Identifying users with a downward trend SQL - mysql

Question

Trying to identify a list of customers who's quantity decreases from their previous purchase.
In this example we see that with each new purchase Mary's quantity decreases over time. However, while Bob shows a decline, he would not yield in the results because on 9/19 he purchased 8 quantities which is greater than his previous purchase of 5.
I'm trying to figure out a query for this for the life of me I can't seem to get it together
Customer PurchaseDate Quantity
Bob 9/1/2021 10
Bob 9/10/2021 6
Bob 9/18/2021 5
Bob 9/19/2021 8
Mary 9/1/2021 10
Mary 9/10/2021 6
Mary 9/18/2021 5
Mary 9/19/2021 3
Frank 9/1/2021 5
Lucus 9/1/2021 5
Lucus 9/10/2021 6
Lucus 9/18/2021 10
End results should be
Customer
Mary

Answer 1

This is a bit tricky, and to find results that are steadily increasing or decreasing you would probably want to use the MATCH_RECOGNIZE clause, which MySQL doesn't (yet) support. This way you can define a pattern whereby each qty is less than than the previous value. Additionally, you could probably do this with a recursive cte, but that would be outside of my abilities.
Here is what I came up with, with the caveat that it only compares the first and last values:
WITH
tbl (customer, purchasedate, quantity) AS (
SELECT * FROM VALUES
('Bob', '9/1/2021', 10),
('Bob', '9/10/2021', 6),
('Bob', '9/18/2021', 5),
('Bob', '9/19/2021', 8),
('Mary', '9/1/2021', 10),
('Mary', '9/10/2021', 6),
('Mary', '9/18/2021', 5),
('Mary', '9/19/2021', 3),
('Frank', '9/1/2021', 5),
('Lucus', '9/1/2021', 5),
('Lucus', '9/10/2021', 6),
('Lucus', '9/18/2021', 10)
)
SELECT
DISTINCT customer
FROM
tbl
QUALIFY
FIRST_VALUE(quantity) OVER (partition BY customer ORDER BY purchasedate)
> LAST_VALUE(quantity) OVER (PARTITION BY customer ORDER BY purchasedate)
Which gives:
CUSTOMER
Bob
Mary
Or, to get strictly decreasing with a known max, you can chain them all together which gets pretty ugly:
WITH
tbl (customer, purchasedate, quantity) AS (
SELECT * FROM VALUES
('Bob', '9/1/2021', 10),
('Bob', '9/10/2021', 6),
('Bob', '9/18/2021', 5),
('Bob', '9/19/2021', 8),
('Mary', '9/1/2021', 10),
('Mary', '9/10/2021', 6),
('Mary', '9/18/2021', 5),
('Mary', '9/19/2021', 3),
('Frank', '9/1/2021', 5),
('Lucus', '9/1/2021', 5),
('Lucus', '9/10/2021', 6),
('Lucus', '9/18/2021', 10)
)
SELECT
DISTINCT customer
FROM
tbl
qualify
(NTH_VALUE(quantity, 1) OVER (partition BY customer ORDER BY purchasedate) >= NTH_VALUE(quantity, 2) OVER (partition BY customer ORDER BY purchasedate))
and ((NTH_VALUE(quantity, 2) OVER (partition BY customer ORDER BY purchasedate) >= NTH_VALUE(quantity, 3) OVER (partition BY customer ORDER BY purchasedate)) or (NTH_VALUE(quantity, 3) OVER (partition BY customer ORDER BY purchasedate) is null))
and ((NTH_VALUE(quantity,3) OVER (partition BY customer ORDER BY purchasedate) >= NTH_VALUE(quantity, 4) OVER (partition BY customer ORDER BY purchasedate)) or (NTH_VALUE(quantity, 4) OVER (partition BY customer ORDER BY purchasedate) is null))
Which gives:
CUSTOMER
Mary
Though for an unknown amount I would think match_recognize would be the best solution (or you could add in some recursion or a custom function).

Answer 2

SELECT Customer
FROM ( SELECT CASE WHEN Customer = #customer AND Quantity > #quantity
THEN 1
ELSE 0
END AS increase_detected,
#customer := Customer Customer,
PurchaseDate,
#quantity := Quantity Quantity
FROM test
CROSS JOIN ( SELECT #customer := NULL, #quantity := NULL ) init_variables
ORDER BY Customer, PurchaseDate
) subquery
GROUP BY Customer
HAVING NOT SUM(increase_detected);
https://dbfiddle.uk/?rdbms=mysql_5.6&fiddle=68b75b0df7fe4b383896e78db0caa569