+= (Addition Assignment) (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).
expression += expression
To view Transact-SQL syntax for SQL Server 2014 and earlier, see Previous versions documentation.
Is any valid expression of any data type in the numeric category except the bit data type.
Returns the data type of the argument with the higher precedence. For more information, see Data Type Precedence (Transact-SQL).
For more information, see + (Addition) (Transact-SQL).