UPDATE query to copy from one table to another based on a secondary field

Question

I altered my table structure today and created the 'Country' fields/table and I need to perform an UPDATE query to bring everything in line.

I have thousands of records. CityID has been populated with the IDs from the City lookup table since the database was created. CountryID is currently empty as a new field.

I have begun to associate each CityID with a CountryID. For example, New York City's CityID = "25" and the USA's CountryID = "1", so these values are inputted into the City table.

I would like to UPDATE Main.CountryID retrospectively, so for any records where Main.CityID = "25", it should automatically insert Main.CountryID = 1

Does that make sense?

My Main table:

MainID  
Name  
Address etc.  
CityID
CountryID (Empty)

My City table

CityID  
City  
CountryID

My Country table

CountryID  
Country

Apologies for protracted question, it's been a long day...

Answer 1

Do an join update of the Main table with the City table:

UPDATE Main a
INNER JOIN City b 
    ON a.CityID = b.CityID
SET a.CountryID = b.CountryID

Answer 2

As you have values in City Table update the Main table column on the basis of join with City Table

Try this, i hope this works fine.

Update t1
   set t1.CountryID = t2.CountryID
   FROM Main AS t1
   INNER JOIN City AS t2 ON t1.CityID = t2.CityID

Answer 3

Try this one. I already try it.

UPDATE mainid a JOIN city b
    ON a.cityID = b.cityID
SET a.countryID = b.countryID

Okay, I will explain it. if You wanna get countryID based on cityID. You should join table MainID with table cityID because in that table there is cityID and countryID . So, join mainid a JOIN city b a.cityID = b.cityID and set a.countryID = b.countryID