postgresql - Using a variable period in an interval in Postgres

Question

I have a relation that maintains monthly historical data. This data is added to the table on the last day of each month. A service I am writing can then be called specifying a month and a number of months prior for which to retrieve the historical data. I am doing this by creating startDate and endDate variables, and then returning data between the two. The problem I am having is that startDate is a variable number of months before endDate, and I cannot figure out how to use a variable period in an interval.

Here is what I have:

    DECLARE
      endDate   TIMESTAMP := (DATE_TRUNC('MONTH',$2) + INTERVAL '1 MONTH') - INTERVAL '1 DAY';
      startDate TIMESTAMP := endDate - INTERVAL $3 'MONTH';

I know that the line for startDate is not correct. How is this properly done?

Answer 1

Use this line:

startDate TIMESTAMP := endDate - ($3 || ' MONTH')::INTERVAL;

and note the space before MONTH. Basically: You construct a string with like 4 MONTH and cast it with ::type into a proper interval.

Edit: I' have found another solution: You can calculate with interval like this:

startDate TIMESTAMP := endDate - $3 * INTERVAL '1 MONTH';

This looks a little bit nicer to me.