EXISTS asks 'is there at least one?'

EXISTS runs a small query for each row and answers yes or no — it doesn't care what it found, only whether anything was there. NOT EXISTS flips it to 'nothing at all', which is a clear way to ask for students who joined no club.

A tiny example

In plain English: Show me the students who have no matching row in the clubs table.

SELECT s.name
FROM students s
WHERE NOT EXISTS (
  SELECT 1
  FROM clubs c
  WHERE c.student_id = s.id
);

What comes back

name
Sana

Sana is the only student with no matching row in the clubs table, so she's the only row.

Related SQL lessons

CASECASE turns values into labels
A window functionA window function adds a value to each row
LAG looks at the row beforeLAG looks at the row before
SUM OVER (ORDER BY ...)SUM OVER (ORDER BY ...) is a running total

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