An overview of table calculations
Table calculations are different from all other calculations in Tableau. Row-level and aggregate calculations, which we considered in the previous chapter, are performed at the data source layer. If you were to examine the queries sent to the data source by Tableau, you'd find the code for your calculations translated into whatever flavor of SQL the data source used.
Table calculations, on the other hand, are performed after the initial query. Here's an extended diagram that shows how aggregated results are stored in Tableau's cache:
Table calculations are performed on the aggregate table of data in Tableau's cache right before the data visualization is rendered. As we'll see, this is important to understand for multiple reasons, including the following:
Aggregation: Table calculations operate on aggregate data. You cannot reference fields in a table calculation without referencing it as an aggregate.
Filtering: Any filters will be applied before table calculations...
