SQL query to calculate average and insert into a table - sql

Question

I'm working on a song archive database and I'm stuck on some queries. I would like to -
Calculate the rating of each user by their average Comments value of score and inserting the rating into Users
Calculating how much Purchases each user has
Calculate the average score of a Song from the Comments table
Calculating how many credits each user has spent on their purchases
Below you can find my tables...
CREATE TABLE Users
(
username NVARCHAR( 30 ) NOT NULL PRIMARY KEY,
pass NVARCHAR( 16 ),
email NVARCHAR( 50 ),
city NVARCHAR( 10 ),
credits INT,
rating INT
)
CREATE TABLE Songs
(
song_id INT NOT NULL IDENTITY ( 1, 1 ) PRIMARY KEY,
song_name NVARCHAR( 30 ),
username NVARCHAR( 30 ),
genre INT,
price INT,
song_length INT,
listens INT
)
CREATE TABLE Genres
(
genre_id INT NOT NULL IDENTITY ( 1, 1 ) PRIMARY KEY,
genre_name NVARCHAR( 16 )
)
CREATE TABLE Purchases
(
purchase_id INT NOT NULL IDENTITY ( 1, 1 ) PRIMARY KEY,
song_id INT,
username NVARCHAR( 30 )
date_purchased DATETIME
)
CREATE TABLE Comments
(
comment_id INT NOT NULL IDENTITY ( 1, 1 ) PRIMARY KEY,
username NVARCHAR( 30 ),
song_id INT,
text NVARCHAR( 30 ),
score INT
)

Answer 1

I answered some of your questions. In addition to the respective queries I arranged them as common table expressions, which I think could be a convenient way to use them...
Calculating how much credits each user has spent on his purchases, might require to know your logic about how users invest their credits.
WITH CTE_PurchasesByUser AS
(
SELECT p.username as username, count(*) as NrOfPurchases
FROM Purchases p
GROUP BY p.username
),
CTE_AverageScoreBySong AS
(
SELECT c.song_id as song_id, (sum(c.score)/count(c.score)) as AverageScore
FROM Comments c
GROUP BY c.song_id
),
CTE_AverageScoreByUser AS
(
SELECT u.username as username, (sum(c.score)/count(c.score)) as AverageScore
FROM Users u
INNER JOIN Comments c ON u.username = c.username
GROUP BY u.username
)
SELECT u.*, ISNULL(bbu.NrOfPurchases,0), asu.AverageScore
FROM Users u
LEFT JOIN CTE_PurchasesByUser bbu ON u.username = bbu.username
LEFT JOIN CTE_AverageScoreByUser asu ON u.username = asu.username
This SQL ran with your tables, yet I didn't test it with data rows...