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Thứ tự thực thi 1 câu lệnh SQL là một phần quan trọng để thực hiện truy vấn cơ sỡ dữ liệu thông qua ngôn ngữ SQL. Giúp lập trình viên như chúng ta có thể dễ dàng kiểm soát business logic của mình. Từ đó đưa ra các dòng truy phấn phức tạp một cách chính xác hơn. Bài viết ngày hôm nay mình muốn chia sẻ về trình tự thực thi của một câu lệnh SQL. Liệu rằng những gì xảy ra bên dưới của nó có giống như trình sự khi viết mã ?Câu lệnh SQL được thực hiện theo trình tự logic (còn được gọi là logical query processing order) thay vì thứ tự chúng xuất hiện trong câu lệnh. Điều này rất quan trọng để hiểu cách SQL Server, MySQL, PostgreSQL, hoặc các hệ thống quản lý cơ sở dữ liệu khác xử lý câu lệnh SQL. Trình tự thực hiện như sau:
1. Thứ tự thực thi 1 câu lệnh SQL với FROM
- Xác định bảng hoặc tập dữ liệu nào sẽ được lấy ra.
- Nếu có phép kết nối (JOIN), dữ liệu được kết hợp tại bước này.
Ví dụ:
SELECT * FROM employees JOIN departments ON employees.department_id = departments.id;
Ở đây, dữ liệu từ employees và departments được kết nối dựa trên department_id.
2. Thứ tự thực thi 1 câu lệnh SQL với WHERE
- Lọc các hàng (row) dựa trên điều kiện.
- Chỉ các hàng thỏa mãn điều kiện trong WHERE mới được chuyển sang bước tiếp theo.
Ví dụ:
SELECT * FROM employees WHERE salary > 5000;
Chỉ các nhân viên có mức lương lớn hơn 5,000 mới được chọn.
3.
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