A
User-Defined Function, or
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.
BASIC language
In some old implementations of the
BASICIn computer programming, BASIC is a family of high-level programming languages. The original BASIC was designed in 1964 by John George Kemeny and Thomas Eugene Kurtz at Dartmouth in New Hampshire, USA to provide computer access to non-science students...
programming language, user defined functions are defined using the "DEF FN" syntax. More modern dialects of BASIC are influenced by the
structured programmingStructured programming can be seen as a subset or subdiscipline of procedural programming, one of the major programming paradigms. It is most famous for removing or reducing reliance on the GOTO statement....
paradigm, where most or all code is written as user defined functions or procedures, and the concept becomes practically redundant.
Databases
In SQL databases, a user-defined function provides a mechanism for
extendingA software extension is a computer program designed to be incorporated into another piece of software in order to enhance, or extend, the functionalities of the latter...
the functionality of the
database serverA database server is a computer program that provides database services to other computer programs or computers, as defined by the client-server model. The term may also refer to a computer dedicated to running such a program...
by adding a function that can be evaluated in SQL statements. The
SQLSQL:2003 is the fifth revision of the SQL database query language. The latest revision of the standard is SQL:2008.-Summary:The SQL:2003 standard makes minor modifications to all parts of SQL:1999 , and officially introduces a few new features such as:* XML-related features * Window functions* the...
standard distinguishes between scalar and table functions. A scalar function returns only a single value (or
NULLNull is a special marker used in Structured Query Language to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd, SQL Null serves to fulfill the requirement that all true relational database management systems support...
), whereas a table function returns a (relational) table comprising zero or more rows, each row with one or more columns.
User-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:
CREATE FUNCTION dbo.CtoF(Celsius FLOAT)
RETURNS FLOAT
RETURN (Celsius * 1.8) + 32
Once created, a user-defined function may be used in
expressionsAn expression in a programming language is a combination of values, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...
in SQL statements. For example, it can be invoked where most other intrinsic functions are allowed. This also includes SELECT statements, where the function can be used against data stored in tables in the database. Conceptually, the function is evaluated once per row in such usage. For example, assume a table named ELEMENTS, with a row for each known chemical element. The table has a column named BoilingPoint for the boiling point of that element, in Celsius. This query:
SELECT Name, CtoF(BoilingPoint)
FROM Elements
would retrieve the name and the boiling point from each row. It invokes the CtoF user-defined function as declared above in order to convert the value in the column to a value in Fahrenheit.
Each user-defined function carries certain properties or characteristics. The SQL standard defines the following properties:
- language - defines the programming language in which the user-defined function is implemented; examples are SQL, C, or Java.
- parameter style - defines the conventions that are used to pass the function parameters and results between the implementation of the function and the database system (only applicable if language is not SQL).
- specific name - a name for the function that is unique within the database. Note that the function name does not have to be unique, considering overloaded functions. Some SQL implementations, such as Microsoft SQL Server, require that function names are unique within a database, and overloaded functions are not allowed.
- determinism - specifies whether the function is deterministic or not. The determinism characteristic has an influence on the query optimizer
The query optimizer is the component of a database management system that attempts to determine the most efficient way to execute a query. The optimizer considers the possible query plans for a given input query, and attempts to determine which of those plans will be the most efficient...
when compiling a SQL statement.
- SQL-data access - tells the database management system whether the function contains no SQL statements (NO SQL), contains SQL statements but does not access any tables or views (CONTAINS SQL), reads data from tables or views (READS SQL DATA), or actually modifies data in the database (MODIFIES SQL DATA).
User-defined functions should not be confused with
stored procedureA stored procedure is a subroutine available to applications accessing a relational database system. Stored procedures are actually stored in the database data dictionary....
s. Stored procedures allow the user to group a set of SQL commands. A procedure can accept parameters and execute its SQL statements depending on those parameters. A procedure is not an expression and, thus, cannot be used like user-defined functions.
Some database management systems allow the creation of user defined functions in languages other than SQL.
Microsoft SQL ServerMicrosoft SQL Server is a relational model database server produced by Microsoft. Its primary query languages are T-SQL and ANSI SQL.-Genesis:...
, for example, allows the user to use
.NET languagesCLI Languages are computer programming languages that are used to produce libraries and programs that conform to the Common Language Infrastructure specifications...
including C# for this purpose. DB2 and Oracle support user-defined functions written in C or Java programming languages.
SQL Server 2000
There are three types of UDF in Microsoft SQL Server 2000:
- Scalar functions.
- Inline table-valued functions.
- Multistatement table-valued functions.
Scalar functions return a single data value (not a table) with RETURNS clause. Scalar functions can use all scalar data types, with exception of timestamp and user-defined data types.
Inline table-valued functions return the result set of a single SELECT statement.
Multistatement table-valued functions return a table, which was built with many TRANSACT-SQL statements.
User-defined functions can be invoked from a query like built-in functions such as OBJECT_ID, LEN, DATEDIFF, or can be executed through an EXECUTE statement like stored procedures.
Performance Notes:
1. On Microsoft SQL Server 2000 a table-valued function which 'wraps' a View may be much faster than the View itself. The following MyFunction is an example of a 'function-wrapper' which runs faster than the underlying view MyView:
CREATE FUNCTION MyFunction
RETURNS @Tbl TABLE
(
StudentID VARCHAR(255),
SAS_StudentInstancesID INT,
Label VARCHAR(255),
Value MONEY,
CMN_PersonsID INT
)
AS
BEGIN
INSERT @Tbl
(
StudentID ,
SAS_StudentInstancesID ,
Label ,
Value ,
CMN_PersonsID
)
SELECT
StudentID ,
SAS_StudentInstancesID ,
Label ,
Value ,
CMN_PersonsID
FROM MyView -- where MyView selects (with joins) the same columns from large table(s)
RETURN
END
2. On Microsoft SQL Server 2005 the result of the same code execution is the opposite: view is executed faster than the 'function-wrapper'.
User-defined functions are subroutines made of one or more Transact-SQL statements that can be used to encapsulate code for reuse.
It takes zero or more arguments and evaluates a return value. Has both control-flow and DML statements in its body similar to stored procedures.
Does not allow changes to any Global Session State, like modifications to database or external resource, such as a file or a network.
Does not support output parameter.
DEFAULT keyword must be specified to pass the default value of parameter.
Errors in UDF cause UDF to abort which, in turn, aborts the statement that invoked the UDF.
CREATE FUNCTION CubicVolume
-- Input dimensions in centimeters
(@CubeLength decimal(4,1),
@CubeWidth decimal(4,1) ,
@CubeHeight decimal(4,1) )
RETURNS decimal(12,3)
AS
BEGIN
RETURN ( @CubeLength * @CubeWidth * @CubeHeight )
END
Data type supported in Microsoft �SQL Server 2000
Like a temporary table used to store results
Mostly used to define temporary variable of type (table) and the return value of a UDF
The scope is limited to function, stored procedure, or batch in which it is defined
Assignment operation is not allowed between (Table) variables
May be used in SELECT, INSERT, UPDATE, and DELETE
CREATE FUNCTION to create UDF
ALTER FUNCTION to change the characteristics of UDF
DROP FUNCTION to remove UDF
External links