User-Defined Functions
Microsoft SQL Server 2000 lets you create user-defined functions. Like any function, a user-defined function is a routine that returns a value. Based on what kind of value it returns, each user-defined function falls into one of three categories: ones that return an updatable table, ones that return a non-updatable table, and ones that return a scalar value.
If a user-defined function contains a single SELECT statement and that statement is updateable, then the tabular result returned by the function is also updateable.
If a user-defined function contains more than one SELECT statement, or contains a SELECT statement that is not updateable, then the tabular result returned by that function is not updateable.
If a function returns a table, you can use that function in the FROM clause of a query. For more information, see How to: Create Queries using Something Besides a Table. If a function returns a scalar value, you can use it in a query anywhere you would use a column name. For more information, see Expressions in Queries.