simple definition what is a user defined function
A user-defined function (UDF) is a function provided by the user of a program or environment, in a context where the usual assumption is that functions are built into the program or environment.ser-defined functions in SQL are declared using the CREATE FUNCTION statement. For example, a function that converts Celsius to Fahrenheit might be declared like this: Once created, a user-defined function may be used in expressions in SQL statements.