When the value of an attribute isn't specified, it takes the value "NULL".
NULL indicates that the value might be absent for the attribute, or if present, is unknown. (Hence a NULL == some other NULL isn't True)
But the DISTINCT keyword distinguishes by text, and not values. So,
- NULL and NULL would not be considered distinct.
- Rajeev and rajeev would've been considered distinct (DISTINCT isn't case sensitive by default)
Here, we have two distinct values for the managers "attribute" — rajeev and NULL.