How to avoid concurrent access to the same record?

Question

MySQL

Hello.
There is a table with records that will be assigned to different users. By default, entries are not associated with any user.
Table view

id | user_id | data
1 | 0 | 123
2 | 0 | 456
3 | 0 | 798

The next free entry in this table must be assigned to the user by changing the user_id and get this changed entry.
I see the algorithm of work as follows: select one free entry and change its user_id.
The problem is as follows, it is possible that there will be several simultaneous requests for a free record. For example, two requests were initiated to assign a free record to a user through requests to the web server. A free record was selected, for example, with id 1. The second request also selected a record with id 1, since it is still free. The first request assigned the user_id value it wanted, and the second request did the same.
As a result, the record is assigned to the user from the second request, and the user from the first one is left with nothing.
How is this situation resolved?

Answer 1

SELECT FOR UPDATE
https://www.laravel.com/docs/5.3/queries#pessimist...