What does "+=" mean in T-SQL

Question

What does the following variable assignment mean in T-SQL?

SET @myvariable += 'test'

Answer 1

In SQL Server 2008 and later, it is shorthand for addition / concatenation and assignment.

set @x += 'test'

is the same as:

set @x = @x + 'test'

Answer 2

+= (Addition Assignment) : 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)

+= (String Concatenation Assignment) : Concatenates two strings and sets the string to the result of the operation. For example, if a variable @x equals 'Adventure', then @x += 'Works' takes the original value of @x, adds 'Works' to the string, and sets @x to that new value 'AdventureWorks'.