EXISTS and NOT EXISTS

EXISTS Operator

Examples

-- students who have paid at least one fee
SELECT name FROM students s
WHERE EXISTS (SELECT 1 FROM fees f WHERE f.roll_no = s.roll_no);

-- students who have NOT paid any fee
SELECT name FROM students s
WHERE NOT EXISTS (SELECT 1 FROM fees f WHERE f.roll_no = s.roll_no);

Summary

• EXISTS = true if the subquery has any row; NOT EXISTS = none.
• Great for "has related record / has no related record" checks.

EXISTS Operator

Examples

-- जिन students ने कम से कम एक fee दी
SELECT name FROM students s
WHERE EXISTS (SELECT 1 FROM fees f WHERE f.roll_no = s.roll_no);

-- जिन students ने कोई fee नहीं दी
SELECT name FROM students s
WHERE NOT EXISTS (SELECT 1 FROM fees f WHERE f.roll_no = s.roll_no);

सारांश

• EXISTS = true अगर subquery में कोई row हो; NOT EXISTS = कोई नहीं।
• "related record है / नहीं है" checks के लिए बढ़िया।