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以下两个查询完全相同，除了国家名称。在查询 1 中是“马来西亚”，在查询 2 中是“印度”。

查询本身是错误的；由于我ac.country在第一个子查询中使用“”，而我实际上需要使用“ ass.country”，但我仍然在查询 1 中得到结果，而在查询 2 中得到 NULL。

有人可以解释为什么会这样。

查询一：

SELECT DATE(DAY) AS daycust,
            (SELECT SUM(Quantity) AS webss 
            FROM ACQ AS ass
            WHERE ass.day = ac.day 
            AND ac.country IN (SELECT DISTINCT aam.country_name FROM accounts AS aam)
            AND country =  'Malaysia') AS webss

FROM analytics_country AS ac

WHERE DAY BETWEEN  '2016-07-01' AND  '2016-08-02'
             AND ac.country IN (SELECT DISTINCT aam.country_name FROM accounts AS aam)
             AND country =  'Malaysia'

GROUP BY daycust
ORDER BY daycust

查询 2：

SELECT DATE(DAY) AS daycust,
            (SELECT SUM(Quantity) AS webss 
            FROM ACQ AS ass
            WHERE ass.day = ac.day 
            AND ac.country IN (SELECT DISTINCT aam.country_name FROM accounts AS aam)
            AND country =  'India') AS webss

FROM analytics_country AS ac

WHERE DAY BETWEEN  '2016-07-01' AND  '2016-08-02'
             AND ac.country IN (SELECT DISTINCT aam.country_name FROM accounts AS aam)
             AND country =  'India'

GROUP BY daycust
ORDER BY daycust

注意：我还想补充一点，不同版本的 MySql 的输出会有所不同。

In Server version: "5.5.30-tokudb-7.0.1-MariaDB-log" -> Both the queries work fine.

In Server version: "5.5.41-tokudb-7.5.5-MariaDB - MariaDB Server" -> Only Query 1 works fine.
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