MostlyFocusedMike

TABLE: cats  
id          name        age         breed          net_worth
----------  ----------  ----------  -------------  ----------
1           Maru        3           Scottish Fold  1000000
2           Hana        1           Tabby          21800
3           Grumpy Cat  4           Persian        181600
4           Lil\' Bub   2           Tortoiseshell  2000000

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MostlyFocusedMike

SELECT SUM(cats.net_worth)
FROM owners
INNER JOIN cats_owners
ON owners.id = cats_owners.owner_id
JOIN cats ON cats_owners.cat_id = cats.id
WHERE cats_owners.owner_id = 2;

# which will return the following
SUM(cats.net_worth)
-------------------