Question

Earlier today I asked this question which arose from A- My poor planning and B- My complete disregard for the practice of normalizing databases. I spent the last 8 hours reading about normalizing databases and the finer points of JOIN and worked my way through the SQLZoo.com tutorials.

I am enlightened. I understand the purpose of database normalization and how it can suit me. Except that I’m not entirely sure how to execute that vision from a procedural standpoint.

Here’s my old vision: 1 table called “files” that held, let’s say, a file id and a file url and appropos grade levels for that file.

New vision!: 1 table for “files”, 1 table for “grades”, and a junction table to mediate.

But that’s not my problem. This is a really basic Q that I’m sure has an obvious answer- When I create a record in “files”, it gets assigned the incremented primary key automatically (file_id). However, from now on I’m going to need to write that file_id to the other tables as well. Because I don’t assign that id manually, how do I know what it is?

If I upload text.doc and it gets file_id 123, how do I know it got 123 in order to write it to “grades” and the junction table? I can’t do a max(file_id) because if you have concurrent users, you might nab a different id. I just don’t know how to get the file_id value without having manually assigned it.

Answer 1

You may want to use LAST_INSERT_ID() as in the following example:

START TRANSACTION;
INSERT INTO files (file_id, url) VALUES (NULL, 'text.doc');
INSERT INTO grades (file_id, grade) VALUES (LAST_INSERT_ID(), 'some-grade');
COMMIT;

The transaction ensures that the operation remains atomic: This guarantees that either both inserts complete successfully or none at all. This is optional, but it is recommended in order to maintain the integrity of the data.

For LAST_INSERT_ID(), the most
recently generated ID is maintained in
the server on a per-connection basis.

It is not changed by another client.
It is not even changed if you update
another AUTO_INCREMENT column with a
nonmagic value (that is, a value that
is not NULL and not 0).

Using
LAST_INSERT_ID() and AUTO_INCREMENT
columns simultaneously from multiple
clients is perfectly valid. Each
client will receive the last inserted
ID for the last statement that client
executed.

Source and further reading:

• MySQL Reference: How to Get the Unique ID for the Last Inserted Row
• MySQL Reference: START TRANSACTION, COMMIT, and ROLLBACK Syntax