Question

我在僵局中。

当我运行以下查询时，它可以工作：

 select DISTINCT l.Seating_Capacity - (select count(*)
                        from tblTrainings t1, tbllocations l
                        where l.locationId = t1.LocationId) as
                        availableSeats
                        from tblTrainings t1, tbllocations l
                        where l.locationId = t1.LocationId

但是，我们想添加一个CASE语句，当Seating_Capacity - 如上所示的总计数= 0时，显示'FULL'消息。

否则，显示剩余号码。

以下是该查询：

                 select DISTINCT case when l.Seating_Capacity - (select count(*)
                from tblTrainings t1, tbllocations l
                where l.locationId = t1.LocationId) = 0 then 'full' else STR(Seating_Capacity) end)
                availableSeats
                from tblTrainings t1, tbllocations l
                where l.locationId = t1.LocationId

我收到'不正确的语法''''接近'结束'

我也收到一条错误，内部Seating_Capacity是无效的列名。

非常感谢您的协助。

我一定是在梦乡，因为我认为它在测试过程中起作用了。

现在，该应用程序已启用，但无效。

提前多多感谢

select DISTINCT l.LocationId,c.courseId, c.coursename, l.Seating_Capacity - (select count(*)
                        from tblTrainings t1
                        where l.locationId = t1.LocationId and c.courseId = t1.courseId) as
                        availableSeats,d.dateid,d.trainingDates,d.trainingtime,c.CourseDescription,
                        i.instructorName,l.location,l.seating_capacity 
                        from tblLocations l
                        Inner Join tblCourses c on l.locationId = c.locationId
                        left join tblTrainings t on l.locationId = t.LocationId and c.courseId = t.courseId
                        Inner Join tblTrainingDates d on c.dateid=d.dateid 
                        Inner Join tblCourseInstructor ic on c.courseId = ic.CourseId  
                        Inner Join tblInstructors i on ic.instructorId = i.instructorId
                        WHERE CONVERT(VARCHAR(10), d.trainingDates, 101) >= CONVERT(VARCHAR(10), GETDATE(), 101)

Answer 1

为避免重复表达，您可以使用WITH clause来简化查询：

WITH (
   -- Start with your query that already works
   SELECT DISTINCT l.Seating_Capacity - (select count(*)
                    from tblTrainings t1, tbllocations l
                    where l.locationId = t1.LocationId) AS availableSeats
   FROM tblTrainings t1, tbllocations l
   WHERE l.locationId = t1.LocationId
) AS source
SELECT
    -- Add a CASE statement on top of it
    CASE WHEN availableSeats = 0 THEN 'Full'
    ELSE STR(availableSeats)
    END AS availableSeats
FROM source

Answer 2

在您的案例陈述结尾处有一个额外的)删除它。

 0 then 'full' else STR(Seating_Capacity) end)
                                            ^^^

Seating_Capacity尝试使用{al} {/ p>等表别名访问它

Answer 3

我认为你的子查询使查询复杂化了。据我所知，以下内容应该可以根据需要使用：

SELECT  AvailableSeats = CASE WHEN l.Seating_Capacity - COUNT(*) = 0 THEN 'Full' 
                            ELSE STR(l.Seating_Capacity - COUNT(*)) 
                        END
FROM    tblTrainings t1
        INNER JOIN tblLocations l
            ON l.LocationID = t1.LocationID
GROUP BY l.Seating_Capacity;

我已将其他人更改为STR(l.Seating_Capacity - COUNT(*))，因为我认为您想知道剩下的座位，而不仅仅是容量？如果我误解了要求，只需将其更改为STR(l.Seating_Capacity)。

我还将ANSI 89隐式连接切换为ANSI 92显式连接，标准更改超过20年，并且plenty of good reasons可以切换到更新的语法。但为了完整起见，上述查询的ANSI 89版本将是：

SELECT  AvailableSeats = CASE WHEN l.Seating_Capacity - COUNT(*) = 0 THEN 'Full' 
                            ELSE STR(l.Seating_Capacity - COUNT(*)) 
                        END
FROM    tblTrainings t1, tblLocations l
WHERE   l.LocationID = t1.LocationID
GROUP BY l.Seating_Capacity;

修改

要调整完整查询，您只需使用已连接的子查询替换select中的子查询：

SELECT l.LocationId, c.courseId, c.coursename, CASE WHEN l.Seating_Capacity - t.SeatsTaken = 0 THEN 'Full' ELSE STR(l.Seating_Capacity - t.SeatsTaken) END AS availableSeats, d.dateid, d.trainingDates, d.trainingtime, c.CourseDescription, i.instructorName, l.location, l.seating_capacity FROM tblLocations l INNER JOIN tblCourses c ON l.locationId = c.locationId LEFT JOIN ( SELECT t.LocationID, t.CourseID, SeatsTaken = COUNT(*) FROM tblTrainings t GROUP BY t.LocationID, t.CourseID ) t ON l.locationId = t.LocationId AND c.courseId = t.courseId INNER JOIN tblTrainingDates d ON c.dateid=d.dateid INNER JOIN tblCourseInstructor ic ON c.courseId = ic.CourseId INNER JOIN tblInstructors i ON ic.instructorId = i.instructorId WHERE d.trainingDates >= CAST(GETDATE() AS DATE);

JOIN倾向于比相关子查询更好地优化（尽管有时优化器可以确定JOIN会起作用），这也意味着您可以多次引用结果（SeatsTaken）而无需重新评估子查询

此外，通过将计数移至子查询，并将联接移至tblTrainings，我认为您无需DISTINCT来提高性能。

最后我改变了这一行：

WHERE CONVERT(VARCHAR(10), d.trainingDates, 101) >= CONVERT(VARCHAR(10), GETDATE(), 101)

要

WHERE d.trainingDates >= CAST(GETDATE() AS DATE);

我不知道你是否这样做，但如果你有一个关于d.TrainingDates的索引，那么通过将其转换为varchar来与今天进行比较，你将删除优化器使用这个索引的能力，因为你只是说今天午夜>=，无需在d.TrainingDates上执行任何转换，您需要做的就是删除GETDATE（）的时间部分，这可以通过强制转换为DATE来完成。关于此的更多内容包含in this article（来自Aaron Bertrand的另一个宝石）

我的CASE陈述错了。知道我做错了什么吗？

3 个答案: