Var¶

Variables are calculated within the scope in which they are written, and then the result of them is stored and used in the rest of the expression.

Var vs Measure¶

Var will calculate the value in advance and store it
Measure can be considered a function and will be calculated in place with current context

Wrong place to use var¶

https://radacad.com/caution-when-using-variables-in-dax-and-power-bi

Using var defined outside of the calculation context will lead to wrong results.

Wrong

Cost =     
    VAR __val = [Amount] * [Price]
    VAR __tbl = SUMMARIZE(tbl, tbl[yr], "@val", __val) //__val calculated for all years
    VAR __tot = SUMX(__tbl, [@val]))
    VAR __is_val = HASONEVALUE(tbl[yr])
    VAR __ret = IF(__is_val, __val, __tot)
RETURN
    __ret

Correct

Cost__ = [Amount] * [Price]
Cost = 
    VAR __is_val = HASONEVALUE(tbl[yr])
    VAR __ret = IF(
        __is_val,
        [Cost__],
        (
            VAR __tbl = SUMMARIZE(tbl, tbl[yr], "@val", [Cost__])
            VAR __tot = SUMX(__tbl, [@val])
        RETURN 
            __tot
        )
    )     
RETURN
    __ret