Nice Question - it's worth understanding this before you write UPDATE statements that you would expect to work in a procedural language, where the value of col1 would be overwritten by col2, meaning you couldn't then update col2 to the value of what col1 was before.

And yes, you do sometimes need to swap values between columns, or at least recalculate a column based on one or more other columns - usually when something has gone wrong!;-)