更新具有特定条件的行时是否需要事务

Question

I am working on a small project and I want to flag some items in a table as "claimed".

So most items would have a claim_id = NULL, when I want to claim an item I will call this command to update the next available item:

update ci_items
set claim_id = ?
where claim_id IS NULL 
order by id 
LIMIT 1;

I think this should always either update the next available item, or nothing if all items have been claimed. In the context of a web application where 2 db connections could run this at the same time, is there any risk that an item gets claimed twice at the same time, one claim_id overriding the previous one?

I am thinking I could use a transaction but the table structure is currently MyISAM and I don't think that supports transactions

Answer 1

What you have show us works well. Individual SQL update queries have an implicit transaction (also known as "autocommit") for their duration.

Go for it.

And don't stop asking yourself this kind of Atomicity, Consistency, Isolation, Durability ( ACID ) question as you design and build your application.