T-SQL：如何保持左连接但过滤查询答案

【问题标题】：T-SQL: How to keep left join but filter queryT-SQL：如何保持左连接但过滤查询
【发布时间】：2016-11-09 17:43:14
【问题描述】：

我正在尝试过滤掉特定用户添加的结果，同时保留每一天，因为此查询将用于报告中的图表。当我删除 t2.createdBy <> 21 时，我得到了所有的日期，但我还需要通过它过滤的结果是正确的。

查询：

SELECT 
    t1.DateFull, COUNT(t2.ContainerWashedDate) as Washes 
FROM 
    DateLookup t1
LEFT JOIN 
    factContainerWash t2 ON t1.Datefull = t2.ContainerWashedDate
WHERE 
    (t1.DateFull >= '10/5/2016') 
    AND (t1.DateFull <= '11/9/2016') 
    AND t2.createdBy <> 21
GROUP BY 
    t1.DateFull
ORDER BY 
    DateFull

结果：

DateFull                 | Washes
-------------------------+-------
2016-10-05 00:00:00.000  | 1231
2016-10-06 00:00:00.000  |  466
2016-10-10 00:00:00.000  |   84
2016-10-12 00:00:00.000  |   75

预期结果：

DateFull                 | Washes
-------------------------+-------
2016-10-05 00:00:00.000  | 1231
2016-10-06 00:00:00.000  |  466
2016-10-07 00:00:00.000  |  655
2016-10-08 00:00:00.000  |   23

【问题讨论】：

但是createdBy 不可能是 21，如果它根本不存在的话。你有一个逻辑问题需要首先解决。

标签： sql-server tsql reporting-services ssrs-2008

【解决方案1】：

以下是三种方法。当我开始回答这个问题时，我意识到可能会发生微妙的不同事情。可能所有这三个都可以工作，但第一个可能并不总是有效。

我怀疑你只是想要一个额外的NULL 比较：

SELECT t1.DateFull, COUNT(t2.ContainerWashedDate) as Washes
FROM DateLookup t1 LEFT JOIN
     factContainerWash t2
     ON t1.Datefull = t2.ContainerWashedDate
WHERE t1.DateFull >= '2016-10-05' and
      t1.DateFull <= '2016-11-09' and
      (t2.createdBy <> 21 or t2.createdBy is null)
GROUP BY t1.DateFull
ORDER BY DateFull;

或者，或者，使用条件聚合：

SELECT t1.DateFull, 
       COUNT(CASE WHEN createdBy <> 21 THEN t2.ContainerWashedDate END) as Washes
FROM DateLookup t1 LEFT JOIN
     factContainerWash t2
     ON t1.Datefull = t2.ContainerWashedDate
WHERE t1.DateFull >= '2016-10-05' and
      t1.DateFull <= '2016-11-09'
GROUP BY t1.DateFull
ORDER BY DateFull;

将条件移至ON 子句也可能完成您需要的工作：

SELECT t1.DateFull, 
       COUNT(t2.ContainerWashedDate) as Washes
FROM DateLookup t1 LEFT JOIN
     factContainerWash t2
     ON t1.Datefull = t2.ContainerWashedDate AND t2.createdBy <> 21
WHERE t1.DateFull >= '2016-10-05' and
      t1.DateFull <= '2016-11-09'
GROUP BY t1.DateFull
ORDER BY DateFull;

【讨论】：

【解决方案2】：

当您在 WHERE 子句中使用 t2.CreatedBy 时，您将 LEFT JOIN 变成了 INNER JOIN。像这样的东西怎么样：

SELECT
    t1.DateFull
    , COALESCE(t2.Washes, 0) AS Washes
FROM
    (
    SELECT
        ContainerWahsedDate
        , COUNT(ContainerWahsedDate) AS Washes
    FROM
        factConainerWash
    WHERE
        ContainerWahsedDate BETWEEN '2016-10-05' AND '2016-11-09'
        AND CreatedBy <> 21
    GROUP BY
        ContainerWashedDate
    ) t2
    LEFT JOIN DateLookup t1 ON t1.DateFull = t2.ContainerWahsedDate
WHERE
    t2.DateFull BETWEEN '2016-10-05' AND '2016-11-09'

【讨论】：