MySQL Count all 和 Count distinct 超过 2 列相同的查询

Question

目前我有以下查询来计算购买特定商品时具有不同(客户用户名和商店 ID)的行数。我如何计算同一查询中 item_id = 3 的总行数？

SELECT count(*)
FROM (
SELECT DISTINCT customer_username, store_id
FROM transactions
WHERE item_id = 3)

Answer 1

你可以尝试这样的事情:

SELECT COUNT(*), SUM(customer_cnt) FROM (
    SELECT customer_username, store_id, COUNT(*) AS customer_cnt
      FROM transactions
     WHERE item_id = 3
     GROUP BY customer_username, store_id
) t;