Question

此代码的Select语句出现错误。

SELECT Track.Name, Track.UnitPrice, 
    Count(*) AS Purchase_count, 
    Purchase_count * Track.UnitPrice AS Total_per_track
FROM Track
JOIN InvoiceLine ON Track.TrackId = InvoiceLine.TrackId
GROUP BY Track.Name
ORDER BY  Total_per_track desc
LIMIT 10;

返回的错误是

Result: no such column: Purchase_count
At line 1:
SELECT Track.Name, Track.UnitPrice, 
    Count(*) AS Purchase_count, 
    Purchase_count * Track.UnitPrice AS Total_per_track
    FROM Track
    JOIN InvoiceLine
    ON Track.TrackId = InvoiceLine.TrackId
    GROUP BY Track.Name
    ORDER BY  Total_per_track desc
LIMIT 10;

Answer 1

您不能在同一子句中引用SELECT子句中定义的别名（例如，在WHERE子句中也不能引用）。您需要重复原始表达式，或者使用派生表（子查询或cte）。

这里的表达很简单，因此重复似乎更相关：

SELECT 
    t.Name, 
    t.UnitPrice, 
    COUNT(*) AS Purchase_count, 
    COUNT(*) * t.UnitPrice AS Total_per_track
FROM Track t
INNER JOIN InvoiceLine il ON t.TrackId = il.TrackId
GROUP BY t.TrackId, t.Name, t.UnitPrice
ORDER BY Total_per_track desc
LIMIT 10;

注意：

我在TrackId子句中添加了UnitPrice和GROUP BY； TrackId可以避免将具有相同Name的两个音轨错误地组合在一起； UnitPrice出现在SELECT子句中，并且不是聚合函数的一部分，因此，最好在GROUP BY子句中也使用它（尽管在功能上确实如此）取决于TrackId）
表别名使查询的读写更短

Answer 2

要按Total_per_track进行排序，您需要使用表表达式。

例如：

select *
from (
  SELECT
    Track.Name, Track.UnitPrice, 
    Count(*) AS Purchase_count, 
    Count(*) * Track.UnitPrice AS Total_per_track
  FROM Track
  JOIN InvoiceLine
  ON Track.TrackId = InvoiceLine.TrackId
  GROUP BY Track.Name
) x
ORDER BY Total_per_track desc
LIMIT 10

选择语句中无法识别的列

2 个答案: