Wednesday, January 31, 2007

A Quick Intro to DB2 SQL Functions

The last post I made to this blog used functions to translate data from one form to another. Functions frequently can be used to overcome issues when you are reading data from your DB2 tables. With that in mind, I thought it might make sense to go over DB2 SQL functions at a basic level - as a review for those who might not have used them before and to refresh the memory of those who have.

Functions can be specified in SQL statements to transform data from one state to another. Two types of functions can be applied to data in a DB2 table using SQL: column functions and scalar functions. Column functions analyze a group of rows and compute a single value for a designated column or expression. For example, the SUM function can be used to add, returning the sum of the values instead of each individual value:

SELECT SUM(SALARY)
FROM EMP
WHERE WORKDEPT = 'A01';

This query will return the total salary for all employees in the A01 department. This is a better solution than writing a program to read all employee rows in department A01 and then adding the salaries. With the SUM function less data is returned to the program, so it will operate much more efficiently.

The second type of function is a scalar function. Scalar functions are applied to a column or expression and operate on a single value. For example, the DAYOFWEEK function reads a single date value and returns an integer in the range of 1 to 7; where 1 is Sunday and 7 is Saturday:

SELECT DAYOFWEEK(HIREDATE)
FROM EMP
WHERE EMPNO = '000100';

This query will return an integer between 1 and 7 that indicates the day of the week when employee number 000100 was hired. Of course, this is just one example of many, many scalar functions supported by DB2. For a complete list of the available functions, refer to Chapter 3 of the IBM DB2 SQL Reference manual. Using scalar functions is generally more efficient than writing your own code to do the same thing.

Speaking of writing your own code, though, you can write your own user-defined functions (UDFs) in addition to the base, system-defined functions. UDFs give you the ability to write your own customized DB2 functions and standardize on that code. Consider creating UDFs for specific algorithms and processing that need to be executed in multiple applications in your organization. By writing the code once, putting it into a DB2 UDF, and then calling it whenever you need it, you will minimize errors and improve reusability.

A user-defined function can be specified anywhere a system-defined function can be specified.

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