Find third last event in table - Incomplete output - sql

Question

As my research into Firebird continues, I've attempted to improve some of my queries. As I use Libreoffice Base, I'm not 100% sure how the data entry code works, but I believe it's something like this:
CREATE TABLE "Data Entry"(
ID int,
Date date,
"Vehicle Type" varchar,
events int,
"Hours 1" int,
"Hours 2" int
);
INSERT INTO "Data Entry" VALUES
(1, '31/12/22', 'A', '1', '0', '1'),
(2, '31/12/22', 'A', '1', '0', '1'),
(3, '29/12/22', 'A', '3', '0', '1'),
(4, '25/06/22', 'B1', '1', '0', '1'),
(5, '24/06/22' , 'B1', '1', '1', '0'),
(6, '24/06/22' , 'B1', '1', '1', '0'),
(7, '31/12/22' , 'B2', '7', '0', '1'),
(8, '29/12/22' , 'C', '1', '0', '1'),
(9, '29/12/22' , 'C', '2', '0', '1'),
(10, '19/01/22' , 'D1', '5', '1', '0'),
(11, '23/01/22' , 'D2', '6', '1', '1'),
(12, '29/07/19' , 'D3', '5', '0', '1'),
(13, '21/12/22' , 'D4', '1', '0', '1'),
(14, '19/12/22' , 'D4', '1', '1', '1'),
(15, '19/12/22' , 'D4', '1', '0', '1'),
(16, '28/12/22' , 'E', '2', '0', '1'),
(17, '24/12/22' , 'E', '3', '0', '1'),
(18, '14/07/07' , '1', '0', '0', '1'),
(19, '22/12/22' , '2', '1', '0', '1');
I tried this through the online Fiddle pages, but it throws up errors, so either I'm doing it incorrectly, or it's because there was no option for Firebird. Hopefully irrelevant, as I have the table already through the front-end.
One of my earlier queries which works as expected is shown below, along with its output:
SELECT
"Vehicle Type",
DATEDIFF(DAY, "Date", CURRENT_DATE) AS "Days Since 3rd Last Event"
FROM
(
SELECT
"Date",
"Events",
"Vehicle Type",
"Event Count",
ROW_NUMBER() OVER (PARTITION BY "Vehicle Type" ORDER BY "Date" DESC) AS "rn"
FROM
(
SELECT
"Date",
"Events",
"Vehicle Type",
SUM("Events") OVER (PARTITION BY "Vehicle Type" ORDER BY "Date" DESC) AS "Event Count"
FROM "Data Entry"
)
WHERE "Event Count" >= 3
)
WHERE "rn" = 1
Vehicle Type
Days Since 3rd Last Event
A
3
B1
191
B2
1
C
3
D1
347
D2
343
D3
1252
D4
14
E
8
In this output, it does not list every vehicle because not all vehicles have an Event Count that is equal to or greater than 3. The new query I am trying to put together is a combination of different queries (omitted to keep things relevant, plus they already work on their own), with a rewrite of the above code as well:
SELECT
"Vehicle Type",
SUM("Hours 1" + "Hours 2") AS "Total Hours",
MAX(CASE
WHEN
"Total Events" = 3
THEN
DATEDIFF(DAY, "Date", CURRENT_DATE)
END
) "Days Since 3rd Last Event"
FROM
(
SELECT
"Vehicle Type",
"Date",
"Hours 1",
"Hours 2",
CASE
WHEN
"Events" > 0
THEN
SUM( "Events")
OVER(
PARTITION BY "Vehicle Type"
ORDER BY "Date" DESC
)
END
"Total Events"
FROM
"Data Entry"
)
GROUP BY "Vehicle Type"
ORDER BY "Vehicle Type"
The expected output should be:
Vehicle Type
Days Since 3rd Last Event
Total Hours
1
1
2
1
A
3
3
B1
191
3
B2
1
1
C
3
2
D1
347
1
D2
343
2
D3
1252
1
D4
14
4
E
8
2
However, the actual output is:
Vehicle Type
Days Since 3rd Last Event
Total Hours
1
1
2
1
A
3
B1
191
3
B2
1
C
3
2
D1
1
D2
2
D3
1
D4
14
4
E
2
Granted, I've mixed and matched code, made some up myself, and copied some parts from elsewhere online, so there's a good chance I've not understood something correctly and blindly added it in thinking it would work, but now I'm at a loss as to what that could be. I've had a play around with changing the values of the WHEN statements and altering the operators between =, >, and >=, but any deviation from what's currently shown above outputs incorrect numbers. At least the three numbers displayed in the actual output are correct.

Answer 1

You could try using two rankings:
the first one that catches last three rows
the second one that catches your last row among the possible three
then get your date differences.
WITH last_three AS (
SELECT "Vehicle Type", "Date",
SUM("Hours 1"+"Hours 2") OVER(PARTITION BY "Vehicle Type") AS "Total Hours",
ROW_NUMBER() OVER(PARTITION BY "Vehicle Type" ORDER BY "Date" DESC) AS rn
FROM "Data Entry"
), last_third AS (
SELECT "Vehicle Type", "Date", "Total Hours",
ROW_NUMBER() OVER(PARTITION BY "Vehicle Type" ORDER BY rn DESC) AS rn2
FROM last_three
WHERE rn <= 3
)
SELECT "Vehicle Type",
DATEDIFF(DAY, "Date", CURRENT_DATE) AS "Days Since 3rd Last Event",
"Total Hours"
FROM last_third
WHERE rn2 = 1
ORDER BY "Vehicle Type"
Check the demo here.
Note: You will get values for the "Vehicle Type" 1 and 2 too. If you can explain the rationale behind having those values empty, this query can be tweaked accordingly.