sum on count 的微妙關係

原處
select sum(col1)
from
(
select count(col1) col1 from tablename
group by columnname
)a

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換自己的運用:
我要統計今天書被出租的本數 (條件自己擴充)
SELECT COUNT(*) AS item_count
FROM (SELECT isbn_bar FROM BOOKS
GROUP BY isbn_bar) AS check_item

也就是說:
利用 ---> select xxx from ( ) as tmp_a
可以運用出更多的統計數據