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Transact-SQL Variables

A Transact-SQL local variable is an object that can hold a single data value of a specific type. Variables in batches and scripts are typically used:

  • As a counter either to count the number of times a loop is performed or to control how many times the loop is performed.

  • To hold a data value to be tested by a control-of-flow statement.

  • To save a data value to be returned by a stored procedure return code or function return value.

NoteNote

The names of some Transact-SQL system functions begin with two at signs (@@). Although in earlier versions of Microsoft SQL Server, the @@functions are referred to as global variables, they are not variables and do not have the same behaviors as variables. The @@functions are system functions, and their syntax usage follows the rules for functions.

The following script creates a small test table and populates it with 26 rows. The script uses a variable to do three things:

  • Control how many rows are inserted by controlling how many times the loop is executed.

  • Supply the value inserted into the integer column.

  • Function as part of the expression that generates letters to be inserted into the character column.

-- Create the table.
CREATE TABLE TestTable (cola int, colb char(3));
GO
SET NOCOUNT ON;
GO
-- Declare the variable to be used.
DECLARE @MyCounter int;

-- Initialize the variable.
SET @MyCounter = 0;

-- Test the variable to see if the loop is finished.
WHILE (@MyCounter < 26)
BEGIN;
   -- Insert a row into the table.
   INSERT INTO TestTable VALUES
       -- Use the variable to provide the integer value
       -- for cola. Also use it to generate a unique letter
       -- for each row. Use the ASCII function to get the
       -- integer value of 'a'. Add @MyCounter. Use CHAR to
       -- convert the sum back to the character @MyCounter
       -- characters after 'a'.
       (@MyCounter,
        CHAR( ( @MyCounter + ASCII('a') ) )
       );
   -- Increment the variable to count this iteration
   -- of the loop.
   SET @MyCounter = @MyCounter + 1;
END;
GO
SET NOCOUNT OFF;
GO
-- View the data.
SELECT cola, colb
FROM TestTable;
GO
DROP TABLE TestTable;
GO

The DECLARE statement initializes a Transact-SQL variable by:

  • Assigning a name. The name must have a single @ as the first character.

  • Assigning a system-supplied or user-defined data type and a length. For numeric variables, a precision and scale are also assigned. For variables of type XML, an optional schema collection may be assigned.

  • Setting the value to NULL.

For example, the following DECLARE statement creates a local variable named @mycounter with an int data type.

DECLARE @MyCounter int;

To declare more than one local variable, use a comma after the first local variable defined, and then specify the next local variable name and data type.

For example, the following DECLARE statement creates three local variables named @LastName, @FirstName and @StateProvince, and initializes each to NULL:

DECLARE @LastName nvarchar(30), @FirstName nvarchar(20), @StateProvince nchar(2);

The scope of a variable is the range of Transact-SQL statements that can reference the variable. The scope of a variable lasts from the point it is declared until the end of the batch or stored procedure in which it is declared. For example, the following script generates a syntax error because the variable is declared in one batch and referenced in another:

USE AdventureWorks2008R2;
GO
DECLARE @MyVariable int;
SET @MyVariable = 1;
-- Terminate the batch by using the GO keyword.
GO 
-- @MyVariable has gone out of scope and no longer exists.

-- This SELECT statement generates a syntax error because it is
-- no longer legal to reference @MyVariable.
SELECT BusinessEntityID, NationalIDNumber, JobTitle
FROM HumanResources.Employee
WHERE BusinessEntityID = @MyVariable;

Variables have local scope and are only visible within the batch or procedure where they are defined. In the following example, the nested scope created for execution of sp_executesql does not have access to the variable declared in the higher scope and returns and error.

DECLARE @MyVariable int;
SET @MyVariable = 1;
EXECUTE sp_executesql N'SELECT @MyVariable'; -- this produces an error

When a variable is first declared, its value is set to NULL. To assign a value to a variable, use the SET statement. This is the preferred method of assigning a value to a variable. A variable can also have a value assigned by being referenced in the select list of a SELECT statement.

To assign a variable a value by using the SET statement, include the variable name and the value to assign to the variable. This is the preferred method of assigning a value to a variable. The following batch, for example, declares two variables, assigns values to them, and then uses them in the WHERE clause of a SELECT statement:

USE AdventureWorks2008R2;
GO
-- Declare two variables.
DECLARE @FirstNameVariable nvarchar(50),
   @PostalCodeVariable nvarchar(15);

-- Set their values.
SET @FirstNameVariable = N'Amy';
SET @PostalCodeVariable = N'BA5 3HX';

-- Use them in the WHERE clause of a SELECT statement.
SELECT LastName, FirstName, JobTitle, City, StateProvinceName, CountryRegionName
FROM HumanResources.vEmployee
WHERE FirstName = @FirstNameVariable
   OR PostalCode = @PostalCodeVariable;
GO

A variable can also have a value assigned by being referenced in a select list. If a variable is referenced in a select list, it should be assigned a scalar value or the SELECT statement should only return one row. For example:

USE AdventureWorks2008R2;
GO
DECLARE @EmpIDVariable int;

SELECT @EmpIDVariable = MAX(EmployeeID)
FROM HumanResources.Employee;
GO
Caution noteCaution

If there are multiple assignment clauses in a single SELECT statement, SQL Server does not guarantee the order of evaluation of the expressions. Note that effects are only visible if there are references among the assignments.

If a SELECT statement returns more than one row and the variable references a nonscalar expression, the variable is set to the value returned for the expression in the last row of the result set. For example, in the following batch @EmpIDVariable is set to the BusinessEntityID value of the last row returned, which is 1:

USE AdventureWorks2008R2;
GO
DECLARE @EmpIDVariable int;

SELECT @EmpIDVariable = BusinessEntityID
FROM HumanResources.Employee
ORDER BY BusinessEntityID DESC;

SELECT @EmpIDVariable;
GO
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