Question

Assuming I have the following:

tableA

id (auto increment primary key) | age | country
1 | 5 | Germany
2 | 6 | Spain
3 | 7 | France
4 | 7 | Spain
5 | 8 | France
6 | 9 | France
7 | 2 | Mexico
8 | 7 | Saudi Arabia
9 | 5 | Saudi Arabia

I want the resulting table:

tableA

id (auto increment primary key) | age | country | country_number
1 | 5 | Germany | 1
2 | 6 | Spain | 2
3 | 7 | France | 3
4 | 7 | Spain | 2
5 | 8 | France | 3
6 | 9 | France | 3
7 | 2 | Mexico | 4
8 | 7 | Saudi Arabia | 5
9 | 5 | Saudi Arabia | 5

Note that “country_number” just a unique number based on whatever a unique country name is.

Using a cursor is slow. How do I make it such that I can do some sort of “group by”? to assign a unique identifier per country? Or is group by not necessary?

Answer 1

query could be (you won’t have 5 for Saudi Arabia, but at least distinct country_number for each country)

select a.id, a.age, a.country, b.minId as country_number
FROM TableA a
INNER JOIN (SELECT MIN(id) as minId, country
            FROM TableA
            GROUP BY country) as b
      ON a.country = b.country

SqlFiddle

To do things properly,

CREATE TABLE COUNTRY(id int PRIMARY KEY auto_increment, name VARCHAR(100))
SELECT distinct COUNTRY as name FROM TableB;

ALTER TABLE TableB Add column countryID int NOT NULL DEFAULT 0;

UPDATE TableB b, country c
set b.countryID = c.Id
where b.country = c.name;

ALTER TABLE TableB drop column country;

ALTER TABLE TableB add  FOREIGN KEY(countryID) REFERENCES country(id); 

see SqlFiddle