Declare Variable for a Query String

Solution 1:

It's possible, but it requires using dynamic SQL.
I recommend reading The curse and blessings of dynamic SQL before continuing...

DECLARE @theDate varchar(60)
SET @theDate = '''2010-01-01'' AND ''2010-08-31 23:59:59'''

DECLARE @SQL VARCHAR(MAX)  
SET @SQL = 'SELECT AdministratorCode, 
                   SUM(Total) as theTotal, 
                   SUM(WOD.Quantity) as theQty, 
                   AVG(Total) as avgTotal, 
                  (SELECT SUM(tblWOD.Amount)
                     FROM tblWOD
                     JOIN tblWO on tblWOD.OrderID = tblWO.ID
                    WHERE tblWO.Approved = ''1''
                      AND tblWO.AdministratorCode = tblWO.AdministratorCode
                      AND tblWO.OrderDate BETWEEN '+ @theDate +')'

EXEC(@SQL)

Dynamic SQL is just a SQL statement, composed as a string before being executed. So the usual string concatenation occurs. Dynamic SQL is required whenever you want to do something in SQL syntax that isn't allowed, like:

• a single parameter to represent comma separated list of values for an IN clause
• a variable to represent both value and SQL syntax (IE: the example you provided)

EXEC sp_executesql allows you to use bind/preparedstatement parameters so you don't have to concern yourself with escaping single quotes/etc for SQL injection attacks.

Solution 2:

DECLARE @theDate DATETIME
SET @theDate = '2010-01-01'

Then change your query to use this logic:

AND 
(
    tblWO.OrderDate > DATEADD(MILLISECOND, -1, @theDate) 
    AND tblWO.OrderDate < DATEADD(DAY, 1, @theDate)
)