DEV is not SQL function.
SQL (structured query language) is a domain-specific language used in programming and designed for managing data held in a relational database management system, or for stream processing in a relational data stream management system.
Functions can be used anywhere in SQL, like AVG, COUNT, SUM, MIN, DATE and so on with select statements. Functions compile every time. Functions must return a value or result. Functions only work with input parameters.
A function is a database object in SQL server. Basically, it is a set of SQL statements that accept only input parameters, perform actions and return the result. The function can return only a single value or a table.