MySQL - referring back to previous row to add calculated column - mysql

Question

I'm new to MySQL and I'm having trouble with the problem below;
Let's call this table runners_table
+---------+-----------+
| race_id | runner_id |
+---------+-----------+
| 10 | A |
| 10 | E |
| 10 | V |
| 23 | G |
| 23 | J |
| 23 | A |
| 67 | E |
| 67 | G |
| 67 | X |
+---------+-----------+
And I want to add a new column like this;
+---------+-----------+--------------+
| race_id | runner_id | prev_race_id |
+---------+-----------+--------------+
| 10 | A | - |
| 10 | E | - |
| 10 | V | - |
| 23 | G | - |
| 23 | J | - |
| 23 | A | 10 |
| 67 | E | 10 |
| 67 | G | 23 |
| 67 | X | - |
+---------+-----------+--------------+
Where prev_race_id looks back and gets the previous race_id for the same runner_id.
To illustrate;
What I'd like to do is, say a runner took part in 6 races out of 10 races that year, in the row relating to his 5th race I want to know the race_id of his 4th race.
I guess I could make a new table for every runner that was a record of the races they took part in, but this would stretch to hundreds of tables... there must be a better way.
Is it possible to do this?

Answer 1

I think the easiest way is using a correlated subquery. Assuming race_id is in numerical order:
select r.*,
(select r2.race_id
from runners_table r2
where r2.race_id < r.race_id and r2.runner_id = r.runner_id
order by r2.race_id desc
limit 1
) as prev_race_id
from runners_table r;
If some other column determines the previous record, then the where and order by would change.

Answer 2

You can create 3 tables, one for race, one for runner and one for the relationship between them. Since you stored the date of every race you can get the previus one easily.
CREATE TABLE Race(
race_id INT,
data VARCHAR(100)
);
CREATE TABLE Runner(
runner_id INT,
data VARCHAR(100)
);
CREATE TABLE Race_Runner(
race_id INT,
runner_id INT,
fecha DATETIME
);
If you want to know the previous race date for a specific runner, try this query:
select race_id
from Race_Runner
where fecha <> (select max(fecha) from Race_Runner)
and runner_id = ? -- the runner you want
order by fecha desc
limit 1
The following query returns the value of each runner with the previous race date:
select runner_id,
LAG(fecha) over (partition by runner_id order by fecha desc)
as previous_date_race
from Race_Runner