چیت‌شیت SQL

1SELECT orders.id, users.name2FROM orders3INNER JOIN users ON users.id = orders.user_id;

1SELECT users.id, orders.id AS order_id2FROM users3LEFT JOIN orders ON orders.user_id = users.id;

1SELECT user_id, COUNT(*) AS order_count2FROM orders3GROUP BY user_id4HAVING COUNT(*) > 1;

1SELECT COUNT(*) AS total,2       AVG(price) AS average_price,3       SUM(price) AS total_price4FROM products;

1INSERT INTO users (name, email)2VALUES ('Sara', 'sara@example.test');

1UPDATE users2SET status = 'active'3WHERE id = 42;

1DELETE FROM sessions2WHERE expires_at < CURRENT_TIMESTAMP;

1PostgreSQL: WHERE id = $12MySQL: WHERE id = ?3SQL Server: WHERE id = @id

1BEGIN;2UPDATE accounts SET balance = balance - 25 WHERE id = 1;3UPDATE accounts SET balance = balance + 25 WHERE id = 2;4COMMIT;

1CREATE TABLE users (2  id INTEGER PRIMARY KEY,3  email VARCHAR(255) NOT NULL UNIQUE,4  created_at TIMESTAMP NOT NULL5);

1CREATE TABLE orders (2  id INTEGER PRIMARY KEY,3  user_id INTEGER NOT NULL REFERENCES users(id)4);

1CREATE INDEX idx_orders_user_created2ON orders (user_id, created_at);

1EXPLAIN2SELECT id, created_at3FROM orders4WHERE user_id = 42;

1WITH recent_orders AS (2  SELECT id, user_id, total3  FROM orders4  WHERE created_at >= DATE '2026-01-01'5)6SELECT user_id, SUM(total) AS lifetime_total7FROM recent_orders8GROUP BY user_id;

1SELECT2  user_id,3  created_at,4  total,5  ROW_NUMBER() OVER (6    PARTITION BY user_id7    ORDER BY created_at DESC8  ) AS order_rank9FROM orders;

1SELECT2  id,3  CASE4    WHEN score >= 80 THEN 'high'5    WHEN score >= 50 THEN 'medium'6    ELSE 'low'7  END AS score_band,8  COALESCE(nickname, name, 'Unknown') AS display_name9FROM users;

1INSERT INTO products (sku, name, price)2VALUES3  ('A-101', 'Notebook', 4.50),4  ('A-102', 'Pencil', 1.25);

1INSERT INTO inventory (sku, quantity)2VALUES ('A-101', 20)3ON CONFLICT (sku)4DO UPDATE SET quantity = EXCLUDED.quantity;

1CREATE TABLE payments (2  id BIGINT PRIMARY KEY,3  amount DECIMAL(12, 2) NOT NULL CHECK (amount >= 0),4  currency CHAR(3) NOT NULL5);