Choosing columns without naming them

Every column chosen so far has been chosen by writing its name. That holds until the table is wide, or until the columns it arrives with change from one month to the next.

Three chapters, each a different question about a column. What is its name shaped like? What kind of thing does it hold? And once you have chosen a set of them, how do you apply one calculation to all of them at once?

This is the part that replaces a family of helper functions in other tools with one word and a written subject.