Beginning DAX with Power BI_ The SQL Pro’s Guide to Better Business Intelligence ( PDFDrive )

Chapter 5
Joins
Table 5-11. Output of NATURALLEFTOUTERJOIN from Listing 5-12
Customer Purchase Date Last Purchase
1 2019-01-01
1 2019-01-07 2019-01-01
1 2019-01-21 2019-01-07
1 2019-01-25 2019-01-21
2 2019-01-05
2 2019-01-12 2019-01-05
2 2019-01-17 2019-01-12
2 2019-01-22 2019-01-17
The final requirement is to have a column that carries a number that shows, for
each customer purchase, how many days have passed since their last purchase. This is
achieved by wrapping the NATURALLEFTOUTERJOIN function with an ADDCOLUMNS
function (Listing 5-15).
Listing 5-15. The Final RETURN Statement from Listing 5-12
RETURN
ADDCOLUMNS(
NATURALLEFTOUTERJOIN('Sales', LastPurchases),
"Days since last purchase",
VAR DaysSince = INT([Purchase Date] -
[Last Purchase])
RETURN IF (
NOT ISBLANK([Last Purchase]),
DaysSince
)
)
The ADDCOLUMNS function appends a single column to the output of the
NATURALLEFTOUTERJOIN function. The “Days since last purchase” parameter
provides a name for the new column, while the last parameter starting with a nested VAR
statement is a DAX expression that returns the difference between the [Purchase Date]
and [Last Purchase] columns.
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