Question

Given this table design in a web application:

CREATE TABLE `invoice` (
  `invoice_nr` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `revision` int(10) unsigned NOT NULL DEFAULT '1',
  PRIMARY KEY (`invoice_nr`,`revision`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci

What’s the most handy and reliable way to insert a new invoice revision and get back the assigned revision number?

The first obvious approach strikes me as unreliable in a shared environment:

SELECT MAX(revision)+1 AS next_revision -- E.g. 2
FROM invoice
WHERE invoice_nr = 31416;

INSERT INTO invoice (invoice_nr, revision)
VALUES (31416, 2);

This alternative looks slightly better (I don’t know if it’s actually better):

INSERT INTO invoice (invoice_nr, revision)
SELECT 31416, MAX(revision)+1
FROM invoice
WHERE invoice_nr = 31416;

… but I can’t know the revision number unless I run a new query:

SELECT MAX(revision) AS last_revision
FROM invoice
WHERE invoice_nr = 31416;

Is there a recommended method?

App runs on PHP with good old mysql extension–mysql_query() et al.

Answer 1

I’ve gathered a couple of techniques from the MySQL manual. I thought I should share them here for the records.

1. Table locking

If you place a lock on the table, other simultaneous processes will be queued. Then, you don’t need any special trick to avoid dupes:

LOCK TABLES invoice WRITE;

SELECT MAX(revision)+1 AS next_revision -- E.g. 2
FROM invoice
WHERE invoice_nr = 31416;

INSERT INTO invoice (invoice_nr, revision)
VALUES (31416, 2);

-- Or INSERT INTO ... SELECT

UNLOCK TABLES;

2. LAST_INSERT_ID(expr)

The LAST_INSERT_ID() function accepts an optional parameter. If set, it returns such value and it also stores it for the current session so next call to LAST_INSERT_ID() will return that value. This is a handy way to emulate sequences:

CREATE TABLE `sequence` (
    `last_value` INT(50) UNSIGNED NOT NULL DEFAULT '1' COMMENT 'Last value used'
);

START TRANSACTION;

UPDATE sequence
SET last_value=LAST_INSERT_ID(last_value+1);

INSERT INTO invoice (invoice_nr, revision)
VALUES (31416, LAST_INSERT_ID());

COMMIT;