Aggregation of data: Aggregation of data refers to the process of viewing numeric values or the measures at a higher and more summarized level of data. When you place a measure on a shelf, Tableau will automatically aggregate your data. You can determine whether the aggregation has been applied to a field or not, by simply looking at the function. This is because the function always appears in front of the field’s name when it is placed on a shelf.
Example: Sales field will become SUM(Sales) after aggregation.
You can aggregate measures using Tableau only for relational data sources. Multidimensional data sources contain aggregated data only. In Tableau, multidimensional data sources are supported only in Windows.
Disaggregation of data: Disaggregation of data allows you to view every row of the data source which can be useful while analyzing measures.
Example: Consider a scenario where you are analyzing results from a product satisfaction survey. Here the Age of participants is along one axis. Now, you can aggregate the Age field to determine the average age of participants, or you can disaggregate the data to determine the age at which the participants were most satisfied with their product.