Drupal’s database abstraction layer lets you write queries that work across different database engines like MySQL, MariaDB, and PostgreSQL. Instead of writing raw SQL tied to a specific engine, you use Drupal’s database API.
The API provides methods for select, insert, update, delete, and more. You build queries using chainable methods, pass parameters safely to prevent SQL injection, and let Drupal generate the appropriate SQL for whatever database you are running on.
For complex operations, you can still write raw SQL through the query method, but you lose portability. Most sites stick with the entity API and the query builder, which together cover the vast majority of data access needs. Direct database access is rarely needed in modern Drupal because the entity system handles most CRUD operations in a portable way.