Question

I’m adding a field to my table called “room_num”. Basically upon insert I’d like to set the default value of the room_num to be the max value of the last room_num + 1. Obviously I’d use auto-increment if this table applied to only one user but, it will apply to many so I have a running unique index as well as a unique field user_index.

I could run a query based on the user_index and return the max for “room_num” but, since I’m already here, is there a way to make the default value some sort of function?

So, for example my table looks like this:

+-------+-------------+---------+-------------+
| index |  room_num   |  title  |  user_index |
+-------+-------------+---------+-------------+
| 0     | 0           | Patio   | 10          |
+-------+-------------+---------+-------------+
| 1     + 1           | Dining  | 10          |
+-------+-------------+---------+-------------+
| 2     + 0           | Bar     | 15          |
+-------+-------------+---------+-------------+
| 3     + 2           | Dining  | 10          |
+-------+-------------+---------+-------------+

So, now I want the default value for room_num of my next insert for user_index 15 to default to max room_num (0) + 1 (which in this case would be 1). But, I’d like to take care of this here:

Rather than querying for it in my application first and THEN inserting the record and in essence making two database connections.

Is this possible?

Answer 1

Well, you could do something like this:

INSERT INTO myTable (title, user_index, room_num)
  SELECT 'MyTitle', 10, COALESCE((MAX(room_num) + 1), 1)
  FROM   myTable
  WHERE  user_index = 10

Obviously, from your application you would just replace 'MyTitle' and the instances of 10 with your desired title and proper user_index.

Note: Based on my five minutes with Google, I don’t think INSERT...SELECT is an atomic operation. So, you’ll probably want to wrap this in a transaction or table lock/unlock to prevent race conditions.