With calculated columns you can attach columns to a connected sheet's returned data and have those columns auto expand and contract as the data is updated. For example, say you want to parse out a date column into separate month and year columns, if you use a calculated column the formulas will auto-extend or contract whenever your data changes.

To add a calculated column, give the new column a header, in the column immediately adjacent to the returned data (or calculated column, if you have already added one):

And then type your formula in the cell immediately below:

When you press enter, we will automatically fill the column down:

The light green color indicates the calculated column is set up correctly. Now whenever you re-run the query for the sheet, the calculated column will auto-extend or contract.

To add another calculated column it must be immediately adjacent to the last calculated column.