- (Negative) (Transact-SQL)

numeric_expression

Is any valid expression of any one of the data types of the numeric data type category, except the date and time category.

Returns the data type of numeric_expression, except that an unsigned tinyint expression is promoted to a signed smallint result.

A. Setting a variable to a negative value

USE tempdb;
GO
DECLARE @MyNumber decimal(10,2);
SET @MyNumber = -123.45;
SELECT @MyNumber;
GO

B. Changing a variable to a negative value

USE tempdb;
GO
DECLARE @Num1 int;
SET @Num1 = 5;
SELECT -@Num1;
GO

Reference