mySQL Query get record that dont match - mysql

Question

I have an sql query that I wish to add another condition to but I cant seem to get it work. The query is simple enough:
SELECT DISTINCT monthly_returns.company_id
FROM monthly_returns, paidreturns
WHERE monthly_returns.company_id = paidreturns.company_id
AND paidreturns.month = '$cdate'
AND paidreturns.paid =0
However I wish to get the records also from the monthly_returns that have not record at all in paidreturns for the give date. I know it would be something like this
SELECT *
FROM monthly_returns
WHERE monthly_returns NOT IN (SELECT * FROM paidreturns WHERE paidreturns.month = '$cdate')
paidreturns.paid =0 is where the bill has not been paid, but equally if there is no record for that date in paidreturns then the bill is also not paid.
The schema
paidreturns table
-id
-company_id
-paid
-month
-total
monthly_returns table
-id
-company_id
-wage_totals
-month

Answer 1

Try this:
SELECT
DISTINCT monthly_returns.company_id
FROM
monthly_returns
LEFT JOIN
paidreturns
ON monthly_returns.company_id = paidreturns.company_id
AND monthly_returns.month = paidreturns.month
WHERE
monthly_returns.month = '$cdate'
AND
(
paidreturns.paid = 0
OR
paidreturns.company_id IS NULL
);
Using a LEFT JOIN, you can find all records from monthly_returns, regardless of whether they matched an entry from paidreturns.
Then, by adding paidreturns.company_id IS NULL to the WHERE clause, you include those unmatched entries in your query.

Answer 2

select company_id
from
(
(
SELECT DISTINCT monthly_returns.company_id
FROM monthly_returns, paidreturns
WHERE monthly_returns.company_id = paidreturns.company_id
AND paidreturns.month = '$cdate'
AND paidreturns.paid =0
)
union
(
SELECT company_id
FROM monthly_returns
WHERE monthly_returns NOT IN (SELECT * FROM paidreturns WHERE paidreturns.month = '$cdate'
)
) as x