Excluding results that are within same month in SQL - sql

Question

I have two tables -- one, a history table that contains a log of some kind of entries, and another (let's call it flags) that contains columns about flags (for a certain account). Both tables contain account IDs.
I want to write a query that only extracts rows from the flag table if the account ID does not already have an entry for that month in the history table (e.g., in the flag table, an entry was entered on April 2, 2019 and in the history table, the account already had an entry recorded on April 1, 2019. The result is, the April 2nd entry should not be pulled up).
I have a query right now that basically looks like this:
SELECT *multiple column names*
FROM flags
WHERE NOT EXISTS (SELECT acc_id FROM history WHERE ...)
This is where I am stuck. With the subquery, I basically want to get the matches where the dates from both tables match (same month and year), and with the WHERE NOT EXISTS, exclude the results from flag that are found in the subquery (essentially I only want results where the date for the entry is not from the same month)
The most important columns are:
the account ID (to correctly associate each log entry to the right account)
date (to only get rows where the month recorded is not already logged in the history table)
I initially used MONTH(), but that only extracts the month of the date. I need it to match both the month and the year because the history table contains a few years of data.
Any help would be greatly appreciated! Thank you in advance!

Answer 1

SELECT *multiple column names*
FROM flags
WHERE NOT EXISTS (
SELECT 1
FROM history
WHERE history.acc_id=flags.acc_id
AND date_trunc('month', history.date) =
date_trunc('month', flags.date)
)
The date_trunc function will work for postgres, which was one of the tags originally. If you aren't using postgres, there may be a similar function in your database, it you could format the dates to just Year-month and compare the resulting strings.