user-defined function
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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. UDFs are usually written for the requirement of its creator.


BASIC language

In some old implementations of the
BASIC Basic or BASIC may refer to: Science and technology * BASIC, a computer programming language * Basic (chemistry), having the properties of a base * Basic access authentication, in HTTP Entertainment * Basic (film), ''Basic'' (film), a 2003 film ...
programming language, user-defined functions are defined using the "DEF FN" syntax. More modern dialects of BASIC are influenced by the
structured programming Structured programming is a programming paradigm aimed at improving the clarity, quality, and development time of a computer program by making specific disciplined use of the structured control flow constructs of selection ( if/then/else) and repet ...
paradigm, where most or all of the code is written as user-defined functions or procedures, and the concept becomes practically redundant.


COBOL language

In the
COBOL COBOL (; an acronym for "common business-oriented language") is a compiled English-like computer programming language designed for business use. It is an imperative, procedural, and, since 2002, object-oriented language. COBOL is primarily ...
programming language, a user-defined function is an entity that is defined by the user by specifying a FUNCTION-ID paragraph. A user-defined function must return a value by specifying the RETURNING phrase of the procedure division header and they are invoked using the function-identifier syntax. See the ISO/IEC 1989:2014 Programming Language COBOL standard for details. As of May 2022, the IBM Enterprise COBOL for z/OS 6.4 (
IBM COBOL IBM has offered the computer programming language COBOL on many platforms, starting with the IBM 1400 series and IBM 7000 series, continuing into the industry-dominant IBM System/360 and IBM System/370 mainframe systems, and then through IBM ...
) compiler contains support for user-defined functions.


Databases

In
relational database management system A relational database (RDB) is a database based on the relational model of data, as proposed by E. F. Codd in 1970. A Relational Database Management System (RDBMS) is a type of database management system that stores data in a structured for ...
s, a user-defined function provides a mechanism for extending the functionality of the
database server A database server is a server which uses a database application that provides database services to other computer programs or to computers, as defined by the client–server model. Database management systems (DBMSs) frequently provide database- ...
by adding a function, that can be evaluated in standard query language (usually
SQL Structured Query Language (SQL) (pronounced ''S-Q-L''; or alternatively as "sequel") is a domain-specific language used to manage data, especially in a relational database management system (RDBMS). It is particularly useful in handling s ...
) statements. The SQL standard distinguishes between scalar and table functions. A scalar function returns only a single value (or
NULL Null may refer to: Science, technology, and mathematics Astronomy *Nuller, an optical tool using interferometry to block certain sources of light Computing *Null (SQL) (or NULL), a special marker and keyword in SQL indicating that a data value do ...
), 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 user-defined function that converts Celsius to Fahrenheit (a temperature scale used in USA) 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 expressions 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 , with a row for each known chemical element. The table has a column named BoilingPoint for the boiling point of that element, in Celsius. The 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 include SQL, C, C# and 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 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 Query optimization is a feature of many relational database management systems and other databases such as NoSQL and graph databases. The query optimizer attempts to determine the most efficient way to execute a given query by considering the pos ...
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 procedure A stored procedure (also termed prc, proc, storp, sproc, StoPro, StoredProc, StoreProc, sp, or SP) is a subroutine available to applications that access a relational database management system (RDBMS). Such procedures are stored in the database d ...
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 Server Microsoft SQL Server is a proprietary relational database management system developed by Microsoft using Structured Query Language (SQL, often pronounced "sequel"). As a database server, it is a software product with the primary function of ...
, for example, allows the user to use .NET languages 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 Microsoft SQL Server is a proprietary relational database management system developed by Microsoft using Structured Query Language (SQL, often pronounced "sequel"). As a database server, it is a software product with the primary function of ...
2000: scalar functions, inline table-valued functions, and 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 A result set is the set of results returned by a query, usually in the same format as the database the query is called on. For example, in SQL, which is used in conjunction with relational databases, it is the result of a SELECT query on a table ...
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 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


Apache Hive

Apache Hive Apache Hive is a data warehouse software project. It is built on top of Apache Hadoop for providing data query and analysis. Hive gives an SQL-like Interface (computing), interface to query data stored in various databases and file systems that i ...
defines, in addition to the regular user-defined functions (UDF), also user-defined aggregate functions (UDAF) and table-generating functions (UDTF). Hive enables developers to create their own custom functions with Java.


Apache Doris

Apache Doris, an open-source real-time analytical database, allows external users to contribute their own UDFs written in C++ to it.


References

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External links


Microsoft SQL Server reference for CREATE FUNCTION
Databases SQL Subroutines