In the query's SQL Editor (not Builder), you can use a UNION operator to stack or combine data from multiple sources into rows. However, a union may create more rows than Spreadsheets or another consumer of the results can support. As an alternative, create a table and append the sources' data as datasets, which you can then slice-and-dice and analyze in a query.

In the Query Builder, you can create a relationship between sources to add a JOIN clause to combine their data into columns (not stacked rows). Learn more in Support Center