Understanding the basic concepts

[daanodinot]

I'm just trying to grasp the basic concepts, but - I have to say - the amount of conflicting information I see makes it really, really confusing.

So let me get this straight about the SUM function:

1. Does it operate in a row context or filter context?
2. Is it an iterator or aggregator function?

Thanks in advance!

[VKB]

🙂 It really frustrates anyone at the beginning! 
In the background evaluation process, DAX treats them as one and the same.
But, for the users, the rule of thumb is:
SUM is an aggregator function and ignores Row context! 
SUMX is there for iterations (simply means row context).

[PabloDeheza]

Hi there!

1. It operates in a filter context when used in a measure.

2. The SUM funcion is an aggregator, SUMX and every other "X" function are iterators.

[daanodinot]

Thank you! Perhaps I phrased point one incorrectly. What I meant to say is: does it create (or work with) a row context or filter context?

Here is a quote from an article that adds to my confusion:

Another common mistake of DAX newbies is thinking that SUM and SUMX are two different functions. This is not the case. SUM is nothing but a simplified version of SUMX: we call it syntax sugar. SUM ( Sales[Quantity] ) is internally translated into SUMX ( Sales, Sales[Quantity] ). Therefore, a regular aggregator like SUM, AVERAGE, MIN or MAX behaves the same way an iterator does.

This is the reason why a calculated column computing SUM ( Sales[Quantity] ) produces the grand total of Sales[Quantity] on each row. […]

[…] The row context does not filter, it iterates.

This definitely implies that SUM creates (works with) a row context and is an iterator.

[PabloDeheza]

I suggest you watch this videos from SQLBI as I believe there is no better content out there.
Hope this clarify this concepts, they are not easy to understand at first.
The Whiteboard Playlist