full-outer-joins-41.psql

/**
  FULL OUTER JOIN will select all columns from both tables weather the column exists in both tables or not.
  It will fill inn the empty columns with null.
 */

SELECT * FROM customer
FULL OUTER JOIN payment
ON customer.customer_id = payment.customer_id
WHERE customer.customer_id IS null
OR payment.payment_id IS null;

-- Rules out that we have information about a customer that has not made a transaction in our payment table.
-- Same if we would do DISTINCT on both tables in two queries.
-- Result is empty since there are as many unique customer_ids as in unique payment_id's

-- Other ways to confirm this query above.
SELECT COUNT(DISTINCT customer_id) FROM payment; -- Returns 599 rows
SELECT COUNT(DISTINCT customer_id) FROM customer; -- Return 599 rows