Understood, thanks. But is that really different from the point of view of the developer? As long as the column is not used in the output it's just a variable.

Understood, thanks. But is that really different from the point of view of the developer? As long as the column is not used in the output it's just a variable. I elaborate because I checked the examples since my first comment: There are two derive sections in introduction.prql. derive [ # This adds columns / variables. gross_salary = salary + payroll_tax, gross_cost = gross_salary + benefits_cost # Variables can use other variables. ] Note the comments about "columns / variables" and "Variables can use other variables", so the distinction is probably not very clear even now. From my point of view that would be let gross_salary = salary + payroll_tax let gross_cost = gross_salary + benefits_cost Then I'd get gross_salary and gross_cost in the result or not, according to the select I'm doing at the end. [1] By the way, in the examples I saw select [name, salary, average_country_salary] which has some unnecessary brackets. Why not just omitting them? select name, salary, average_country_salary Same for the parentheses in aggregate average_country_salary = (average salary) Maybe omitting [ ] and ( ) would make the parser too complex but I'm all in to offload work to computers instead of to developers :-) And why not only one type of brackets? Example: the join clause join average_salaries [==country] which gets compiled to JOIN average_salaries ON newest_employees.country = average_salaries.country Why square brackets there instead of round ones? That's a condition and we're conditioned to put conditions inside (), as in if () {} else {} And omit == by default, so join average_salaries (country) [1] Edit: maybe you can add a negative select, to remove columns from the output. Maybe: remove col1, col2 That would be handy where there are 100 columns and one wants to remove only 5 of them. No need to enumerate the other 95.