如何根据mysql中的组添加自动递增id

Question

table格式如下：

indexer group name id
1       abc   a
2       abc   b
3       xyz   c
4       abc   e
5       xyz   d

现在我希望它像，

indexer group name id
1       abc   a    1
2       abc   b    2
3       xyz   c    1
4       abc   e    3
5       xyz   d    2

"id" 应根据 "group"

自动递增

Answer 1

试试这个：

update yourtable t1
join (
    select
          tt.indexer, @rowno := if(@grp = `group`, @rowno + 1, 1) as id, @grp := `group`
    from (select * from yourtable order by `group`, indexer) tt
    cross join (select @rowno := 0, @grp := null) t
) t2
on t1.indexer = t2.indexer
set t1.id = t2.id

Demo Here

已编辑：

如果你想插入一个新行，你必须这样做：

insert into yourtable
select '$indexer', '$group', '$name', coalesce(max(id), 0) + 1
from yourtable
where name = '$name'

Answer 2

如果你想使用内置函数，你必须使用 MyISAM table。但请注意，那些不支持事务并使用 table 级别的锁等。您可能想阅读相关内容。如果您觉得不错，请按照以下步骤操作。如果要使用InnoDB或其他引擎，则必须自己编写解决方案，即存储过程。

引自manual：

For MyISAM tables, you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.
CREATE TABLE animals (
    grp ENUM('fish','mammal','bird') NOT NULL,
    id MEDIUMINT NOT NULL AUTO_INCREMENT,
    name CHAR(30) NOT NULL,
    PRIMARY KEY (grp,id)
) ENGINE=MyISAM;

INSERT INTO animals (grp,name) VALUES
    ('mammal','dog'),('mammal','cat'),
    ('bird','penguin'),('fish','lax'),('mammal','whale'),
    ('bird','ostrich');

SELECT * FROM animals ORDER BY grp,id;

Which returns:

+--------+----+---------+
| grp    | id | name    |
+--------+----+---------+
| fish   |  1 | lax     |
| mammal |  1 | dog     |
| mammal |  2 | cat     |
| mammal |  3 | whale   |
| bird   |  1 | penguin |
| bird   |  2 | ostrich |
+--------+----+---------+
In this case (when the AUTO_INCREMENT column is part of a multiple-column index), AUTO_INCREMENT values are reused if you delete the row with the biggest AUTO_INCREMENT value in any group. This happens even for MyISAM tables, for which AUTO_INCREMENT values normally are not reused.

If the AUTO_INCREMENT column is part of multiple indexes, MySQL generates sequence values using the index that begins with the AUTO_INCREMENT column, if there is one. For example, if the animals table contained indexes PRIMARY KEY (grp, id) and INDEX (id), MySQL would ignore the PRIMARY KEY for generating sequence values. As a result, the table would contain a single sequence, not a sequence per grp value.

如何根据mysql中的组添加自动递增id

How to add auto increment id according to a group in mysql

mysql

auto-increment