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僕のaccounts.idは1でフォロワ数は

SELECT COUNT(1) FROM follows WHERE target_account_id = 1;

で数えられてそのうち僕がフォローしてる数は

WITH followers AS (SELECT account_id FROM follows WHERE target_account_id = 1) SELECT COUNT(1) FROM follows, followers WHERE follows.account_id = 1 AND follows.target_account_id = followers.account_id;

みたいな感じっぽい（もっと単純にできないかな…）で下を上で割ると40%くらいでしたｽﾏﾇｳ