Aggregation & Grouping
Subqueries
Query Inside a Query
💡 Subquery, EK, "NESTED, DOLL" hai — EK, query, ke, ANDAR, EK, DUSRI, query, hoti, hai, JISKA, RESULT, OUTER, query, USE, karti, hai — JAISE, PEHLE, EK, CHHOTA, KAAM, PURA, karke, PHIR, USKE, RESULT, ko, DUSRE, KAAM, mein, USE, karna.
Subquery, WHERE, clause, mein, (filter, condition, ke, roop, mein), FROM, clause, mein, (EK, DERIVED, table, ki, tarah), YA, SELECT, clause, mein, (EK, COMPUTED, column, ki, tarah) use, ho, sakti, hai.
Correlated, subquery, OUTER, query, ke, columns, ko, REFERENCE, karti, hai — HAR, OUTER, row, ke, liye, ALAG, se, EXECUTE, hoti hai, JISSE, YE, PERFORMANCE-EXPENSIVE, ho, sakti, hai.
-- WHERE clause mein subquery:
SELECT name FROM users
WHERE id IN (SELECT user_id FROM orders WHERE amount > 1000);
-- Correlated subquery:
SELECT name, (SELECT COUNT(*) FROM orders WHERE orders.user_id = users.id) AS order_count
FROM users;🪆
Subquery, EK, "NESTED, DOLL" hai — EK, query, ke, ANDAR, EK, DUSRI, query, hoti, hai, JISKA, RESULT, OUTER, query, USE, karti, hai — JAISE, PEHLE, EK, CHHOTA, KAAM, PURA, karke, PHIR, USKE, RESULT, ko, DUSRE, KAAM, mein, USE, karna.
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⚡ झट से Recap
- Subquery = query, ke, andar, ek, dusri, query
- WHERE, FROM, YA, SELECT, clause, mein, use, ho, sakti, hai
- Correlated subqueries, performance-expensive, ho, sakti, hain — joins, aksar, behtar, hote, HAIN
इस page में (2 subtopics)
EXISTS, ek, subquery, ke, ROWS, HONE/NA, HONE, ko, CHECK, karta hai — VALUES, KHUD, RETURN, nahi, karta, isliye, LARGE, result, sets, ke, saath, aksar, IN, se, FASTER, hai.
SELECT name FROM users u
WHERE EXISTS (SELECT 1 FROM orders WHERE orders.user_id = u.id);Subquery, SELECT, clause, mein, use, ho, sakti, hai, EK, COMPUTED, column, ki, tarah — LEKIN, HAR, ROW, ke, liye, ALAG, se, EXECUTE, hoti, hai, isliye, LARGE, tables, PAR, SLOW, ho, sakta, hai.
Common Mistake: SELECT, clause, mein, subqueries, N+1, query, PATTERN, jaisa, PERFORMANCE, PROBLEM, create, kar, sakte, HAIN — JOIN, ya, window, function, aksar, BETTER, alternative, hai.