PowerBi: Days between purchase on multiple rows - powerbi

Question

I'm trying to calculate the daydiff between when the first (H1) and the second (H2) product have been bought for each customer (ID)
I can do it if the dates are on the same line, but not on mulitple rows. There are 5.6mil rows in all with 30 different products, but right now its only between H1 and H2
The result i'm after is something like

Answer 1

For a data source with 5.6M rows, I would only recommend doing it though native server-side query unless the calculations are not required to evaluate in respect to some filter context. Else, you can do it through the following DAX measure.
If you have a table like this and you want the DateDIff by ID for each preceding PurchaseDate
| ID | Product | PurchaseDate |
|----|---------|--------------|
| 10 | H1 | 2021-09-15 |
| 10 | H2 | 2021-09-19 |
| 20 | H1 | 2021-05-01 |
| 20 | H2 | 2021-06-05 |
| 20 | H3 | 2021-07-08 |
you can write the following measure
Measure =
VAR _curentlyVisibleDateById =
MAX ( 'fact'[PurchaseDate] )
VAR _immediatelyPrecedingDateById =
CALCULATE (
CALCULATE (
MAX ( 'fact'[PurchaseDate] ),
'fact'[PurchaseDate] < _curentlyVisibleDateById
),
ALLEXCEPT ( 'fact', 'fact'[ID] )
)
VAR _diffbyId =
DATEDIFF ( _immediatelyPrecedingDateById, _curentlyVisibleDateById, DAY )
RETURN
_diffbyId
Edit
If you want to limit calculations to only specific groups of Product; i.e. only for Products H1,H2,H3 then please use this
Measure =
VAR _curentlyVisibleDateById =
CALCULATE (
MAX ( 'fact'[PurchaseDate] ),
'fact'[Product] IN { "H1", "H2", "H3" }
)
VAR _immediatelyPrecedingDateById =
CALCULATE (
CALCULATE (
MAX ( 'fact'[PurchaseDate] ),
'fact'[PurchaseDate] < _curentlyVisibleDateById
),
ALLEXCEPT ( 'fact', 'fact'[ID] ),
'fact'[Product] IN { "H1", "H2", "H3" }
)
VAR _diffbyId =
DATEDIFF ( _immediatelyPrecedingDateById, _curentlyVisibleDateById, DAY )
RETURN
_diffbyId