sql - Problem with MySql INSERT MAX()+1

Question

I have a single table containing many users. In that table I have column called user_id (INT), which I want increment separately for each person. user_id MUST start at 1

I've prepared a simple example:

Showing all names
+--------------+-----------------------+
| user_id      | name                  |
+--------------+-----------------------+
| 1            | Bob                   |
| 1            | Marry                 |
| 2            | Bob                   |
| 1            | John                  |
| 3            | Bob                   |
| 2            | Marry                 |
+--------------+-----------------------+


Showing only where name = Bob
+--------------+-----------------------+
| user_id      | name                  |
+--------------+-----------------------+
| 1            | Bob                   |
| 2            | Bob                   |
| 3            | Bob                   |
+--------------+-----------------------+

The following query will do this, but it will only work if 'Bob' already exists in the table...

INSERT INTO users(user_id, name) SELECT(SELECT MAX(user_id)+1 from users where 
name='Bob'), 'Bob';

If Bob does not exist (first entry) user_id is set to 0 (zero). This is the problem. I need the user_id to start from 1 not 0.

Answer 1

You can use something like this:

INSERT INTO users (user_id, name)
SELECT 1 + coalesce((SELECT max(user_id) FROM users WHERE name='Bob'), 0), 'Bob';

But such query can lead to a race condition. Make sure you are in a transaction and you lock the users table before running it. Otherwise you might end up with two Bobs with the same number.