Column not recognized in select statement - sql

Question

Im getting an error in the Select statement of this code.
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;
The Error returning is
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

You can't refer to an alias defined in the SELECT clause in the same clause (nor in the WHERE clause for example). You need to either repeat the original expression, or use a derived table (subquery or cte).
Here the expression is simple enough so repeating seems more relevant:
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;
Notes:
I added TrackId and UnitPrice to the GROUP BY clause; TrackId is there to avoid wrongly grouping together two tracks that would have the same Name; UnitPrice appears in the SELECT clause and is not part of an aggregate function, so it is a good practice to have it in the GROUP BY clause too (although, it does seem to be functionally dependant on TrackId)
table aliases make the query shorter to read and write

Answer 2

In order to order by Total_per_track you need to use a table expression.
For example:
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