VAR different result than without it

[Credo]

I've been slowly learning to use variables however I've run into a result that I dont understand 

I have a table with Sales by Manager that is filtered by TopN 10 Managers based on sales. 

I have the following measures, one just a sales total, another is two display a total that accounts for all sales not just top 10 managers. 

Sum of sales =
SUM ( 'FactSales'[Sales] )

Sales Top 10 =
IF (
    HASONEVALUE ( 'FactSales'[Manager] );
    'FactSales'[Sum of sales];
    CALCULATE ( 'FactSales'[Sum of sales]; ALL ( 'FactSales'[Manager] ) )
)

This measure displays the correct total for all sales

I tried writting the same measure with variables

Sales Top 10 =
VAR Sales =
    SUM ( 'FactSales'[Sales] )
RETURN
    IF (
        HASONEVALUE ( 'FactSales'[Manager] );
        Sales;
        CALCULATE ( Sales; ALL ( 'FactSales'[Manager] ) )
    )

However it only shows the total for top 10. 

Whats causing this behaviour? 

[OwenAuger · Accepted solution]

Hi @Credo

When a variable is defined with VAR, it is evaluated in the context where it is defined, then it is effectively a constant when used in any subsequent expressions. Currently there is no way to define a measure locally within another measure, so I would recommend sticking with Sum of Sales as a separate measure and referencing that within Sales Top 10.

To explain further, in your second example, after the variable Sales is defined as SUM ( 'FactSales'[Sales] ), the expression CALCULATE ( Sales; ALL ( 'FactSales'[Manager] ) ) gives exactly the same result as Sales, since Sales is effectively a constant, and modifying filters makes no difference to a constant.

Regards,

Owen