Functions contain a series of instructions, which are performed when the function is called, generally returning one result upon completion. MariaDB Enterprise Server provides support for a number of functions, including stored functions, built-in functions, user-defined functions, and aggregate functions.
They differ from procedures in how they are commonly used. Functions are often called in SELECT statements to perform underlying logic and return one value only to the query. Procedures are typically not used in SELECT statements.
Additional information is available in the MariaDB Knowledge Base: