题解 | #列出供应商及其可供产品的数量#

SELECT v.vend_id,
COUNT(prod_id) AS prod_id
FROM Vendors v
LEFT JOIN Products p
ON v.vend_id = p.vend_id
GROUP BY v.vend_id
ORDER BY v.vend_id ASC
# 产品个数，计数的是prod_id，不是vend_id