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.
Note
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
Declaring a Transact-SQL Variable
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, this 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, this script generates a syntax error because the variable is declared in one batch and referenced in another:
USE AdventureWorks;
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 EmployeeID, NationalIDNumber, Title
FROM HumanResources.Employee
WHERE EmployeeID = @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
Setting a Value in a Transact-SQL Variable
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. This batch, for example, declares two variables, assigns values to them, and then uses them in the WHERE clause of a SELECT statement:
USE AdventureWorks;
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 AdventureWorks;
GO
DECLARE @EmpIDVariable int;
SELECT @EmpIDVariable = MAX(EmployeeID)
FROM HumanResources.Employee;
GO
Warning
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 this batch @EmpIDVariable is set to the EmployeeID value of the last row returned, which is 1:
USE AdventureWorks;
GO
DECLARE @EmpIDVariable int;
SELECT @EmpIDVariable = EmployeeID
FROM HumanResources.Employee
ORDER BY EmployeeID DESC;
SELECT @EmpIDVariable;
GO
See Also
Other Resources
DECLARE @local\_variable (Transact-SQL)
SET @local\_variable (Transact-SQL)
SELECT (Transact-SQL)