Question

I have two tables, table_a and table_b:

table_a:

city 
--------
tokoyo 
london 

table_b:

zipcode  
-------
23675
11290

How to combine table_b to table_a, like this:

table_a:

city          zipcode
--------      --------
tokoyo        23675
london        11290

Answer 1

Without a common column to join on, I don’t think you will be able to trust the results (as far as I know, MySQL does not guarantee that rows are returned in the order they are inserted).

But to be technical, I think you can achieve this in the following way: Don’t actually do this, see below

ALTER TABLE table_a ADD COLUMN zipcode unsigned int;
ALTER TABLE table_a ADD COLUMN id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT NOT NULL;
ALTER TABLE table_b ADD COLUMN id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT NOT NULL;
UPDATE table_a JOIN table_b USING(id) SET table_a.zipcode = table_b.zipcode;

Forgive me if my syntax is off, it’s been a while.

This technique leverages what I think MySQL does when you add a non-null auto_increment column: it fills in the values for you, one unique value per row. Now you have a joining column, and can perform the update.

However, I’ll state this again to be very clear: If there really is no additional columns between the tables, you have no completely accurate way to join these two tables. You would need either a common join column, or at least each having an auto_increment column and the guarantee that all zipcodes and cities where entered in appropriate pairs in transactions.