SQL JOINing a Table to Itself

Getting two sets of information from one table in a select statement often leads people to write subselects, but it really doesn't matter that this is the same table twice, we can just give it a new alias and treat it as if it were a different table. This is one of those techniques where, once you've seen it, it's really obvious, but until that point it can be very confusing. I explained this to someone else recently, so I thought I'd capture it here in case it's helpful to anyone else.

Consider that tried-and-tested example: employees and managers. Here's the staff table from the database (today's imaginary data isn't particularly imaginative, sorry):

mysql> select * from staff;
+----+------------+-----------+------------+
| id | first_name | last_name | manager_id |
+----+------------+-----------+------------+
|  1 | Hattie     | Hopkins   |          4 |
|  2 | Henry      | Hopkins   |          4 |
|  3 | Harry      | Hopkins   |          5 |
|  4 | Helen      | Hopkins   |       NULL |
|  5 | Heidi      | Hopkins   |          4 |
|  6 | Hazel      | Hopkins   |          1 |
+----+------------+-----------+------------+
6 rows in set (0.00 sec)

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