In the selection from the dept_manager table, why NULL AS emp_no?

Hi Anas!
Thanks for reaching out.

UNION and UNION ALL could be applied to two tables that have the same structure (i.e. we could have split e.g. the 'departments' table into two, and then use UNION to combine the two parts), so we can create additional columns with NULL values for both tables, so that the "new" tables with the NULL columns would fit and thus can be combined with UNION / UNION ALL.

Hope this helps.
Best,
Tsvetelin