如何在查询中包含零计数结果

Question

我有这两个 table：

运营商：

Id Nome
--+----
 1 JDOE
 2 RROE
 3 MMOE

来电：

Id CallDate OpId
--+--------+----
 1 20161228    2
 2 20161228    3
 3 20161228    2
 4 20161228    3
 5 20170104    1
 6 20170104    2
 7 20170104    1

这个查询：

SELECT Operators.id, Operators.Nome, Count(Calls.OpId) AS CountCalls
FROM Operators LEFT JOIN Calls ON Operators.id = Calls.OpId
GROUP BY Calls.CallDate, Operators.id, Operators.Nome
HAVING Calls.CallDate=20170104;

哪个 returns:

Id Nome CountCalls
--+----+----------
 1 JDOE          2
 2 RROE          1

我怎样才能做到 return 这个呢？

Id Nome CountCalls
--+----+----------
 1 JDOE          2
 2 RROE          1
 3 MMOE          0

也就是说，如何在任何查询中也包括 main table 的零结果，它在左连接 table 中没有出现，至少在查询过滤条件定义的数据片中？

这是 Access 2013。

我读过 this answer 但看不出它与我正在做的有何不同。

Answer 1

因为您在 HAVING 子句中引用了 Calls.CallDate，所以您要删除没有调用的运算符。如果没有调用，则 CallDate 将是 NULL，而 NULL=20170104 不正确，因此排除这些行。您需要将此谓词移动到您的连接子句中：

SELECT Operators.id, Operators.Nome, Count(Calls.OpId) AS CountCalls
FROM Operators LEFT JOIN Calls ON (Operators.id = Calls.OpId AND Calls.CallDate=20170104)
GROUP BY Calls.CallDate, Operators.id, Operators.Nome;

你也不需要按 Calls.CallDate 分组，因为你只有一个，所以你可以使用：

SELECT Operators.id, Operators.Nome, Count(Calls.OpId) AS CountCalls
FROM Operators LEFT JOIN Calls ON (Operators.id = Calls.OpId AND Calls.CallDate=20170104)
GROUP BY Operators.id, Operators.Nome;

顺便说一句 HAVING 是错误的运算符。 HAVING 用于过滤聚合，因为您没有过滤聚合，您应该简单地使用 WHERE

SELECT Operators.id, Operators.Nome, Count(Calls.OpId) AS CountCalls
FROM Operators LEFT JOIN Calls ON Operators.id = Calls.OpId
WHERE Calls.CallDate=20170104
GROUP BY Calls.CallDate, Operators.id, Operators.Nome;

如果你想在 CountCalls 上筛选，你可以使用 HAVING，例如，如果你只想要调用超过 1 次的操作员，你可以使用：

SELECT Operators.id, Operators.Nome, Count(Calls.OpId) AS CountCalls
FROM Operators LEFT JOIN Calls ON Operators.id = Calls.OpId
WHERE Calls.CallDate=20170104
GROUP BY Calls.CallDate, Operators.id, Operators.Nome
HAVING Count(Calls.OpId) > 1;

这只会return

Id Nome CountCalls
--+----+----------
 1 JDOE          2

Answer 2

尝试以下查询：

SELECT o.id, o.name, count(c.opid) as countcalls
FROM Operators o LEFT JOIN calls c ON o.id = c.opid AND c.calldate=20170104
GROUP BY c.calldate, o.id, o.name
ORDER BY o.id;

输出：

+------+------+------------+
| id   | name | countcalls |
+------+------+------------+
|    1 | JDOE |          2 |
|    2 | RROE |          1 |
|    3 | MMOE |          0 |
+------+------+------------+

如何在查询中包含零计数结果

How to include zero-count results in query

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outer-join