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SQL Server 2000 Stored Procedure & XML Programming
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@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 values. 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 result set from the Select statement and to assign a variable in the same Select statement:
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Make, @Model = Model From Equipment Where EquipmentId = 2 Select -- wrong
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Assigning Values with the Set Statement
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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:
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Set @LastName = 'Johnson'
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Use of the Set statement is preferable, since it makes code more readable and reduces the opportunity to make a mistake (assign a variable and return a result set at the same time).
C h a p t e r 4 : B a s i c Tr a n s a c t - S Q L P r o g r a m m i n g C o n s t r u c t s
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
Displaying the Values of Variables
The value of a variable can be displayed to the user by using a Select or a Print statement:
Select @LastName Print @FirstName
It is possible to include a local variable in a result set that will be returned to the user:
make "Selected make", Model "Selected Model", @Model "Original model" From Equipment Where EquipmentId = 2 Select
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 SQL Server 7.0 and SQL Server 2000 documentation, Microsoft refers to them as scalar functions, meaning that they return just one value. Since you can still find references to global variables in some documentation, and since I would like to use some of them in this chapter, I will review them both here and in the next chapter, which is dedicated to functions.
SQL Server 2000 Stored Procedure & XML Programming
Global variable names begin with an @@ prefix. You do not need to declare them, since the server constantly maintains them. They are system-defined functions and you cannot declare them. Let s review the principal global variables/scalar functions.
@@identity
This is a function/global variable that you will use frequently. It is also a feature that generates many of the questions on Usenet newsgroups. One column in each table can be defined as the Identity column, and the server will automatically generate a unique value in it. This is a standard technique in Microsoft SQL Server for generating surrogate keys (keys whose values are just numbers and do not carry any information). Usually, such columns will be set to assign sequential numbers:
Create table Eq (EqId int identity(1,1), Make varchar(50), Model varchar(50), EqTypeId int)
The @@identity global variable allows you to get the last identity value generated in the current session. It is important to read the value as soon as possible (that is, in the next Transact-SQL statement). Otherwise, it might happen that you initiate, for example, another stored procedure or a trigger that inserts a record to a different table with an Identity column. In such a case, SQL Server overwrites the number stored in @@identity with the new value. In the following example, a record will be inserted and a new identifier will immediately be read:
Declare @intEqId int Insert into Eq(Make, Model, EqTypeId) Values ('ACME', 'Turbo', 2) Select @intEqId = @@identity
If one Transact-SQL statement inserts several records into a table with an Identity column, @@identity will be set to the value from the last record:
Declare @intEqId int Insert into Equipment(Make, Model, EqTypeId) Select Make, Model, EqTypeID From NewEquipment Select @intEqId = @@identity
You will use this function very often. One of the most common types of stored procedures that you will write will just insert a record and return its new key to the caller.
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