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SQL Expressions

A Review of Built-Functions


Built-In Functions Fundamentals


Introduction


While your primary job as a database developer consists of creating lists, probably your second most important job is to assist your users with the various assignments they must perform on your application. One way you can assist is to use functions that perform otherwise complex tasks. We introduced and described functions in the previous lesson. To assist your development with the different tasks of a database, Transact-SQL ships with various already created and tested functions. You just need to be aware of these functions, their syntax, and the results they produce.

To help you identify the functions you can use, they are categorized by their types and probably their usefulness.

Because of their complexities, some values can be easily recognized or fixed. For example, a date such as January 6, 1995 is constant and can never change. This type of value is referred to as deterministic because it is always the same. In the same way, a time value such as 5PM is constant and cannot change. There are other values that cannot be known in advance because they change based on some circumstances. For example, the starting date of the school year changes from one year to another but it always occurs. This means that, you know it will happen but you don't know the exact date. Such a value is referred to as non-deterministic.

To support determinism and non-determinism, Transact-SQL provides two broad categories of functions. A function that always returns the same or known value is referred to as deterministic. A function whose returned value may depend on a condition is referred to as non-deterministic.

Casting a Value



In most cases, a value the user submits to your database is primarily considered a string. This is convenient if that's what you are expecting. If the value the user provides must be treated as something other than a string, for example, if the user provides a number, before using such a value, you should first convert it to the appropriate type, that is, from a string to the expected type.

To assist with conversion, you can use either the CAST() or the CONVERT() function. The syntax of the CAST() function is:

CAST(Expression AS DataType)
The Expression is the value that needs to be cast. The DataType factor is the type of value you want to convert the Expression to. The DataType can be one of those we reviewed in Lesson 4.

In the following example, two variables are declared and initialzed as strings. Because they must be involved in a multiplication, each is converted to a Decimal type:

DECLARE @StrSalary Varchar(10),
@StrHours Varchar(6),
@WeeklySalary Decimal(6,2)
SET @StrSalary = '22.18';
SET @StrHours = '38.50';

SET @WeeklySalary = CAST(@StrSalary As Decimal(6,2)) *
CAST(@StrHours As Decimal(6,2));
SELECT @WeeklySalary;
GO


Converting a Value



Like CAST(), the CONVERT() function is used to convert a value. Unlike CAST(), CONVERT can be used to convert a value its original type into a non-similar type. For example, you can use CONVERT to cast a number into a string and vice-versa.

The syntax of the CONVERT() function is:

CONVERT(DataType [ ( length ) ] , Expression [ , style ])
The first argument must be a known data type, such as those we reviewed in Lesson 4. If you are converting the value into a string (varchar, nvarchar, char, nchar) or a binary type, you should specify the number of allowed characters the data type's own parentheses. As reviewed for the CAST() function, the Expression is the value that needs to be converted.

Here is an example:

-- Square Calculation
DECLARE @Side As Decimal(10,3),
@Perimeter As Decimal(10,3),
@Area As Decimal(10,3);
SET @Side = 48.126;
SET @Perimeter = @Side * 4;
SET @Area = @Side * @Side;
PRINT 'Square Characteristics';
PRINT '-----------------------';
PRINT 'Side = ' + CONVERT(varchar(10), @Side, 10);
PRINT 'Perimeter = ' + CONVERT(varchar(10), @Perimeter, 10);
PRINT 'Area = ' + CONVERT(varchar(10), @Area, 10);
GO






source : http://blogs.msdn.com/sharepoint/default.aspx