Find average SQL statement multiple tables - sql

Question

Trying to get the average with a amount of times a user has purchases brocolli and then with the price at that time. A 0 if the user has not purchased any. This is working and saying it can't see the Name column. What am I missing?
SELECT U.ID, U.NAME, COALESCE(AVG(P.PRICE),0) AS SELLPRICE FROM USERS AS U
LEFT JOIN PURCHASES AS P ON U.ID=P.USERID AND P.FoodId=1 GROUP BY U.ID;
EDIT
SELECT P.USERID, U.NAME, AVG(P.PRICE) AS "Sell Price"
FROM PURCHASES AS P INNER JOIN USERS AS U ON CASE WHEN P.ID NOT NULL THEN
"Sell Price" ELSE 0 WHERE P.FOODID=1
I also tried simplifying to just use the purchasers table and get wrong results but maybe I can tweek this as it runs.
SELECT AVG(A.Price),ID FROM PURCHASES AS A WHERE FOODID=1 GROUP BY ID;
To be honest this was in part this issue with the compiler I was using as it was browser site compiler so even when I had it working on my machine it was giving different results on the site. I ended using an inner join on the two tables.
Update
This ran correctly for the answer. Thank you.
SELECT U.ID, U.NAME, AVG(P.PRICE) FROM USERS AS U LEFT JOIN PURCHASES AS P ON U.ID = P.USERID AS P AND P.FoodId=1 GROUP BY U.ID, P.NAME;

Answer 1

If I understand correctly, you are trying to get the average purchase price of broccoli for each user. You are closer to achieving this with your second query, you just need to group by the UserId column (to get the average per user), not the Id column (this would give you the average per purchase - kind of meaningless) of the Purchases table. So, i think all you need is:
SELECT AVG(PRICE), USERID FROM PURCHASES WHERE FOODID=1 GROUP BY USERId
If you want to add some user information to your output, like their name, you will need to join with the Users table:
SELECT AVG(P.PRICE), P.USERID, U.NAME
FROM PURCHASES AS P INNER JOIN USERS AS U ON P.USERID = U.ID
WHERE P.FOODID=1
GROUP BY P.USERId, U.NAME
Your first query I'm afraid has more than one syntax issues. For example, the name aver_output doesn't correspond to a table or subquery column. If you were trying to name your average price column, you would need the AS keyword (or just lose the comma). Also, you have a subquery and next to it you have a table (Users) without any correlation between them. You must specify how you want to join the two, i.e. whether you want an inner, outer, left or right join. In most systems you can also use a comma to indicate a join, but you don't even have that.
In any case, even if you do fix the syntax, the subquery is unnecessary, as you can achieve the same thing without one.
Edit (after edits to the original post):
The best way to include the users who have not ever purchased any broccoli, is to perform a left join to the PURCHASES table, as in your last attempt. However, you need to group by the name of the user, because it appears in your select list. Grouping by the name of the product is not necessary in this case. So, I suggest:
SELECT U.ID, U.NAME, AVG(P.PRICE) FROM USERS AS U LEFT JOIN PURCHASES AS P ON U.ID = P.USERID AND P.FoodId=1 GROUP BY U.ID, U.NAME;

Answer 2

You need a LEFT JOIN of USERS to PURCHASES and GROUP BY user:
SELECT
U.ID, U.NAME,
COALESCE(AVG(P.PRICE), 0) AS "Sell Price",
COUNT(P.USERID) AS "Number of purchases"
FROM USERS AS U LEFT JOIN PURCHASES AS P
ON P.USERID = U.ID AND P.FOODID = 1
GROUP BY U.ID, U.NAME

Answer 3

SELECT U.Id, AVG(U.NAME)
FROM (SELECT PRICE, USERID, ID
FROM PURCHASES WHERE FOODID=1) P
Join USERS U on U.Id = P.USERId
GROUP BY U.ID
But actually, all you should need is
SELECT U.Id, AVG(U.NAME)
FROM PURCHASES P Join USERS U
on U.Id = P.USERId
WHERE P.FOODID = 1
GROUP BY U.ID
except I still don't see why you are asking for average of Name column...
What exactly are you trying to do?