Explanation of the code:

Both tables (june and july) are joined on the matching columns. Because it's a FULL OUTER join, all the records will be in the result, even if there is no matching row in the other table. If there is no matching row, the missing columns in the result will be filled with NULL values. In the code both columns are combined into a single column. By using the COALESCE function, the first non-NULL value is displayed.

To calculate the difference a COALESCE function is used to convert the NULL values to the number 0.