In my last post here, I talked about the importance of built-in DB2 functions and how they can be used to simplify development. But DB2 also allows us to create our own, user-defined functions to programmatically extend the functionality of DB2.
A user-defined function, or UDF for short, is procedural functionality that you code up and add to DB2. The UDF, after coded and implemented, extends the functionality of DB2 SQL by enabling users to specify the UDF in SQL statements just like built-in SQL functions.
User-defined functions are ideal for organizations wanting to utilize DB2 and SQL to perform specialized routines with your own rules, business logic and data transformation procedures.
UDFs are similar in functionality to application subroutines, but user-defined functions are different because they can be used inside SQL statements. The only way that user-defined functions can be executed is from within an SQL statement. This gives them great power. A user-defined function is not a substitute for an application subroutine, subprogram, or procedure. Instead, user-defined functions extend the functionality of the SQL language.
There are two basic ways to create a user-defined function: You can code your own program from scratch (using either a host programming language or SQL) or you can edit an existing function. When coded, the CREATE FUNCTION statement registers the user-defined function to DB2 for use.
Types of User-Defined Functions (UDFs)
DB2 supports five different types of UDFs depending upon the manner in which they are coded and the type of functionality they support.
- The first type is the external scalar UDF. Recall, from the prior post, that scalar functions are applied to a column or expression and operate on a single value. Furthermore, external functions are referred to as external because they are written and developed outside of (or external to) DB2. External UDFs must be written in a host programming language. DB2 user-defined functions can be written in Assembler, C, C++, COBOL, Java, or PL/I.
- The next type of UDF is the external table UDF. Remember that table functions, when called, return an entire table. A table function is specified in the WHERE clause of a SELECT statement taking the place of a table, view, synonym, or alias.
- A third type of user-defined function can be created from another existing function. This is a sourced function. A sourced function is based on a function that already exists -— it can be based on a built-in function (scalar or column) or another user-defined function that has already been created. External functions are coded from scratch, whereas sourced functions are created based on a pre-existing function and inherit the attributes of the underlying source function.
- DB2 also supports the creation of SQL scalar user-defined functions. SQL scalar functions return a scalar value and are written entirely using SQL. Fairly extensive functionality can be delivered via SQL scalar functions using SQL procedural language.
- The fifth, and final type of UDF supported by DB2 was introduced as of DB2 10 for z/OS: the SQL table UDF. SQL table functions are written exclusively in SQL but return a table as a result.
There are a lot of details involved in coding and using UDFs, such as schema management, configuring WLM for execution, SQL restrictions, and more. But I will not be digging into those details in today’s post. I invite you to research the topic and learn how to extend the functionality of your DB2 environment using UDFs.