Question

我的目标是编写一个查询，返回包含3个标志字段的所有类别，如下所示：

ID |恩|茹| FR

语言标志必须为ON或OFF，具体取决于是否与该类别相关联的课程记录。

我目前的陈述是：

SELECT c.ID,
    (CASE WHEN c.ID IN (SELECT c.ID FROM Lessons AS l WHERE l.Category_Id = c.ID AND l.Language_Id = 1) THEN 1 ELSE 0 END) AS En,
    (CASE WHEN c.ID IN (SELECT c.ID FROM Lessons AS l WHERE l.Category_Id = c.ID AND l.Language_Id = 2) THEN 1 ELSE 0 END) AS Ru,
    (CASE WHEN c.ID IN (SELECT c.ID FROM Lessons AS l WHERE l.Category_Id = c.ID AND l.Language_Id = 3) THEN 1 ELSE 0 END) AS Fr
FROM LessonCategories AS c

问题是这个查询非常慢，因为Lessons表有超过60,000条记录，而且我已经运行了3次。

我正在寻找一种方法来提高查询效率。我想过在分类和课程之间使用分组，但我不确切知道如何以及它是否可能。

更快速查询的伪代码是：

SELECT c.[ID], 
    COUNT(l.Language_Id = 1) > 0 AS En
    COUNT(l.Language_Id = 2) > 0 AS Ru
    COUNT(l.Language_Id = 3) > 0 AS Fr
FROM CategoryTreeView AS c
INNER JOIN Lessons AS l ON l.Category_Id = c.ID
GROUP BY c.[ID]

有可能使用有效的t-sql表达这个吗？或者是处理这种查询的更好方法吗？

P.S。如果它有帮助，我不关心得到一个按位标志字段而不是3个语言字段。

感谢。

Answer 1

嗯，你只需要结合你的两个查询;）

SELECT c.ID, 
       SUM(case when l.Language_Id = 1 then 1 else 0) AS En
       SUM(case when l.Language_Id = 2 then 1 else 0) AS Ru
       SUM(case when l.Language_Id = 3 then 1 else 0) AS Fr
FROM CategoryTreeView AS c
INNER JOIN Lessons AS l ON l.Category_Id = c.ID
GROUP BY c.ID

Answer 2

试试这个

SELECT c.ID,
    (CASE WHEN l.Language_Id = 1 THEN 1 ELSE 0 END) AS En,
    (CASE WHEN l.Language_Id = 2 THEN 1 ELSE 0 END) AS Ru,
    (CASE WHEN l.Language_Id = 3 THEN 1 ELSE 0 END) AS Fr
FROM LessonCategories AS c
INNER JOIN Lessons l ON c.ID = l.CategoryId

顺便说一句，目前，您正在通过应用SELECT语句对CASE子句进行过滤。通过将过滤语句移动到WHERE之前执行的SELECT子句，可以显着提高任何查询的性能。

Answer 3

你可以在第二个例子中使用计数：

COUNT(case when l.Language_Id = 1 then 1 else null end) as EnCount
COUNT(case when l.Language_Id = 2 then 1 else null end) as RuCount

但实际上，您希望在第一个示例中使用exists：

case when exists(SELECT 0 FROM Lessons AS l WHERE l.Category_Id = c.ID AND l.Language_Id = 1) then 1 else 0 end as En

通过适当的索引，三个exists将更快。

Answer 4

一种完全不同的方法尝试转轴。

DECLARE @ctw TABLE(id int, languageid int) 

insert @ctw values(1,1)
insert @ctw values(1,1)
insert @ctw values(1,2)
insert @ctw values(1,3)
insert @ctw values(2,1)
insert @ctw values(2,3)

SELECT * FROM 
(
SELECT id, CASE languageid WHEN 1 THEN 'en' WHEN 2 THEN 'ru' WHEN 3 THEN 'fr' END language
FROM @ctw) a
PIVOT
(count(language) 
FOR language
in([en],[ru],[fr]) 
)AS p ORDER BY id

测试在这里http://data.stackexchange.com/stackoverflow/q/107960/

是否可以在t-sql中对组进行子查询？

4 个答案: