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The major difference between the two types of variables is their scope. The scope of local variables is a batch (a set of T-SQL statements that is sent to SQL Server and executed simultaneously). This restriction implicitly includes a single stored procedure
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(because stored procedures are defined in a batch). This is a significant limitation. However, several workarounds can be used as solutions to this problem. A stored procedure cannot access variables defined in other stored procedures. One way to pass values to and from stored procedures is to use parameters. Keep in mind that you are passing only the values associated with the variables, not references, as you can in some other programming languages. Another way to transfer value between stored procedures or between batches is the use of more permanent database objects such as tables or temporary tables. Let s review basic operations with local variables.
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Before you can do anything with a local variable, you need to declare it. Declaration consists of the reserved word Declare and a list of variables and their respective data types. The names of variables must comply with the rules for identifiers: They must begin with @:
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Declare @LastName varchar(50)
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It is possible to define several variables in a single Declare statement. You just need to separate them with commas:
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Declare @LastName varchar(50), @FirstName varchar(30), @BirthDate smalldatetime
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You can also define variables based on user-defined data types:
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Declare @OfficePhone phone
NOTE
You cannot define the nullability of the variable, as you can with table columns. This does not mean that variables cannot contain null values. In fact, before assignment, the value of each variable is null. It is also possible to explicitly set the value of each variable to null.
Assigning Values with the Select Statement
There are several ways to assign a value to a local variable. In early versions of SQL Server, the only way to do this was to use a modification of the Select statement:
Select @LastName = 'Smith'
It is also possible to assign several variables in the same statement:
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
Select
@LastName = 'Smith', @FirstName = 'David', @BirthDate = '2/21/1965'
NOTE
It is necessary to assign a value of an appropriate data type to the variable; however, there are some workarounds. In some cases, the server will perform an implicit conversion from one data type to another. SQL Server also includes a set of functions for explicit conversion. Convert() and Cast() can be used to change the data type of the value (see 5). Some data types are not compatible, so explicit conversion is the only solution.
Quite often, variables are assigned values from the result set of the Select statement:
@Make = Equipment.Make, @Model = Equipment.Model, @EqType = Equipment.EqType From EqType INNER JOIN Equipment ON EqType.EqTypeId = Equipment.EqTypeId Where EquipmentId = 2 Select
There are some potential problems associated with this approach. How will the server assign values if the result set contains multiple records, or no records If more than one record is returned in the result set, 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 result set. 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 result set 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:
Declare @make varchar(50), @model varchar(50), @EqType varchar(50) @Make = 'ACME', @Model = 'Turbo',
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