Who Manages Nobody

HR is drawing up an individual-contributor list: everyone who is nobody's manager. A person manages someone when their emp_id appears in some other row's manager_id.

Show exactly these 3 columns, in this order: name, department, position. Order by department ascending, then name ascending.

NOT EXISTS is how you ask this. It takes a correlated subquery — one that mentions the outer row — and keeps the row when that subquery finds nothing. What the subquery *selects* is irrelevant, which is why SELECT 1 is the convention: the question is only whether a matching row exists.

Now the part worth the card. The obvious alternative looks equivalent:

WHERE e.emp_id NOT IN (SELECT manager_id FROM employees)

Run it. It returns zero rows — not an error, not a warning, just nothing. Seven employees have no manager, so manager_id is NULL for them, and NOT IN against a list containing NULL can never be true: SQL cannot promise your id differs from a value it does not know. NOT EXISTS compares row by row and is unbothered.

This is the single most common way a correct-looking anti-join returns a silently empty answer. [The anti-join explained](/blog/sql-anti-join/) has the long version.