How to compare one row to others in DAX in Excel - excel

Question

I have this table which has foreign keys from several other keys:
Basically, this table shows which students registered in which module run by which teacher in what term.
I want to query the following:
How many students have registered for more than one module run by a given tutor?
It will look something like this:
For example, Vasiliy Kuznetsov runs two modules: FunPro and NO. If one student registers for both of them, he is counted as one.
My sql oriented mind is telling me this: Count all the rows in which student_id and tutor_id are the same. For example, in one row student_id is 5 and tutor_id is 10, and the same is true for the third row. Then, I count it as one.
How can I do that with DAX formulas?

Answer 1

RowCount:=
COUNTROWS( ModuleRegistration )
StudentsWithTwoOrMoreRegistrations:=
COUNTROWS(
FILTER(
VALUES( ModuleRegistration[Student_ID] )
,[RowCount] >= 2
)
)
I refer to arguments positionally, thus the first argument to a function is (1), the second (2), and so on.
So, [RowCount] is trivial.
[StudentsWithTwoOrMoreRegistrations] is a bit more involved. DAX, being a functional language, is best understood inside-out.
FILTER() takes a table expression in (1) and evaluates a boolean predicate, (2), for each row in (1). It returns all rows from (1) for which (2) evaluates to true.
Our FILTER()'s (1) is VALUES( ModuleRegistration[Student_ID] ). VALUES() returns the unique rows from a field based on current filter context (it respects slicers and filters in the pivot table). Thus, we will return some subset of the unique list of [Student_ID]s.
Our FILTER()'s (2) is [RowCount] >= 2. For each [Student_ID] in (1), we'll evaluate [RowCount], checking how many times that student appears in ModuleRegistration. [RowCount] is evaluated in the combination of filter context from the pivot table (the [Faculty Name] field in your sample pivot provides filter context) and row context from FILTER()'s (1). Thus it counts how many times the student appears in ModuleRegistration for the [Faculty Name] on the pivot table row.
We check that [RowCount] is >= 2.
You've not indicated if your measure needs to handle grand totals, or how you might want to see that. If you need more help for the grand total to get it to behave the way you like, let me know.
Edit for grand total
There are a few ways you might want to handle grand totals. I'm gong to assume that you want a unique count of students.
StudentsWithTwoOrMoreRegistrations:=
COUNTROWS(
SUMMARIZE(
FILTER(
SUMMARIZE(
ModuleRegistration
,ModuleRegistration[Tutor_ID]
,ModuleRegistration[Student_ID]
)
,[RowCount] >= 2
)
,ModuleRegistration[Student_ID]
)
)
WTF happened to our measure?
Let's examine:
Starting with the innermost SUMMARIZE(). SUMMARIZE() navigates relationships outward from the table in (1) and groups by the columns listed in (2)-(N) (these don't have to be from the table in (1), but must be reachable by navigating relationships).
This is equivalent to the following in SQL:
SELECT
mr.Tutor_ID
,mr.Student_ID
FROM ModuleRegistration mr
We use FILTER() on this table like earlier. [RowCount] is evaluated in the combination of filter context from the pivot table and the row in the table, defined by our SUMMARIZE() above.
Now our row context is instead of just a student, a student-tutor pair. This pair will have a [RowCount] >= 2 when the student has taken more than one module from a tutor.
Our FILTER() returns the pairs which have a [RowCount] >= 2. This output table has two fields, [Tutor_ID] and [Student_ID], but we want to count distinct [Student_ID]s out of this.
Thus, we use the table from FILTER() as our (1) in the outer SUMMARIZE(). We group only by the values of [Student_ID]. We then count the rows of this table.
When only one [Faculty_Name] is in context, e.g. on a pivot table row, then our inner SUMMARIZE() is grouping by a single value of [Tutor_ID] and whatever [Student_ID]s are associated with it. This is identical to our earlier measure.
When we have many [Tutor_ID]s in context, like in the grand total, then we'll see the appropriate behavior of only counting each [Student_ID] once.