+= (Add EQUALS) (Transact-SQL)
Adds two numbers and sets a value to the result of the operation. For example, if a variable @x equals 35, then @x += 2 takes the original value of @x, add 2 and sets @x to that new value (37).
Returns the data type of the argument with the higher precedence. For more information, see Data Type Precedence (Transact-SQL).