What is the 'expression' in COUNTAX and COUNTX (DAX)? - powerbi

Question

The documentation for COUNTAX (DAX) and COUNTX (DAX)
states that the second argument is an expression that is evaluated for each row.
See: https://learn.microsoft.com/en-us/dax/countax-function-dax
This is exactly what I need, but I cannot figure out what the 'expression' should look like.
The example given in Microsoft's documentation is this:
=COUNTAX(FILTER('Reseller',[Status]="Active"),[Phone])
But the second argument ([Phone]) does not look like an expression.
An expression in my expectation is something like "value > 3 AND value <= 10"
What kind of expression can be used here?

Answer 1

In the example [Phone] is the expression evaluated for each row in the resulting table. To clarify, since COUNTAX and COUNTX return the count of nonblank rows, the count of nonblank values in the [Phone] column is the computed expression. After the FILTER function is applied to the table, the expression would be equivalent to COUNT([Phone]) for this context. Using the Server Timings feature in Dax Studio the text representation of what is passed to the storage engine can be viewed. In the case of COUNTX you will see a query with IS NOT NULL in the WHERE clause for the column used as the expression, as would be [Phone] in this case, with the COUNT function selected since any rows with a blank [Phone] will have already been filtered out.
The statement below is an example query from the Server Timings feature in DAX Studio using the example measure from your question. As you can see, there are two filters in the WHERE clause. The first on the Status column, to return only rows that are active. The second is to eliminate null values in the Phone column. This leaves the COUNT aggregate function to count all rows that have an active Status and a non-blank value in the Phone column, which is equivalent to a count of the Phone column with the active Status. The query here is only a text representation of the requests sent to the storage engine, thus the syntax displayed won't be actual SQL, but will give you a better idea of how the DAX is being processed.
SET DC_KIND="AUTO";
SELECT
COUNT ( )
FROM 'Reseller'
WHERE
'Reseller'[Status] = 'Active' AND
'Reseller'[Phone] IS NOT NULL;
'Estimated size ( volume, marshalling bytes ) : 224012, 1392082'

Answer 2

usually in dax the term expression means Column or Formula, so you could use either a column or a formula in the countax/countx or in any other functions that are evaluated for each row (generally functions that end in x :D )

Answer 3

Thank you #userfl89, for the enlightening and analytic explanation.
I see now that <expression> is actually a WHERE clause. Now it starts making sense why Microsoft choosed to use the word 'expression' here.
Still, it all is full of contradictions.
The syntax for Count is
COUNT(<column>)
Then, [Phone], in COUNT([Phone]) is of type <column>. I assume this is also just a WHERE clause towards the storage engine?
Compare with COUNTAX(FILTER('Reseller',[Status]="Active"),[Phone]), where [Phone] is of type <expression>.
The real logic in COUNTAX is in fact the FILTER-function.
The <expression> is always a WHERE clause that but selects the non-blanc values. OK, it is an 'expression', but it is always the same one. Right?
I cannot but conclude that Microsoft's type system in their syntax description is a bit messy and makes it hard for me (as a starter on PowerBI) to make my own DAX queries based on their documentation.
Or they should come up with better examples to compensate for the confusing syntax descriptions