SQL cơ bản: SELECT, WHERE, JOIN cho người mới bắt đầu

-- Lấy tất cả cột
SELECT * FROM employees;

-- Lấy các cột cần thiết
SELECT name, email, department FROM employees;

-- Alias
SELECT name AS ho_ten, salary AS luong FROM employees;

-- So sánh
SELECT * FROM employees WHERE salary > 15000000;

-- Nhiều điều kiện
SELECT * FROM employees
WHERE department = 'IT' AND salary > 20000000;

-- LIKE — tìm kiếm pattern
SELECT * FROM employees WHERE name LIKE '%Nguyễn%';

-- IN — nhiều giá trị
SELECT * FROM employees WHERE department IN ('IT', 'HR', 'Finance');

-- BETWEEN — khoảng
SELECT * FROM orders WHERE order_date BETWEEN '2026-01-01' AND '2026-12-31';

SELECT * FROM employees ORDER BY salary DESC;  -- Giảm dần
SELECT * FROM employees ORDER BY department ASC, salary DESC;

-- INNER JOIN: Chỉ lấy dữ liệu khớp cả 2 bảng
SELECT e.name, d.department_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.id;

-- LEFT JOIN: Lấy tất cả từ bảng trái
SELECT e.name, o.order_total
FROM employees e
LEFT JOIN orders o ON e.id = o.employee_id;

SELECT
    department,
    COUNT(*) AS so_nhan_vien,
    AVG(salary) AS luong_tb,
    MAX(salary) AS luong_cao_nhat
FROM employees
GROUP BY department
HAVING COUNT(*) > 5;