mysql - insert into table select max(column_name)+1

Question

I have this mysql table built like this:

CREATE TABLE `posts` (
    `post_id` INT(10) NOT NULL AUTO_INCREMENT,
    `post_user_id` INT(10) NOT NULL DEFAULT '0',
    `gen_id` INT(10) NOT NULL DEFAULT '0',
    PRIMARY KEY (`post_user_id`, `post_id`)
)
COLLATE='utf8_general_ci'
ENGINE=MyISAM;

When I do:

insert into posts (post_user_id) values (1);
insert into posts (post_user_id) values (1);
insert into posts (post_user_id) values (2);
insert into posts (post_user_id) values (1);
select * from posts;

I get:

post_id | post_user_id | gen_id
1                  1     0
2                  1     0
1                  2     0
3                  1     0

A unique post_id is generated for each unique user.

I need the gen_id column to be 1 2 3 4 5 6 etc. How can I increment this column when I do an insert. I tried the one below, but it won't work. What's the right way to do this?

insert into posts (post_user_id,gen_id) values (1,select max(gen_id)+1 from posts);
//Select the highest gen_id and add 1 to it.

Answer 1

Try this:

  INSERT INTO posts (post_user_id,gen_id) 
  SELECT 1, MAX(gen_id)+1 FROM posts;