Thanks. ;-)

I first had this idea 12 years ago, during a first year C programming lecture at University. The professor declared in the class that if you want to swap the values of two variables, you have to do it with a temporary variable:

X=5
Y=6

# to swap X and Y, introduce a temporary variable Z
Z=X
X=Y
Y=Z
[download]

I challenged his declaration and provided the following solution in 2 steps without a third variable:

X=X+Y
Y=X-Y
X=X-Y

# I know this solution is not perfect:
# when X and Y are big, there is a danger of overflow.
[download]

The lecturer was amazed and gave me an 'A' for the subject. *grin*

The solution set order_id=11-order_id where order_id in (5,6) is just another variant of the same trick.

Update: Thanks rkg to point out the missing bit. My memory got rusty. The original challenge was to swap the values without using a third temporary variable.