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.
- The names of some Transact-SQL system functions begin with two at signs (@@). Although in earlier versions of SQL Server, the @@functions are referred to as global variables, @@functions aren't variables, and they don't have the same behaviors as variables. The @@functions are system functions, and their syntax usage follows the rules for functions.
- You can't use variables in a view.
- Changes to variables aren't affected by the rollback of a transaction.
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, 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 AdventureWorks2014; 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
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. The following batch, for example, declares two variables, assigns values to them, and then uses them in the
WHERE clause of a
USE AdventureWorks2014; 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 AdventureWorks2014; GO DECLARE @EmpIDVariable INT; SELECT @EmpIDVariable = MAX(EmployeeID) FROM HumanResources.Employee; GO
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 non-scalar 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 AdventureWorks2014; GO DECLARE @EmpIDVariable INT; SELECT @EmpIDVariable = BusinessEntityID FROM HumanResources.Employee ORDER BY BusinessEntityID DESC; SELECT @EmpIDVariable; GO