INNER JOINs return only records with matching keys in both tables. LEFT JOINs return records from the first table if they are referenced by the second table, while RIGHT JOINs return records from the second table if they are referenced by the first table. FULL OUTER JOINs return all records from both tables, even if there is no match. WHERE can be used to filter results to show only records with NULL keys, and UNION combines the results of two queries into one result set. With a good understanding of joins, developers can write powerful and efficient queries in MySQL.