mySQL - update multiple columns with a select returning multiple rows

Question

I have a table of postcodes and I want to update each postcode with its 3 nearest neighbours. Ie to fill in the blanks in this table:

postcode  nearestPostco         


        

Answer 1

Anytime I see a table with columns that have 1-up counters after their names, I get concerned.

In general, it is a Bad Idea (TM) to store data that can be calculated from data that is already stored. What happens if your application all of a sudden needs the 4 closest postal codes? What if the postal code boundaries change?

Assuming the distance calculation isn't very complex, you'll be better off in the long run not explicitly storing this data.