更改键和外键

Question

I have two tables. First one is dogs:

oldId   name
-----   -----
'AX01'  Fido
'AX02'  Pluto
...      ....

and another called serviced:

Id  ServicedDog  Descripton
--- ------------ ----------
1   'AX01'        'Dog bath'
2   'AX01'        'Dog Vaccined'
...
22  'AX02'        'Dog bath'
...

The problem we have is that we need to convert the oldId column (CHAR(4)) of table dogs to an auto incremented INT one and update the respective ServicedDog column of table serviced to have the new values (it is a foreign key). Another of the problems I have is the data type for ServicedDog is CHAR(4) and I will need to change the data type of this column to INT. Please, tell me, how can this be done?. Thanks

Answer 1

(Without seeing SHOW CREATE TABLE for what you have, I am making some guesses.)

Is it really called oldid now?

-- Add the desired id (and populate it);
-- Replace the existing PK:
ALTER TABLE dogs
    ADD COLUMN id SMALLINT UNSIGNED AUTO_INCREMENT NOT NULL,
    DROP PRIMARY KEY,
    ADD PRIMARY KEY(id)
    ADD INDEX(oldid);

-- add new dog_id (but do not populate)
ALTER TABLE serviced
    ADD COLUMN dog_id SMALLINT UNSIGNED NOT NULL,
    ADD INDEX(dog_id);

-- establish new linkage:
UPDATE   serviced
    JOIN dogs  ON serviced.oldid = dogs.oldid
    SET  serviced.dog_id = dogs.id;

Notes:

• Assumes oldid is really the current name of that column.
• Assumes no more than 64K dogs ever ( SMALLINT UNSIGNED ); change the declaration if you might have more.
• Keeps an index on oldid -- for the conversion; you may want to drop the index and maybe even the column.