Reconnecting two sql tables using mutual id from third table

Question

Suppose we have three SQL tables A,B and C:

TABLE A { id, c_id}

TABLE B {id, c_id}

TABLE C {id}

So A and B have foreign keys from table C. I created a column in Table A that will reference Table B:

ALTER TABLE A ADD COLUMN b_id INTEGER;
ALTER TABLE A ADD FOREIGN KEY (b_id) REFERENCES B(id);

All the tables are in production and filled with data.

What i need to figure out is how to populate this new column for every row and i need a SQL query or a hint that will help me to remap A to B based on a c_id in both of them.

So i need to get just one of many b_id from B for each row in A based on a mutual c_id and insert it for each row.

The business rule says just to use any (first) b_id when there are more of them.

After that i'll drop the c_id in Table A.

Answer 1

I think it will be:

UPDATE A  
SET b_id = t.b_ID
FROM (select min(id) as b_Id,c_id FROM B Group by c_id ) AS t
WHERE A.c_id = t.c_id 

SQLFiddle demo