Help with a measure

[gtamir]

Hi, The following is a measure from Marco Ruso. It works perfectly, but I want to understand it deeper. Thanks in advance.

1- In the Variable "FirstPurchaseCustomers" is the ADDCOLUMNS a virtual column? If yes, to which table? Sales?

New Customers =

VAR FirstPurchaseCustomers =

ADDCOLUMNS (

ALL ( Sales[CustomerKey] ),

"DateOfFirstBuy", CALCULATE ( MIN ( Sales[Order Date] ), ALLEXCEPT ( Sales, Sales[CustomerKey] ) )

)

VAR NewCustomers =

FILTER (

FirstPurchaseCustomers,

CONTAINS ( VALUES ( 'Date'[Date] ), 'Date'[Date], [DateOfFirstBuy] )

)

RETURN

COUNTROWS ( NewCustomers )

[amitchandak]

FirstPurchaseCustomers is table  based on sales, addcolumns create a table

https://docs.microsoft.com/en-us/dax/addcolumns-function-dax

The filter will again return the filtered table as

https://docs.microsoft.com/en-us/dax/filter-function-dax

and after filter, you are only getting new customers. So countrows of the table is returned.

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[gtamir]

So it is a virtual table. 

[d_gosbell]

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@gtamir wrote:

So it is a virtual table. 

---

Yes, it logically creates a virtual 2 column table with the CustomerKey and DateOfFirstBuy columns

[gtamir]

@d_gosbell Thank you. So it stays in memory as long as the project is on and you can refer to it, as I understand.

I didn't find any article about virtual tables. Do you know of anything written about the issue?

[d_gosbell · Accepted solution]

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@gtamir wrote:

So it stays in memory as long as the project is on and you can refer to it, as I understand.

---

No, the "table" only exists in memory for a few milliseconds while the measure is being evaluated in the context of a query. You cannot refer to it outside of the measure.  "virtual tables" is not a specific term, I'm using "virtual" as an adjective to describe the fact that the table variable is temporary and not persisted anywhere. It only exists for the duration of the evaluation of the measure (this is the way all variables defined using the VAR keyword behave).