mysql - retrieving a value using sub-select - mysql

Question

SELECT snr, nsname FROM type
WHERE snr in (SELECT snr FROM diet
WHERE foodnr in (SELECT foodnr FROM food WHERE foodtype = 'FISH' ));
there are more animals that eat the same kind of food BUT eat different types of food in my database, how do i get the animals that only eat that type of food and nothing else.
bah this is impossible to answer without my entire database i feel like an ass just asking this because its so vague but what i am trying to do is i need to filter out the animals that have more foodtypes besides FISH

Answer 1

JOIN the tables instead, like so:
SELECT t.snr, t.nsname
FROM type t
INNER JOIN diet d ON t.snr = d.snr
INNER JOIN food f ON t.foodnr = f.foodnr
WHERE t.foodtype = 'FISH'
GROUP BY t.foodtype
HAVING COUNT(DISTINCT t.foodtype) = 1;

Answer 2

Note I'm assuming the following about your schema:
TYPE: snr, nsname
DIET: snr, foodnr
FOOD: foodnr, foodtype
You can use a NOT EXISTS:
SELECT t.snr, t.nsname
FROM type t
INNER JOIN diet d ON t.snr = d.snr
INNER JOIN food f ON d.foodnr = f.foodnr
WHERE f.foodtype = 'FISH'
AND NOT EXISTS
(
SELECT 1
FROM diet dd
INNER JOIN food ff ON dd.foodnr = ff.foodnr
WHERE ff.foodtype <> 'FISH'
AND dd.snr = t.snr
)
Or you can use an anti-join pattern:
SELECT t.snr, t.nsname
FROM type t
INNER JOIN diet d ON t.snr = d.snr
INNER JOIN food f ON d.foodnr = f.foodnr AND f.foodtype = 'FISH'
LEFT OUTER JOIN diet dd ON t.snr = dd.snr AND f.foodnr <> dd.foodnr
WHERE f.foodtype = 'FISH'
AND dd.snr IS NULL