Variables

Syntax#

DECLARE @VariableName DataType [ = Value ] ;
SET @VariableName = Value ;

Declare a Table Variable

DECLARE @Employees TABLE
(
    EmployeeID INT NOT NULL PRIMARY KEY,
    FirstName NVARCHAR(50) NOT NULL,
    LastName NVARCHAR(50) NOT NULL,
    ManagerID INT NULL
)

When you create a normal table, you use CREATE TABLE Name (Columns) syntax. When creating a table variable, you use DECLARE @Name TABLE (Columns) syntax.

To reference the table variable inside a SELECT statement, SQL Server requires that you give the table variable an alias, otherwise you’ll get an error:

Must declare the scalar variable “@TableVariableName”.

i.e.

DECLARE @Table1 TABLE (Example INT)
DECLARE @Table2 TABLE (Example INT)

/*
-- the following two commented out statements would generate an error:
SELECT *
FROM @Table1
INNER JOIN @Table2 ON @Table1.Example = @Table2.Example

SELECT *
FROM @Table1
WHERE @Table1.Example = 1
*/

-- but these work fine:
SELECT *
FROM @Table1 T1
INNER JOIN @Table2 T2 ON T1.Example = T2.Example

SELECT *
FROM @Table1 Table1
WHERE Table1.Example = 1

Updating a variable using SET

DECLARE @VariableName INT
SET @VariableName = 1
PRINT @VariableName

1

Using SET, you can only update one variable at a time.

Updating variables using SELECT

Using SELECT, you can update multiple variables at once.

DECLARE @Variable1 INT, @Variable2 VARCHAR(10)
SELECT @Variable1 = 1, @Variable2 = 'Hello'
PRINT @Variable1
PRINT @Variable2

1

Hello

When using SELECT to update a variable from a table column, if there are multiple values, it will use the last value. (Normal order rules apply - if no sort is given, the order is not guaranteed.)

CREATE TABLE #Test (Example INT)
INSERT INTO #Test VALUES (1), (2)

DECLARE @Variable INT
SELECT @Variable = Example
FROM #Test
ORDER BY Example ASC

PRINT @Variable

2

SELECT TOP 1 @Variable = Example
FROM #Test
ORDER BY Example ASC

PRINT @Variable

1

If there are no rows returned by the query, the variable’s value won’t change:

SELECT TOP 0 @Variable = Example
FROM #Test
ORDER BY Example ASC

PRINT @Variable

1

Declare multiple variables at once, with initial values

DECLARE 
  @Var1 INT = 5, 
  @Var2 NVARCHAR(50) = N'Hello World', 
  @Var3 DATETIME = GETDATE()

Compound assignment operators

Supported compound operators:

+= Add and assign

-= Subtract and assign

*= Multiply and assign

/= Divide and assign

%= Modulo and assign

&= Bitwise AND and assign

^= Bitwise XOR and assign

|= Bitwise OR and assign

Example usage:

DECLARE @test INT = 42;
SET @test += 1;
PRINT @test;    --43
SET @test -= 1;
PRINT @test;    --42
SET @test *= 2
PRINT @test;    --84
SET @test /= 2;
PRINT @test;    --42

Updating variables by selecting from a table

Depending on the structure of your data, you can create variables that update dynamically.

DECLARE @CurrentID int = (SELECT TOP 1 ID FROM Table ORDER BY CreateDate desc)


DECLARE @Year int = 2014
DECLARE @CurrentID int = (SELECT ID FROM Table WHERE Year = @Year)

In most cases, you will want to ensure that your query returns only one value when using this method.