How to retreive stack from Database - database

Question

I have this table called 'Stack'.
+---------------+-------+-------------+
| Stack_Counter | value | Stack_Depth |
+---------------+-------+-------------+
| 1 | 3 | 1 |
| 2 | 0 | 2 |
| 3 | 0 | 1 |
| 4 | | 0 |
| 5 | 3 | 1 |
| 6 | 3 | 2 |
| 7 | 1 | 3 |
| 8 | 2 | 2 |
| 9 | 4 | 1 |
| 10 | 2 | 2 |
| 11 | 0 | 3 |
| 12 | 0 | 2 |
| 13 | 0 | 1 |
| 14 | 2 | 2 |
| 15 | 2 | 3 |
| 16 | 1 | 4 |
| 17 | 1 | 3 |
| 18 | 2 | 2 |
| 19 | 1 | 3 |
| 20 | 0 | 4 |
+---------------+-------+-------------+
I want to find out the stack array in Stack_Counter '20'.
So the correct answer should be
+---------------+-------+-------------+
| Stack_Counter | value | Stack_Depth |
+---------------+-------+-------------+
| 13 | 0 | 1 |
| 18 | 2 | 2 |
| 19 | 1 | 3 |
| 20 | 0 | 4 |
+---------------+-------+-------------+
Basically , this is to find out consecutive rows in selected Stack_Depth.
Is there any way to acheive it?

Answer 1

... and here's a generic all-SQL solution:
SELECT Stack_Counter, value, Stack_Depth
FROM
(SELECT *, RANK() OVER (
PARTITION BY Stack_Depth
ORDER BY Stack_Counter DESC) rank
FROM stack)
WHERE rank=1 AND Stack_Depth > 0;

Answer 2

An SQL-only solution may be more trouble than it's worth (either because of its complexity or its tediousness), but here is one that works with SQLite:
SELECT * FROM (SELECT * FROM stack WHERE Stack_Depth=1 ORDER BY Stack_Counter DESC LIMIT 1)
UNION ALL
SELECT * FROM (SELECT * FROM stack WHERE Stack_Depth=2 ORDER BY Stack_Counter DESC LIMIT 1)
UNION ALL
SELECT * FROM (SELECT * FROM stack WHERE Stack_Depth=3 ORDER BY Stack_Counter DESC LIMIT 1)
UNION ALL
SELECT * FROM (SELECT * FROM stack WHERE Stack_Depth=4 ORDER BY Stack_Counter DESC LIMIT 1);

Answer 3

The window functions I mentioned in comments proved to be more difficult than I thought... for myself at the time. Peak's answer is an elegant solution using the rank() window function, just what I had originally intended. In the mean time, sqlite also supports recursive CTE (Common Table Expressions; WITH statement):
WITH RECURSIVE
latest (id, level) AS (
VALUES (20, (SELECT Stack_Depth FROM stack WHERE Stack_Counter = 20))
UNION ALL
SELECT (SELECT max(Stack_Counter)
FROM stack
WHERE Stack_Depth = level - 1
AND Stack_Counter <= 20),
level - 1
FROM latest
WHERE level - 1 > 0
)
SELECT stack.*
FROM stack INNER JOIN latest
ON stack.Stack_Counter = latest.id
ORDER BY stack.Stack_Counter
There are three places that I had to insert the desired stack level, but these could all be replaced with a named SQL parameter if you're calling this from a prepared statement in the host language.
And if you're not interested in choosing a particular Stack_Counter value, rather just want the result from the entire table, then replace the VALUES clause with a SELECT like
WITH RECURSIVE
latest (id, level) AS (
SELECT * FROM (SELECT Stack_Counter, Stack_Depth FROM stack ORDER BY Stack_Counter DESC LIMIT 1)
UNION ALL
SELECT (SELECT max(Stack_Counter)
FROM stack
WHERE Stack_Depth = level - 1),
level - 1
FROM latest
WHERE level - 1 > 0
)
SELECT stack.*
FROM stack INNER JOIN latest
ON stack.Stack_Counter = latest.id
ORDER BY stack.Stack_Counter