How to select the lowest value from selection - mysql

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.

Answer 2

You want the min level that the user can see:
select
u.id as UserId,
MIN(l.id) as LevelId
from
Users u
inner join Levels l on
u.exp >= l.min_exp
group by
u.Id
If you want it just for a specific user:
select
u.id as UserId,
MIN(l.id) as LevelId
from
Users u
inner join Levels l on
u.exp >= l.min_exp
where
u.id = 5
group by
u.Id

Answer 3

The question is phrased unclearly, but from the context, it looks like you are looking for the highest level that a person has achieved given their current exp and the min_exp.
Since you know the id of the user you are looking for, the query can be
select users.id as uid,users.exp,levels.id as levelid from users,levels where users.id = 4 and levels.min_exp <= users.exp order by levelid desc limit 1
if you make sure you have proper indexes on both tables
alter table users add primary key (id)
alter table levels add primary key (id)
alter table levels add unique key(min_exp);
(non primary key indexes would be fine too)
Then the query is extremely efficient:
explain select users.id as uid,users.exp,levels.id as levelid from users,levels where users.id = {{uid}} and levels.min_exp <= users.exp order by levelid desc limit 1;
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 1 | SIMPLE | levels | index | min_exp | PRIMARY | 4 | NULL | 1 | Using where |
----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+`