Power BI: calculated aggregate column using related tables

Question

I have the following two tables:

I would like to add a calculated column to the Author table showing the total number of pages in all the books the author has written.

In SQL I would solve the problem by writing a view like this (or using that code in a trigger to populate the calculated column):

SELECT
    "Author"."Author ID"
    (SELECT sum("Page count") FROM Book WHERE "Author ID" = "Author"."Author ID") AS "Total pages" 
FROM "Author";

How to achieve something like that in Power BI?

Answer 1

Since you want to add the column to the dimension table (one side of the one-to-many relationship) you'll have to use RELATEDTABLE() instead of the RELATED() function:

Page Count = SUMX(
    RELATEDTABLE(Book), 
    Book[Page Count]
)

The row context in your calculated column gets transfered to a filter context in the fact table (Books). This feature is know as context transition .

Of cause your SQL would rather return a table like this one:

Author Pages = 
SELECTCOLUMNS(
    Author, 
    "Author ID", Author[Author ID],
    "Page Count", SUMX(
        RELATEDTABLE(Book), 
        Book[Page Count]
    )
)

However, if you are just interested in visualizing the numbers you don't need any of the above expressions, but just have to drag Author ID and Page Count into a Table visual.