当多个用户尝试在网站的MySQL数据库中同时更改相同的数据时会发生什么？

Question

Say a PHP script appends some text to an already existing data in a MySQL database. Say the existing data is abc. Now one user wants to append 123 and another wants to append xyz. Both of them run the script at the same time. Whose will be appended first? Will the final result be abc123xyz or abcxyz123?

At a worse case, say the script first takes the data, appends the given text to it, and then replaces the old data in the database. Then whose change will 'survive' here? Will the result be abc123 or abcxyz?

Sorry if it has been asked before.

Answer 1

Depends on the engine you use. For InnoDB MySQL performs row-level lock only for writes. There will be a tiny minute difference in hitting the database with the query

It will execute the first query.Then it will execute the second query so the data will be either 123abc or 123xyz depending upon which query executed first.