MySQL database merge of two databases

Question

I set up two Wordpress blogs a while ago, both obviously having different databases. I've more recently merged these databases into one by changing the tables prefixes, therefore these two 'entities' have the same amount of tables and the same names (as they originate from a Wordpress install) but with different prefixes, ie:

Blog1_tabledata1
Blog1_tabledata2
Blog1_tabledata3
Blog1_tabledata4
Blog2_tabledata1
Blog2_tabledata2
Blog2_tabledata3
Blog2_tabledata4

I have now realised that I need to merge these two databases (where they're both using the same tables) so that they can be used in the same Wordpress instance (later separated by tags etc).

What would be the most simple way of doing this?

(Please note I am asking this from a MySQL standpoint - this is not a Wordpress question!)

Answer 1

If you absolutely are not looking for a wordpress solution, that means you are not looking at all domains. By this, I mean that you are not looking at what the data means . This could be a problem. but nevertheless:

1. figure out the foreign keys. If the tables are MyIsam instead of InnoDB, they will be implicit. Figure out what ID points to what field.
2. select from one DB and insert into the other. This mean you add the rows of one table to the equivalent of the target database. Auto-increment rows will be fine. But for foreign key (explicit AND implicit) fields here is were the trouble starts.
3. If you insert, say a user, the user gets a new ID -> You have to find the equivalent of the userid in the old db so you can insert the foreign keys with the right ID. this is tricky and without making this a wordpress-question there is no more help we can give you: just figure out what rows they should be :). it is database // domain specific. (with that I mean you can't just figure that out by looking at the fields, you must know some of what they mean)
4. If the db is correct, this should work, but I'm not sure if you get into trouble with duplicates (all should go on ID, and you fixed that in step 3 with unique and connected id's. but if your domain doesn't want two accounts, two pages or two whatevers (tags?) to have the same name, you still have a problem. But again, this is domain specific logic and you're specifically asking not to go there.