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Basic Transact-SQL Programming Constructs
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There are some potential problems associated with this approach. The question is: How will the server assign values if the resultset contains multiple records, or no records If more than one record is returned in the resultset, a variable will be assigned the values from the last record. The only trouble is that we cannot predict which record will be the last because this position depends on the index that the server uses to create the resultset. It is possible to create workarounds to exploit these facts (that is, to use hints to specify an index or use minimum and/or maximum functions to assign extreme values). The recommended solution, however, is to narrow the search criteria so that only one record is returned. The other behavior that might cause unexpected results is the case in which a resultset does not return any records. It is a common belief and expectation of many developers that the variable will be set to null. This is absolutely incorrect. The content of the variable will not be changed in this case. Observe the following example, or try to run it against the Asset database:
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Declare @make varchar(50), @model varchar(50), @EqType varchar(50) Select @Make = 'ACME', @Model = 'Turbo', @EqType = 'cabadaster' Select @Make = make, @Model = Model, @EqType = EqType.EqType From EqType INNER JOIN Equipment ON EqType.EqTypeId = Equipment.EqTypeId Where EquipmentId = -1 Select @make make, @model model, @EqType EqType
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Since the Equipment table does not have a record with the identifier set to 1, the variables will keep their original value. Only if the values of the variables were not previously set will they continue to contain a null value. The variable can be assigned with any Transact-SQL expression such as a constant, or a calculation, or even a complete Select statement that returns a single value:
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Select @Make = make, @Model = Model, @EquipmentName = Make + ' ' + Model, @EqType = (select EqType from EqType where EqTypeId = Equipment.EqTypeId) From Equipment Where EquipmentId = 2
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There is one combination of statements and expressions that will result in a syntax error. It is not possible to return a resultset from the Select statement and to assign a variable in the same Select statement:
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Select make, @Model = Model From Equipment Where EquipmentId = 2
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The value of a variable can be displayed to the user using a Select or a Print statement:
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Select @LastName Print @FirstName
It is possible to include a local variable in a resultset that will be returned to the user:
Select make "Selected make", Model "Selected Model",
4:
Basic Transact-SQL Programming Constructs
@Model "Original model" From Equipment Where EquipmentId = 2
Assigning Values with the Set Statement
In SQL Server 2000 and SQL Server 7.0, the syntax of the Set statement has been expanded to support the assignment of local variables. In earlier versions, it was possible to use the Set statement only to declare cursor variables. Today, Microsoft is proclaiming this as a preferred method for assigning variables:
Set @LastName = 'Johnson'
There is just one problem with the Set statement it is not possible to assign several values with one statement. You will be forced to write code like this:
Set Set Set @Make = 'ACME' @Model = 'Turbo' @EqType = 'cabadaster'
Assigning Values in the Update Statement
The ability to set the values of local variables in an Update statement is a feature that is buried deep in the oceans of SQL Server Books Online. It is an element that was designed to solve concurrency issues when code needs to read and update a column concurrently.
Update Inventory Set @mnsCost = Cost = Cost * @fltTaxRate Where InventoryId = @intInventoryId
Global Variables
Global variables constitute a special type of variable. The server maintains the values in these variables. They carry information specific to the server or a current user session. They can be examined from anywhere, whether from a stored procedure or a batch. In the
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