Mysql auto-increment a column with one specific primary key

Question

Let's say I have a table with following scheme: (Note the Post Number column)

-- UserID  -- Post Id  -- Post Number*

-- 4       -- 85       -- 1
-- 4       -- 86       -- 2
....
-- 5       -- 9362     -- 1
-- 4       -- 9363     -- 3

Is there a way to auto_increment a column independently from all other entries and only respect those with the same primary key? If so, how would someone implement this?

Thanks a lot.

Answer 1

SET New.PostId := IFNULL(
      ( SELECT  MAX(PostId)+1
            FROM  SoQuestion
            WHERE  UserId = New.UserId ), 1);

This should avoid the bug wherein a DELETE messes with COUNT(*) .

Answer 2

In order to achieve what you're looking for, you have to use triggers . There's no other direct way to accomplish this task (I guess).

I did try a fast demo now:

Create Table SoQuestion (
  UserId int,
  PostId int,
  PostNumber int null
 );

 CREATE TRIGGER inc_post_num 
 BEFORE INSERT ON SoQuestion
 FOR EACH ROW
 set New.PostNumber = (select num 
                       From (select count(*) as num 
                             from SoQuestion 
                             where UserId = New.UserId) as b) 
                     + 1;


insert into SoQuestion (UserId, PostId) Values (1,1);
insert into SoQuestion (UserId, PostId) Values (1,10);
insert into SoQuestion (UserId, PostId) Values (1,20);
insert into SoQuestion (UserId, PostId) Values (2,1);
insert into SoQuestion (UserId, PostId) Values (2,10);
insert into SoQuestion (UserId, PostId) Values (3,1);
insert into SoQuestion (UserId, PostId) Values (4,1);

select * FROM SoQuestion;

And here's the output that I got:

UserId | PostId | PostNumber |
==============================  
1      | 1      | 1          |
1      | 10     | 2          |
1      | 20     | 3          |
2      | 1      | 1          |
2      | 10     | 2          |
3      | 1      | 1          |
4      | 1      | 1          |

Here's the demo .

---

After going through the Auto_Increment documentation, I found another way to achieve this without using triggers. The idea is about creating an Auto_Increment column and add it with another column as PRIMARY KEY . In our case it would be UserId and the AUTO_INCREMENT would be PostNumber and they both form the primary key. This is how:

Create Table SoQuestion (
  UserId int,
  PostId int,
  PostNumber int NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (UserId, PostNumber)
 );

insert into SoQuestion (UserId, PostId) Values (1,1);
insert into SoQuestion (UserId, PostId) Values (1,10);
insert into SoQuestion (UserId, PostId) Values (1,20);
insert into SoQuestion (UserId, PostId) Values (2,1);
insert into SoQuestion (UserId, PostId) Values (2,10);
insert into SoQuestion (UserId, PostId) Values (3,1);
insert into SoQuestion (UserId, PostId) Values (4,1);

select * FROM SoQuestion;

This would give us the same output that the first way gave:

UserId | PostId | PostNumber |
==============================  
1      | 1      | 1          |
1      | 10     | 2          |
1      | 20     | 3          |
2      | 1      | 1          |
2      | 10     | 2          |
3      | 1      | 1          |
4      | 1      | 1          |

And here's the demo for the second way.