计算单列百分比

Question

So the outcome i would like is for '% Of Tracks' to display each genres percentage of the total number of tracks

SELECT Genre.Name, Count (Genre.GenreID) as 'No Of Tracks', 
(Genre.GenreID) as '% of Tracks' 
From Genre, Track
Where Genre.GenreID = Track.GenreID
Group by Genre.Name

Not sure if i set it up correctly would appreciate some advice

Answer 1

SELECT Genre.Name, COUNT(Genre.GenreID) AS 'No Of Tracks', 
    ROUND(100.0 * COUNT(Genre.GenreID) / (SELECT COUNT(*) FROM Tracks), 2) AS '% of Tracks' 
FROM Genre
LEFT JOIN Track
    ON Track.GenreID = Genre.GenreID
GROUP BY Genre.Name

Note that I have replaced your join with an explicit one. Also, the query SELECT COUNT(*) FROM Genre is not correlated, so the query optimizer should be able to take advantage of this when executing.

Answer 2

You have to divide the count of Track s per Genre with the total number of Tracks :

SELECT
    g.Name,
    COUNT(*) AS 'No of Tracks',
    (COUNT(*) * 100.0) / (SELECT COUNT(*) FROM Tracks) AS '% of Tracks'
FROM Genre g
LEFT JOIN Tracks t
    ON t.GenreID = g.GenreID
GROUP BY g.GenreID, g.Name

Notes:

1. Use JOIN syntax instead of the old-style (using comma).
2. Use meaning full table aliases to improve readability.

Answer 3

To get the number of tracks per genre you need to group the tracks by GenreID and divide that count by the total number of tracks:

SELECT Genre.Name AS Genre, 
       CONVERT(nvarchar, CAST(100.00*COUNT(Tracks.TrackId)/(SELECT COUNT(*) FROM Tracks) AS decimal(8,2))) + '%' AS 'Of Total Tracks'
FROM Genre
INNER JOIN Tracks ON (Genre.GenreId = Tracks.GenreId)
GROUP BY Tracks.GenreId, Genre.Name

This gives you (for example) this result:

Genre      Of Total Tracks
---------- -------------------------------
Classic    16.67%
Pop        33.33%
Rock       50.00%