Question

I have a table with three filled columns named “Name”, “City” and “Occupation”.
I want to create a new column in the same table that contains the number of people who have the same occupation.

"Name" | "City" | "Occupation"
------------------------------
Amy    | Berlin | Plumber
Bob    | Berlin | Plumber
Carol  | Berlin | Lawyer
David  | London | Plumber

I want to have a table that contains:

"Name" | "City" | "Occupation" | "Number"
---------------------------------------
Amy    | Berlin | Plumber      | 2
Bob    | Berlin | Plumber      | 2
Carol  | Berlin | Lawyer       | 1
David  | London | Plumber      | 1

How does the SQL Query that creates the new columns have to look like? I want to actually create a new column in the database that I can access later.

Answer 1

Simple self-join:

SELECT t0.Name, t0.City, t0.Occupation, COUNT(*) AS Number
FROM sometable AS t0
JOIN sometable AS t1 ON t1.Occupation=t0.Occupation
GROUP BY t0.Name, t0.City, t0.Occupation

If Name is a primary key you can just group by that alone instead, since the other columns would have a functional dependency on it. Of course Name wouldn’t normally be a very good primary key.

(You might need COUNT(*)-1 if you want the number of other people doing the job rather than the total. It’s unclear; the numbers in your example don’t add up either way.)

If you must change your schema (and I wouldn’t recommend this kind of denormalisation unless you are absolutely certain you need it; see comments), you can do it with an UPDATE JOINed to Michael’s subselect:

ALTER TABLE sometable ADD COLUMN Number INTEGER NOT NULL;
UPDATE sometable AS t0 JOIN (
    SELECT Occupation, COUNT(*) AS Number
    FROM sometable
    GROUP BY Occupation
) AS t1 ON t1.Occupation=t0.Occupation
SET t0.Number= t1.Number;