SQL expert Larry Rockoff introduces a way to perform calculations on the individual data items retrieved from a database.
Anyone familiar with Microsoft Excel is probably aware that functions provide a huge amount of functionality for the typical spreadsheet user. Without the ability to use functions, most of the data available in spreadsheets would be of limited value. The same is true in the world of SQL. Familiarity with SQL functions will greatly enhance your ability to generate dynamic results for anyone viewing data or reports generated from SQL.
This chapter covers a wide variety of some of the most commonly used functions in four different categories: character functions, date/time functions, numeric functions, and conversion functions. Additionally, we’ll talk about composite functions—a way of combining multiple functions into a single expression.
What Is a Function?
Similar to the calculations covered in the previous chapter, functions provide another way to manipulate data. As was seen, calculations can involve multiple fields, either with arithmetic operators such as multiplication, or by concatenation. Similarly, functions can involve data from multiple values, but the end result of a function is always a single value.
What is a function? A function is merely a rule for transforming any number of input values into one output value. The rule is defined within the function and can’t be altered. However, the user of a function is allowed to specify any desired value for the inputs to the function. Some functions may allow some of the inputs to be optional. That means that the specification of that particular input isn’t required. Functions can also be designed to have no inputs. However, regardless of the type or number of input values, functions always return precisely one output value when the function is invoked.
There are two types of functions: scalar and aggregate. The term scalar comes from mathematics and refers to an operation that is done on a single number. In computer usage, it means that the function is performed on data in a single row. For example, the LTRIM function removes spaces from one specified value in one row of data.
In contrast, aggregate functions are meant to be performed on a larger set of data. For example, the SUM function can be used to calculate the sum of all the values of a specified column. Because aggregate functions apply to larger sets or groups of data, we will leave discussion of this type of function to Chapter 9, “Summarizing Data.”
Every SQL database offers dozens of scalar functions. The actual functions vary widely between databases, in terms of both their names and how they work. As a result, we will cover only a few representative examples of some of the more useful functions.
The most common types of scalar functions can be classified under three categories: character, date/time, and numeric. These are functions that allow you to manipulate character, date/time, or numeric datatypes. In addition, we will talk about some useful conversion functions that can be used to convert data from one datatype to another.