How to replicate Excel COUNTIF formula?

[JollyRoger01]

I am struggling to get my head around the basic layout of DAX, even a simple COUNTIF formula. I have two tables in my data model connected by a field called System. I just wanted to calculate in this table how many rows are in Table 1 where the two row values for System are the same.

[Fowmy · Accepted solution]

@JollyRoger01 

I guess you have a one-to-many relationship from this table to table1. Add the following calculated column to the table on the one side

Count System = 
CALCULATE(
    COUNTA(Table1[System]),
    RELATEDTABLE(Table1)
)

[JollyRoger01]

Thank you, that seems to work. Though I am a little confused as to what the function is actually doing. How does it know that I am trying to match the two columns? Is that implied from the one to many relationship already?

[Fowmy]

@JollyRoger01 

Yes, since you already have a valid relationship, CALCULATE activates it here. 

[JollyRoger01]

Okay, I understand that. But how would I do this if I didn't have a relationship?

[Fowmy]

@JollyRoger01 

You can do it this way:

Count System =
VAR __system = [System]
RETURN
    COUNTROWS ( FILTER ( ALL ( Table1[System] ), Table1[System] = __system ) )

[JollyRoger01]

I can't seem to make that one work. Are you sure that is in the right format?