in future week.
Query that I've added below returns PurchaseOrder due for a particular Commodity, AmountDue and its DeliveryDate.
Obviously it only returns PO_Dates that are in the table. What I want is to also include dates where no PO is expected, i.e. null for those cell.
To me one possibility is to LEFT JOIN the dataset with set of dates of future week on Date column, that will eventually make the result null where no Purchase Order is expected.
In Firebird I don't know how to select list of week long dates and then use it in the join.
SELECT
PURCHASE_ORDER_DET.COMMODITYID AS COM_ID,
PURCHASE_ORDER_DET.DELIVERYDATE + CAST ('29.12.1899' AS DATE) as DLV_DATE,
SUM(PURCHASE_ORDER_DET.REQQUANTITY) as DLV_DUE
FROM
PURCHASE_ORDER_DET
LEFT JOIN PURCHASE_ORDER_HDR on PURCHASE_ORDER_HDR.POH_ID =
PURCHASE_ORDER_DET.POH_ID
WHERE
PURCHASE_ORDER_DET.COMMODITYID = 1
AND PURCHASE_ORDER_HDR.STATUS in (0,1,2)
AND PURCHASE_ORDER_DET.DELIVERYDATE + CAST ('30.12.1899' AS TIMESTAMP) >= '3.01.2019'
AND PURCHASE_ORDER_DET.DELIVERYDATE + CAST ('30.12.1899' AS TIMESTAMP) <= '9.01.2019'
AND PURCHASE_ORDER_DET.DELETED is NULL
Group by
PURCHASE_ORDER_DET.COMMODITYID,
PURCHASE_ORDER_DET.DELIVERYDATE
DataSet
COM_ID DLV_DATE DLV_DUE
1 3.01.2019 50.000000
1 5.01.2019 10.000000
Expected
COM_ID DLV_DATE DLV_DUE
1 3.01.2019 50.000000
1 4.01.2019 null
1 5.01.2019 10.000000
1 6.01.2019 null
1 7.01.2019 null
1 8.01.2019 null
1 9.01.2019 null

Answer 1

Ignoring your odd use of datatypes*, there are several possible solutions:
Use a 'calendar' table that contains dates, and right join to that table (or left join from that table). The downside of course is having to populate this table (but that is a one-off cost).
Use a selectable stored procedure to generate a date range and join on that.
Generate the range in a recursive common table expression in the query itself
Option 1 is pretty self-explanatory.
Option 2 would look something like:
CREATE OR ALTER PROCEDURE date_range(startdate date, enddate date)
RETURNS (dateval date)
AS
BEGIN
dateval = startdate;
while (dateval <= enddate) do
BEGIN
suspend;
dateval = dateval + 1;
END
END
And then use this in your query like:
select date_range.dateval, ...
from date_range(date'2019-01-03', date'2019-01-09') -- use date_range(?, ?) for parameters
left join ...
on date_range.dateval = ...
Option 3 would look something like:
WITH RECURSIVE date_range AS (
SELECT date'2019-01-03' dateval -- start date, use cast(? as date) if you need a parameter
FROM rdb$database
UNION ALL
SELECT dateval + 1
FROM date_range
WHERE dateval < date'2019-01-09' -- end date use ? if you need a parameter
)
SELECT *
FROM date_range
LEFT JOIN ...
ON date_range.dateval = ...
Recursive common table expressions have a maximum recursion depth of 1024, which means that it isn't suitable if you need a span wider than 1024 days.
*: I'd suggest that you start using DATE instead of what looks like the number of days since 30-12-1899. That avoids having to do awkward calculations like you do now. If you do need those number of days, then you can for example use datediff(DAY FROM date'1899-12-30' TO somedatevalue) or somedatevalue - date'1899-12-30' to convert from date to that numeric value.