First, define the names of the variables and what type of data they will contain.
Second, set the values of the variables.
Third, reference the variables in the query wherever you normally would use the explicit value.
The first step is to define the names and types of variable you are using. There is one naming convention you must follow. The name of the variable must begin with an @ symbol. After that, any combination of letters and numbers is allowed. The variable may have any appropriate name. The datatype may be any valid datatype supported by SQL Server. There is a list of them at http://msdn.microsoft.com/en-us/library/ms187752(SQL.90).aspx. The syntax for defining the names and types is as follows:
DECLARE @variable AS datatype [, @variable2 AS datatype ... , @variableN AS datatype];
You may declare as many variables as needed in one DECLARE statement. Note the semicolon at the end of the statement. It is usually optional, but it clarifies the end of a SQL statement.
After defining the variables, you set their values. Unlike the DECLARE satement, the SET statement may only affect one variable at a time:
SET @variable = value;
Again, the semicolon at the end is usually optional. However, if you are going to use a Common Table Expression to define a "view" before the main query, you must include the semicolon after the SET statement.
Finally, you use the variable in the query. Put the @variable reference anyplace where you would normally type in the specific value. In the future, you can update the value by changing the variable assignment in the SET statement instead of searching through the query for the explicit value.
Here is an example of using two variables, @StartDate and @EndDate to search for all the giving recorded in the CNHst table between two dates.
declare @StartDate as datetime, @EndDate as datetime
set @StartDate = '1/1/2010';
set @EndDate = '3/31/2020';
C.NameCounter, sum(C.Amount) as TotalGiving
Shelby.CNHst as C
datediff(d, @StartDate, C.CNDate) >= 0 and datediff(d, C.CNDate, @EndDate) >= 0
Even in this simple example, you can see that it is easier to update the dates at the top of the query rather than updating them in the WHERE clause. Once you are comfortable using variables, you will find them immensely helpful in making your queries easy to update over time.