Get the average with quantity - sql

Question

I have a problem to calculate easily some simple average. My table :
id / user / action / data
1 / a / unit_price / 40
2 / a / quantity / 1
3 / b / unit_price / 70
4 / b / quantity / 2
Unit_price is a price for a user and quantity is quantity. So there i should get :
(40 + 70 + 70) / 3 = 60
If i do an
(AVG(action) WHERE action = unit_price)
I get :
(70+40)/2 = 55
If I do an
(SUM(action) WHERE action = unit_price) / (SUM(action) WHERE action = quantity)
I get :
110 / 3 = 36.6
The easiest way I found is to don't put the unit_price but the global price then make a division in the PHP code to get the unit_price, but I was hoping SQL could do something for me.

Answer 1

select coalesce(sum(quantity * unit_price) /sum(quantity), 0) from
(select
sum(case when action='unit_price' then data else 0 end) as unit_price,
sum(case when action='quantity' then data else 0 end) as quantity
from test
group by user) as a
SqlFiddle

Answer 2

You can use something like this which basically pivots the data to a more usable format and then gets the values that you need:
select avg(unit_price) AvgUnitPrice,
sum(unit_price*quantity)/sum(quantity) AvgPrice
from
(
select user,
max(case when action = 'unit_price' then data end) unit_price,
max(case when action = 'quantity' then data end) quantity
from table1
group by user
) x;
See SQL Fiddle With Demo

Answer 3

Ok, obviously your table design is not optimal, you should have unit_price and quantity as separate columns. But, working with what you have, try this:
SELECT SUM(A.data*B.data)/SUM(B.data) Calculation
FROM ( SELECT user, data
FROM YourTable
WHERE action = 'unit_price') AS A
INNER JOIN (SELECT user, data
FROM YourTable
WHERE action = 'quantity') AS B
ON A.user = B.user

Answer 4

I would join the table to itself in order to get the two records beloning together on one line
SELECT
SUM(unit_price * quantity) / SUM(quantity) AS average_unit_price
FROM
(SELECT
U.data AS unit_price, Q.data AS quantity
FROM
theTable U
INNER JOIN theTable Q
ON U.user = Q.user
WHERE
U.action = 'unit_price' AND
Q.action = 'quantity')
If you have more than two records per user and the ids of the both records are consequtive, then you would have to change the WHERE-clause to
WHERE
U.action = 'unit_price' AND
Q.action = 'quantity' AND
U.id + 1 = Q.id
Note:
If you calculate AVG(unit_price * quantity) you get the average sum per user.
(1*40 + 2*70) / 2 = 90
If you calculate SUM(unit_price * quantity) / SUM(quantity) you get the average unit price.
(1*40 + 2*70) / 3 = 60

Answer 5

Something like this should work; syntax might not be perfect since I didn't try it out, but you get the main idea at least.
SELECT sumUnitPrice.sum / sumQuantity.sum
FROM
(
(SELECT SUM(data) as sum
FROM WhateverTheHellYourTableIsNamed
WHERE action = 'unit_price') sumUnitPrice
(SELECT SUM(data) as sum
FROM WhateverTheHellYourTableIsNamed
WHERE action = 'quantity') sumQuantity
)

Answer 6

Your table design doesn't look good.
Make 2 tables instead:
ITEM
ItemId int not null PK,
Name varchar(200) not null,
UnitPrice decimal (10,2) not null
SALES
SalesId int not null PK,
ItemId int not null FK,
Quantity decimal(10,2)
PK - primary key, FK - foreign key
Average:
select
I.Name, avg(I.UnitPrice * S.Quantity) as avgSales
from
Sales S
join Items I on I.ItemId = S.ItemId
group by
I.Name