Question

我正在尝试按评分选择产品评论数，评分为0 - 5.

以下基本选择有效，但不会给出基础表中不存在的评级计数。

SELECT Rating, COUNT(*)  AS 'Reviews' FROM  ProductReviews 
WHERE   ProductID = 'product1' 
GROUP BY Rating

我尝试使用CTE生成缺少的结果，使用外部联接加入到Review表，只要我尝试包含“group by”表达式，结果就会回落以匹配基本查询的结果

（我已经检查过CTE确实生成了所有必需值）。

BEGIN

DECLARE @START AS INT = 0;
DECLARE @END AS INT = 5;

WITH CTE_Ratings AS
(
    SELECT @START as cte_rating
    UNION ALL
    SELECT 1 + cte_rating
    FROM CTE_Ratings
    WHERE cte_rating < @END
)


SELECT
    cte_rating AS 'ReviewRating'
    , ISNULL(COUNT(*), 0) AS 'ReviewCount'
FROM CTE_Ratings
LEFT OUTER JOIN Reviews ON Reviews.Rating = cte_rating
WHERE ProductReviews.ProductID = 'product1'
    AND cte_rating BETWEEN @START AND @END
    GROUP BY cte_rating
END

（我还尝试构建一个包含所需值的临时表，加入到Review表中，结果相同）。

如果是上述两个查询，结果为：

Rating  Reviews
0       1
3       3
4       9
5       47

而我想要获得相同数据的是：

Rating  Reviews
0       1
1       0
2       0
3       3
4       9
5       47

有人建议何时添加Group By聚合函数导致查询失败，或者如何改进？

Answer 1

WHERE将OUTER JOIN更改为INNER JOIN，因为您正在对外部表中的可选行进行过滤。
因此，将外部表过滤器移动到JOIN

此外，您已经在CTE的开始和结束之间进行过滤

WITH ...
-- CTE here
SELECT
    C.cte_rating AS 'ReviewRating'
    , ISNULL(COUNT(PR.Rating), 0) AS 'ReviewCount'
FROM
   CTE_Ratings C
   LEFT OUTER JOIN
   ProductReviews PR ON C.cte_rating= PR.Rating AND PR.ProductID = 'product1'
GROUP BY
   C.cte_rating

更清楚的是，你实际上是在做这个

WITH ...
-- CTE here
SELECT
    C.cte_rating AS 'ReviewRating'
    , ISNULL(COUNT(PR.Rating), 0) AS 'ReviewCount'
FROM
   CTE_Ratings C
   LEFT OUTER JOIN
   (
      SELECT PR.Rating
      FROM ProductReviews
      WHERE ProductID = 'product1'
   ) PR ON C.cte_rating= PR.Rating
GROUP BY
   C.cte_rating

CTE with Group通过返回与基本查询相同的结果

1 个答案: