When we have multiple data sources and we want to integrate those to visualize in our Tableau dashboard, we use blending. It creates a connection between primary and secondary data source by a left join kind of a link. By default, Tableau does a left join considering all data elements from primary data source. Shuffling of various options can be performed between data sources. There are two ways of blending – automatically defined relationship and manually defining the blend. Each data source contains its set of dimensions and measures. We can not have specific set of dimensions and measures which will limit us to create groups, formulas etc across the data. Data sources can be of multiple RDBMS such as Oracle, SQL Server, DB2 etc. and other data sources such as spreadsheets etc. Blending also avoids joining of duplicate data as it will be linked via a common dimension. One can change the linking field (dimension or measure) or add more linking fields to include additional rows of data from the secondary data source in the blend, changing the aggregated values. It's no surprise that this one pops up often in Tableau questions for interview.