MySQL-组内计数器 [英] MySQL - Counter within group

问题描述

我想根据升序变量为组中的每一行添加一个计数器.我有一个解决方案，但如果组中的某些变量相等，则它不起作用:

I would like to add a counter for each row within a group according an ascending variable. I have a solution but it does not work if some variable within groups are equal:

CREATE TABLE tb (
g CHAR(1)
, x  INTEGER
);

INSERT INTO tb (g, x)
VALUES
('a',1)
, ('a',2)
, ('a',10)
, ('b',1)
, ('b',1)
, ('b',10)
;

SELECT g,x, (SELECT COUNT(*)
           FROM tb b
           WHERE a.g = b.g
                 AND a.x > b.x
           ) + 1 AS counter
FROM tb a
;    

我得到的是:

--------------------
| g | x  | counter |
--------------------
| a |  1 |    1    |  
| a |  2 |    2    |  
| a | 10 |    3    |
| b |  1 |    1    |
| b |  1 |    1    |
| b | 10 |    3    |
--------------------

但是我想得到:

--------------------
| g | x  | counter |
--------------------
| a |  1 |    1    |  
| a |  2 |    2    |  
| a | 10 |    3    |
| b |  1 |    1    |
| b |  1 |    2    |
| b | 10 |    3    |
--------------------

有什么主意可以解决这个问题吗?非常感谢您的提示.
佐丹奴

Any idea how I can solve this problem? Thanks a lot for hints.
giordano

推荐答案

用户定义的变量在解决此问题时变得很方便.这应该起作用:

User Defined Variables become handy when solving this issues. This should work:

select g, x, counter from (
    select g, x,
        @counter := if (g = @prev_g, @counter + 1, 1) counter,
        @prev_g := g
    from tb, (select @counter := 0, @prev_g := null) init
    order by g, x
) s

如果您真的不介意第四栏，那么可以安全地删除外部选择.这样可以大大提高性能.

If you don't really mind the fourth column then you could safely remove the outer select. That would improve performance a lot.

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