SELECT c.customer_id, c.name, o.order_id, o.product
FROM customers c
INNER JOIN orders o
  ON c.customer_id = o.customer_id;

SELECT c.customer_id, c.name, o.order_id, o.product
FROM customers c
LEFT JOIN orders o
  ON c.customer_id = o.customer_id;

SELECT c.customer_id, c.name
FROM customers c
LEFT JOIN orders o
  ON c.customer_id = o.customer_id
WHERE o.order_id IS NULL;

SELECT
  e.emp_id,
  e.emp_name,
  e.title,
  m.emp_name AS manager_name,
  m.title AS manager_title
FROM employees e
LEFT JOIN employees m
  ON e.manager_id = m.emp_id
ORDER BY e.emp_id;

SELECT c.name, o.order_id, o.product
FROM customers c
LEFT JOIN orders o
  ON c.customer_id = o.customer_id;

DELETE FROM customers
WHERE customer_id = 1;

DELETE FROM customers
WHERE customer_id = 1;

DELETE FROM users
WHERE user_id = 10;

BEGIN;

SELECT stock
FROM products
WHERE product_id = 1
FOR UPDATE;

INSERT INTO orders (user_id, total, status)
VALUES (101, 500, 'pending');

INSERT INTO order_items (order_id, product_id, qty, price)
VALUES (9001, 1, 2, 250);

UPDATE products
SET stock = stock - 2
WHERE product_id = 1;

COMMIT;

ROLLBACK;

BEGIN;

UPDATE customers
SET name = 'New Name'
WHERE id = 10;

INSERT INTO audit_logs (
  table_name,
  record_id,
  action,
  old_data,
  new_data,
  changed_by,
  changed_at
)
VALUES (
  'customers',
  '10',
  'UPDATE',
  '{"name":"Old Name"}',
  '{"name":"New Name"}',
  99,
  CURRENT_TIMESTAMP
);

COMMIT;

SELECT TRIM(first_name) AS first_name_clean,
       TRIM(last_name) AS last_name_clean
FROM customers;

SELECT LOWER(email) AS email_clean
FROM customers;

SELECT name, COALESCE(phone, '-') AS phone_display
FROM customers;

SELECT COUNT(*) AS total_orders,
       SUM(total) AS revenue,
       AVG(total) AS avg_order
FROM orders;

SELECT ROUND(AVG(total), 2) AS avg_order
FROM orders;

SELECT order_id,
       total,
       CASE
         WHEN total >= 10000 THEN 'high'
         WHEN total >= 5000 THEN 'medium'
         ELSE 'low'
       END AS order_level
FROM orders;