sql

SQL so‘rovi qaysi ma’lumot kerakligini aytadi, qanday topishni emas.

SELECT name, score
FROM students
WHERE city = 'Toshkent'
ORDER BY score DESC
LIMIT 3;

Bajarilish tartibi

FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMIT. Shuning uchun WHERE da COUNT(*) ishlatib bo‘lmaydi — guruh shartini HAVING ga yozing.

JOIN

SELECT c.name, SUM(o.amount) AS total
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.id
GROUP BY c.id, c.name;

LEFT JOIN buyurtmasiz mijozlarni ham qoldiradi (ularda o.amount — NULL). NULL bilan solishtirish IS NULL orqali, = NULL emas.