User-defined function
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User Defined Function, or UDF is an extension to a SQL Server (like MySQL) that can be compiled for use in a SQL query. For example, if it is necessary to obtain a random number for use as a primary key, and MySQL does not support that function, a new, external function could be written and compiled to accomplish this task. One sample of such query could be:
SELECT GETRANDOMKEY();
Essentially, this external function becomes part of the server itself.