A user defined function is a collection of Transact-SQL statements that stored in the SQL Server. A user defined function will return data when executed.

A user defined function works in the same way as a system function. It can be used as a scalar expression or a temporary table depending on the turning data format. A system function is provided as part of the SQL Server. But a user defined function must be created and managed by yourself.

If you are using a group of statements to calculate the same data repeatedly in different places, you should consider to create a user defined function for that group of statements.

User defined functions are also called functions.