Join with positions - mysql

Question

I have got tables baskets, fruits and basket_fruits (join-table: basket_id-fruit_id).
How can I return a position of each fruit in basket so I will get something like
+---------------------------------------+
| basket_id | fruit_id | fruit_position |
|---------------------------------------|
| 1 | 2 | 1 |
| 1 | 5 | 2 |
+---------------------------------------+
Fruit position is just a number of a row in a returned joined table (it is not a column).
Schema:
baskets: id, title
fruits: id, title
basket_fruits: id, basket_id, fruit_id

Answer 1

MySQL does not support ranging functions so you'll have to use subqueries:
SELECT basket_id, fruit_id,
(
SELECT COUNT(*)
FROM basket_fruit bfi
WHERE bfi.basket_id = bf.basket_id
AND bfi.fruit_id <= bf.fruit_id
) AS fruit_position
FROM basket_fruit bf
WHERE basket_id = 1
or use session variables (faster but relies on implementation details which are not documented and may break in future releases):
SET #rn = 0;
SELECT basket_id, fruit_id, #rn := #rn + 1 AS fruit_position
FROM basket_fruit bf
WHERE basket_id = 1
ORDER BY
fruit_id

Answer 2

I do not see any column in basket_fruits table that I would consider weightable. If you simply want to add some numbers to the data in that table, you could try this (this allows each basket to have its own weights counting from 1):
SET #current_group = NULL;
SET #current_count = NULL;
SELECT
id, basket_id, fruit_id,
CASE
WHEN #current_group = basket_id THEN #current_count := #current_count + 1
WHEN #current_group := basket_id THEN #current_count := 1
END AS fruit_position
FROM basket_fruits
ORDER BY basket_id, id
Sample input:
+----+-----------+----------+
| id | basket_id | fruit_id |
+----+-----------+----------+
| 2 | 2 | 5 |
| 6 | 2 | 1 |
| 9 | 1 | 2 |
| 15 | 2 | 3 |
| 17 | 1 | 5 |
+----+-----------+----------+
Sample output:
+----+-----------+----------+----------------+
| id | basket_id | fruit_id | fruit_position |
+----+-----------+----------+----------------+
| 9 | 1 | 2 | 1 |
| 17 | 1 | 5 | 2 |
| 2 | 2 | 5 | 1 |
| 6 | 2 | 1 | 2 |
| 15 | 2 | 3 | 3 |
+----+-----------+----------+----------------+

Answer 3

SQL provides no guarantees on the order of the returned rows. Therefore fruit_position is likely to be different when queried from time to time. Most likely this will happen due to DML activity on your table.
If you really need some ordering, you should pick:
Use existing columns as ordering key, like fruit name (if exists)
Create a special field, like seq_nr that will specify ordering for your fruits.