In the last record I don't have an accepted date; how does an inner join work?

An INNER JOIN returns only the rows that have matching values in both tables. If a row in one table has no corresponding match in the other table, that row is excluded from the result set. In your example, the last record lacks an accepted date, which means the row in the table that stores accepted dates does not have a matching entry for that user_id_sender and user_id_receiver pair. When you perform an INNER JOIN on these two tables, the row without a match will be omitted. If you want to keep all rows from the first table regardless of whether there is a match in the second table, you should use a LEFT JOIN (or RIGHT JOIN depending on the direction). The LEFT JOIN will return all rows from the left table and NULLs for columns from the right table where no match exists. This is useful when you need to see all sent messages and indicate whether they were accepted or not.