MySql INSERT MAX()+ 1的问题 [英] Problem with MySql INSERT MAX()+1

问题描述

我有一个包含许多用户的表.在该表中，我有一个名为user_id(INT)的列，我想为每个人分别增加. user_id必须从1开始

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

我准备了一个简单的示例:

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                   |
+--------------+-----------------------+

以下查询将执行此操作，但仅在表中已经存在鲍勃"的情况下才有效...

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';

如果Bob不存在(第一项)，则user_id设置为0(零).这就是问题.我需要user_id从1而不是0开始.

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.

推荐答案

您可以使用以下内容:

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

但是这样的查询会导致竞争状况.在运行之前，请确保您正在事务中并锁定了用户表.否则，您可能会得到两个具有相同编号的Bobs.

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.

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