mysql - 如何通过左连接获得计数？

Question

sqlfiddle

上的结构表

如果我使用查询：

SELECT 
    ei.ItemIdn as ItemIdn,
    ei.UserIdn as UserIdn

FROM items ei

WHERE ei.UserIdn = '0000' 
LIMIT 0, 25

我得到了4行，但是如果我想为每一行获取计数观众：

SELECT 
    ei.ItemIdn as ItemIdn,
    ei.UserIdn as UserIdn,
    ifnull(count(cv.ItemIdn),0) as CountShow

FROM items ei

    left join CountViewes as cv on ei.ItemIdn = cv.ItemIdn

WHERE ei.UserIdn = '0000' 
LIMIT 0, 25

我只有一排。

请告诉我如何获取所有行以及每行count(cv.ItemIdn)？

Answer 1

我相信你的查询需要以GROUP BY结束，因为它包含一个聚合函数。

GROUP BY ei.ItemIdn, ei.UserIdn

见这里：http://sqlfiddle.com/#!2/24d19/32/0

Answer 2

我认为你只是缺少GROUP BY子句 - http://sqlfiddle.com/#!2/24d19/26

SELECT 
    ei.ItemIdn as ItemIdn,
    ei.UserIdn as UserIdn,
    COUNT(cv.ItemIdn) as CountShow
FROM items ei
LEFT JOIN CountViewes as cv on ei.ItemIdn = cv.ItemIdn
WHERE ei.UserIdn = '0000' 
GROUP BY ItemIdn, UserIdn
LIMIT 0, 25