這是用戶、產品和月份的表格。
CREATE TABLE USERS (user VARCHAR(1),product VARCHAR(1),month INT);
INSERT INTO USERS (user,product,month) VALUES
("A","X",9),("A","X",10),("A","X",10),("A","Y",11),
("B","Y",12),("B","X",10),("C","X",10),("D","Z",11),("D","X",9);
如何僅計算某個產品的用戶數量（也可能在特定月份）？
uj5u.com熱心網友回復：
加入派生表，X通過使用聚合并檢查最小產品是否等于最大產品，您可以讓用戶僅使用產品。然后使用count(DISTINCT ...).
SELECT count(DISTINCT u1.user)
FROM USERS u1
INNER JOIN (SELECT u2.user
FROM USERS u2
GROUP BY u2.user
HAVING min(u2.product) = max(u2.product)
AND min(u2.product) = 'X') upx
ON upx.user = u1.user;
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