Unique document number for the month

Question

I need to create an invoice number in format:

CONSTANT_STRING/%d/mm/yyyy

mm - Month (two digits) yyyy - Year

Now, the %d is the number of the invoice in the specific month. Another words, this number is reseted every month.

Now I am checking in database what is the highest number in current month. Then after its incrementation I am saving the row.

I need the whole number to be unique. However, it sometimes happens that it is being duplicated (two users save in the same time).

Any suggestions?

Answer 1

Put a unique index on the field and catch the database error when trying to save the second instance. Also, defer getting the value until the last possible moment.

Answer 2

One solution is SELECT ... FOR UPDATE, which blocks the row until you update it, but can cause deadlocks with a serios multitasking application.

The best way is to fetch the number and increment it in a transaction and then start the work. This way, the row is not locked for long.

Look into BEGIN WORK and COMMIT.

Answer 3

Use the primary key (preferably an INT) of the invoice table or assign a unique number to each invoice, e.g. via uniqid.

PS. If you are using uniqid, you can increase the uniqueness by setting more_entropy parameter to true.

Answer 4

set the id all in one query.

$query = 'INSERT INTO table (invoice_number) VALUES (CONCAT(\''.CONSTANT.'\', \'/\', (SELECT COUNT(*) + 1 AS current_id FROM table WHERE MONTH(entry_date) = \''.date('n').'\' AND YEAR(entry_date) = \''.date('Y').'\'), \'/\', \''.date('m/Y').'\'))';