Select all rows that have multiple sub-relations satisfying some constraints - sql

Question

Let’s say I have a list of baskets that can contain fruits with a certain weight:
Table baskets
(id, name)
----------------
1, 'apples, oranges and more'
2, 'apples and small oranges'
3, 'apples and bananas'
4, 'only oranges'
5, 'empty'
Table basket_fruits
(id, basket, fruit, weight)
----------------
1, 1, 'apple', 2
2, 1, 'apple', 3
3, 1, 'orange', 2
4, 1, 'banana', 2
5, 2, 'apple', 2
6, 2, 'orange', 1
7, 3, 'apple', 2
8, 3, 'banana', 2
9, 4, 'orange', 2
SQL Fiddle with this data
I’m struggling to come up with reasonably efficient queries for these two scenarios:
I want to fetch all baskets that contain at least one apple AND at least one orange, each above a given weight. So the expected result for weight >= 2 is
1, 'apples, oranges and more'
and for weight >= 1 it’s
1, 'apples, oranges and more'
2, 'apples and small oranges'
I want to fetch all baskets that contain no fruit above a given weight. So for weight >= 2 I would expect
5, 'empty'
and for weight >= 3 it should return
2, 'apples and small oranges'
3, 'apples and bananas'
4, 'only oranges'
5, 'empty'
The weight constraint is just a placeholder for "each sub-relation must meet certain constraints". In practice, we need to restrict the sub-relation by date range, status, etc. but I didn’t want to complicate the example any further.
(I’m using postgresql, in case the solution needs to be database-specific.)

Answer 1

I strongly recommend using group by and having for this purpose.
For your first question, this query should work:
SELECT b.name
FROM baskets b INNER JOIN
basket_fruits bf
ON b.id = bf.basket
GROUP BY b.name
HAVING SUM( (bf.fruit = 'apple' AND bf.weight >= 2)::int ) > 0 AND
SUM( (bf.fruit = 'orange' AND bf.weight >= 2)::int ) > 0 ;
The second is a little more complicated, because there are no rows. But a left join and coalesce() suffice so you can express it in the same format:
SELECT b.name
FROM baskets b LEFT JOIN
basket_fruits bf
ON b.id = bf.basket
GROUP BY b.name
HAVING SUM( (COALESCE(bf.weight, 0) >= 2)::int ) = 0

Answer 2

Here are my solutions so far:
All baskets containing both fruits with weight >= 2 (thanks to Gordon Linoff’s suggestions):
SELECT b.* FROM baskets b
INNER JOIN (
SELECT basket FROM basket_fruits
WHERE weight >= 2
GROUP BY basket
HAVING SUM((fruit = 'apple')::int) > 0 AND SUM((fruit = 'orange')::int) > 0
) bf ON b.id = bf.basket
SQL-Fiddle
All baskets without fruits with weight >= 2:
SELECT b.* FROM baskets b
LEFT JOIN (
SELECT basket, fruit FROM basket_fruits
WHERE weight >= 2
) bf ON b.id = bf.basket
WHERE fruit IS NULL
SQL-Fiddle
If anyone has more efficient ideas, I’d love to hear them.

Answer 3

SELECT b.name
FROM baskets b
INNER JOIN basket_fruits f
ON b.id = f.basket
GROUP BY b.name
HAVING SUM(f.wage) >= 3
OR b.id = 5
Is it what you expected?