SQL (Structured Query Language) is a standardized language used to manage relational databases. In OutSystems, you can use SQL to perform various database operations, such as retrieving, inserting, updating, and deleting data.

OutSystems provides a visual interface to work with databases, but it also allows you to use SQL directly to interact with the underlying database. This is particularly useful when you need to perform complex operations that are not possible through the visual interface.

You can use SQL in OutSystems in several ways:

1. SQL Actions: You can create a custom SQL action that contains one or more SQL statements. These actions can be reused in multiple parts of your application.

2. SQL Queries: You can create a custom SQL query that retrieves data from the database based on a specific set of conditions. These queries can also be reused in multiple parts of your application.

3. Raw SQL: You can use raw SQL statements directly in your code, for example, in an expression or a server action.

In OutSystems, you can work with databases using a variety of relational databases, including Microsoft SQL Server, Oracle, and MySQL. The SQL syntax you use may vary slightly depending on the database type you are using, but the basic concepts are the same.

It's important to keep in mind that working with raw SQL can have security implications, as it allows direct access to the underlying database. You should take steps to prevent SQL injection attacks by properly escaping user input and validating data before using it in a SQL statement.