Forum Discussion
Advocate IIICan anyone explain why this works?
Hi experts,
I'm reading the tutorial here:
https://www.daxpatterns.com/new-and-returning-customers/
In the first meausre it presents:
=COUNTROWS (
FILTER (
ADDCOLUMNS (
VALUES ( Sales[CustomerKey] ),
"PreviousSales", CALCULATE (
COUNTROWS ( Sales ),
FILTER (
ALL ( 'Date' ),
'Date'[FullDate] < MIN ( 'Date'[FullDate] )
)
)
),
[PreviousSales] = 0
)
)I don't understand why
ALL ( 'Date' ),
'Date'[FullDate] < MIN ( 'Date'[FullDate] )works. Since we already used ALL('Date') to remove all filters from 'Date', even after placing it into a pivotal table, wouldn't this always return an empty table?
I see your point. I was similarly bewildered when first presented with this type of formulae.
There are two tricky subjects here, by order of importance:
1. What 'Date'[Full Date] actually refers to.
2. The nuances of how ALL( ) actually works
The behaviour of ALL depends on where it is used. It should probably have two different names. Since you're already familiar with the Italian gurus, check out this article for details. ALL can work either by removing filters or by ignoring filters. It sounds like the same but it sure ain't.
Let us walk through your example:
The filter context
We have 'Date'[Full Date] in the rows of our pivot table. That determines the filter context.
Inside FILTER( )
FILTER( ) first computes the table it will operate on. In this case, ALL('Date'). That is the full 'Date' table in your data model. Let's call it Table=ALL('Date').
Nota bene: ALL( ) here ignores filter context but it does NOT remove it, so filter context will be effective everywhere else.
Table has a 'Date'[Full Date] column as well, an instance different from the one in the pivot that we marked in red above. We'll use purple for Table's : 'Date'[Full Date]. So we have one name referring to two different instances. We'll have to see how we tell them apart.
FILTER( ) and filter context interaction
Once it has Table, FILTER( ) starts scanning it to check whether each row complies with the condition:
'Date'[FullDate] < MIN ( 'Date'[FullDate] )
'Date'[FullDate] is the value in the current row of Table. MIN('Date'[FullDate]), however, is the minimum of the values of 'Date'[Full Date] coming from the pivot table (filter context). This is the crux.
Conclusions
You do have a valid point in that MIN('Date'[FullDate]) could actually be referring to MIN('Date'[FullDate]), in which case every row would certainly be filtered out.
How does DAX discriminate between 'Date'[FullDate] and 'Date'[FullDate]?
Basically, when you use the "naked" column, i.e. the column on its own, it's 'Date'[FullDate]. In any other case you'll be referring to 'Date'[Full Date]. It's been built that way so that you can refer to both instances and build powerful code like the one you've shown.
As a final point, bear in mind that filter context affects both arguments in
FILTER(<table>; <filter expression>)
In this case the filter context is ignored in the first argument because we are using ALL( ). It'd be another story if we were using VALUES( ).
Does that help? (I do hope so, as it took some time to put together :smileywink:)
9 Replies
- Greg_Deckler
Community Champion
The ALL is there so that, regardless of visualization context, it gets all of the rows in the Date table. Then, it filters those rows down to the rows where FullDate is less than the current MIN of the FullDate as determined by visualization context.
So, in the visual, let's say that you have a matrix or table or column chart that uses Date, perhaps in the form of Month. For the Month of February in that visual, the MIN('Date'[FullDate]) will be 2/1/2018 let's say. So, what that filter clause does is make sure to eliminate all filter context from Dates and then imposes it's own so in the scenario above it will return all dates from 1/1/2018 to 1/31/2018.
- AlB
I think it would return all dates in the 'Date' table prior to 02/01/2018, not just those prior to 02/01/2018 in the year 2018. Or am I mistaken?
What makes you think it would always return an empty table?
- Greg_Deckler
AlB - Yes, correct, sorry I had an unstated assumption there that we were dealing with a date table that just had 2018 dates in it.
markus_zhangThanks! The thing is, in FILTER():
FILTER ( ALL ( 'Date' ), 'Date'[FullDate] < MIN ( 'Date'[FullDate] ) )ALL('Date'), as you said, already removes all filter context regardless of Pivot Table (sorry forgot to mention this is in Excel 2016), then I think both 'Date'[FullDate] are under the same context (that is, no context at all), so it will always returns empty, as none of the FullDate is going to be smaller than the minimum of the FullDate.
However, as you mentioned, it actually means FullDate that are smaller than the current MIN(), which is influenced by the Pivot Table. This is the part that confuses me. If ALL() already removes the filter context, how come it comes back in the same function? And if it comes back, why does the first 'Date'[FullDate] not get influenced? For a calculated column, I'd use EARLIER() for the second 'Date'[FullDate], but I'm not sure how to do this in a measure.
