When writing a DAX expression, one can avoid repeating the same expression and greatly enhance the code readability by using variables. For example, look at the following expression:
VAR TotalSales = SUM ( Sales[SalesAmount] ) VAR TotalCosts = SUM ( Sales[TotalProductCost] ) VAR GrossMargin = TotalSales - TotalCosts RETURN GrossMargin / TotalSales
Variables are defined with the VAR keyword. After you define a variable, you need to provide a RETURN section that defines the result value of the expression. One can define many variables, and the variables are local to the expression in which they are defined.
A variable defined in an expression cannot be used outside the expression itself. There is no such thing as a global variable definition. This means that you cannot define variables used through the whole DAX code of the model.
Variables are computed using lazy evaluation. This means that if one defines a variable that, for any reason, is not used in the code, the variable itself will never be evaluated. If it needs to be computed, this happens only once. Later uses of the variable will read the value computed previously. Thus, variables are also useful as an optimization technique when used in a complex expression multiple times.
Variables are an important tool in DAX. As you will learn in Chapter 4, variables are extremely useful because they use the definition evaluation context instead of the context where the variable is used. In Chapter 6, “Variables,” we will fully cover variables and how to use them. We will also use variables extensively throughout the book.