Question

I’m using MySQL 5.1

table users:

id | exp
---------
5  | 5
4  | 9

table levels:

id|min_exp
----------
1 | 0
2 | 5
3 | 8

How to select user with his levels.id?

I need to join table users and levels.
This must be valid:

users.exp >= levels.min_exp

and min_exp should be lowest as it can be.

expected output:

users.id|users.exp|levels.id|levels.min_exp
   5    |    5    |    2    |      5
   4    |    9    |    3    |      8                             

Answer 1

If there are no gaps in level.id, you could get better performance by joining levels twice and avoiding aggregates:

select users.id, users.exp, current_level.id, current_level.min_exp
from levels current_level
join levels next_level on next_level.id = current_level.id + 1
join users on users.exp >= current_level.min_exp
    and users.exp < next_level.min_exp

If you need the min/max experience bracket frequently, I’d recommend making the current_level/next_level join a view.

Edit: It just occurred to me that this fails for users at maximum level. Depending on how you want to handle that (do users stop gaining XP at max or continue gaining it without increasing level?), you could add a dummy record above max level in levels or make the next_level join an outer join.