Question

I have a strange problem. I got some data for cities, regions and countries in CSV format and imported them into MySQL tables.

I have 3 tables and their fields

1. City    : id, name, country_code, region_number
2. Region  : region_number, country_code, name
3. Country : country_code, name

Now things get a little complicated, as I added an auto-generated id column to the region table, so the region x for country y would be unique.

The thing is: Now i am trying to update city field region_number to hold this unique value (the new id column in region) so I can have relations city->region.

The relation region->country or country->region is OK.

Is it possible to write an update query that would update city region_code (or fill some new column, eg. region_id) with correct values?

If not an query, what could I use to get the correct values into the cities table?

I have arround 3 million records!

Answer 1

If I understant correctly, I think you are looking for something like this:

UPDATE
  City inner join Region
  on City.country_code = Region.country_code
     and City.region_number = Region.region_number
SET
  City.new_column = Region.id

However, since there’s a relation already between City and Region, I am not sure this is the right thing to do, since it will make the table not normalized.