Most efficient way to migrate a MySQL column from a VARCHAR to a foreign key in another table

Question

I have a few huge (10s of millions of rows in 1 table) legacy tables that have a few columns I should definitely have set up to be foreign keys when I created them, but without a time machine, I have to migrate them.

What's in these columns is like a short "key" for something else. A real example of one of these new tables would be:

+----+-----+---------+
| id | key |  name   |
+----+-----+---------+
|  1 | bf  | BetFair |
|  2 | bd  | BetDaq  |
+----+-----+---------+

And a current row, in the current table has something like,

. (bet_id=1234, odds=2.1, source='bf') (bet_id=1235, odds=2.15, source='bd') .

And what I want the eventual outcome to be,

. (bet_id=1234, odds=2.1, source_id=1) (bet_id=1235, odds=2.15, source_id=2) .

I know how to do this in multiple steps, create the new tables, add all the data from the source tables to the new tables with GROUP BY / DISTINCT , and eventually setting the new foreign key id columns with commands like,

UPDATE BetsTable SET source_id=1 WHERE source='bf' ,

I'm just wondering if there's more of a "one-shot", efficient SQL command to update the entire table in one step, rather than multiple.

Answer 1

If you just want to change the data, try this:

UPDATE BetsTable b
JOIN NewTable n ON n.key = b.source
SET b.source = n.id

If you realy need to make the source column a foreign key, you will need to change it's data type first. But in that case I'm pretty sure it would be more efficient to rebuild the table.