How to denormalize a n_n relation in sql with view - mysql

Question

I have 2 tables
account
id | desc
Prices
id | desc
Table account stores users info while table prices stores several prices depending type of service.
Now, I need to assign prices that apply to each account.
I would like to display a result containg prices and an extra column that tells (in form of a list) the accounts that apply that service...
I was thinking on
CREATE TABLE `account` (
`id_account` smallint(2) unsigned PRIMARY KEY AUTO_INCREMENT,
`user` VARCHAR(55) ,
`pass` VARCHAR(55) ,
`descr` VARCHAR(250)
);
INSERT INTO account VALUES
(1,'67395' , 'pass1','DrHeL'),
(2,'12316' , 'pass2','DeHrL'),
(3,'92316' , 'pass3','EfL');
CREATE TABLE `prices`(
`id_price` smallint(2) unsigned PRIMARY KEY AUTO_INCREMENT,
`service` VARCHAR(40),
`cost_1_1Kg` double ,
`cost_4_1Kg` double ,
`cost_8_1Kg` double
);
INSERT INTO prices VALUES
(1,'laundry', 1.50, 2.00,5.00),
(2,'walk.' , 2.50, 3.00,4.00);
CREATE TABLE `account_prices` (
`id_account` smallint(2) unsigned NOT NULL,
`id_price` smallint(2) unsigned NOT NULL,
`descr` VARCHAR(250)
) ;
INSERT INTO account_prices VALUES
(1,1,'apply SERVICE WITH ID 1 AND SERVICE WITH ID 2'),
(2,1,'apply SERVICE WITH ID 1 AND SERVICE WITH ID 2'),
(3,1,'apply SERVICE WITH ID 1 AND SERVICE WITH ID 2'),
(1,2,'apply SERVICE WITH ID 1 AND SERVICE WITH ID 2'),
(2,2,'apply SERVICE WITH ID 1 AND SERVICE WITH ID 2'),
(3,2,'apply SERVICE WITH ID 1 AND SERVICE WITH ID 2');
This gives me
ID_ACCOUNT ID_PRICE DESCR USER PASS SERVICE COST_1_1KG COST_4_1KG COST_8_1KG
1 1 apply SERVICE WITH ID 1 AND SERVICE WITH ID 2 67395 pass1 laundry 1.5 2 5
2 1 apply SERVICE WITH ID 1 AND SERVICE WITH ID 2 12316 pass2 laundry 1.5 2 5
3 1 apply SERVICE WITH ID 1 AND SERVICE WITH ID 2 92316 pass3 laundry 1.5 2 5
1 2 apply SERVICE WITH ID 1 AND SERVICE WITH ID 2 67395 pass1 walk. 2.5 3 4
2 2 apply SERVICE WITH ID 1 AND SERVICE WITH ID 2 12316 pass2 walk. 2.5 3 4
3 2 apply SERVICE WITH ID 1 AND SERVICE WITH ID 2 92316 pass3 walk. 2.5 3 4
However I would like somethink like:
ID_PRICE SERVICE COST_1_1KG COST_4_1KG COST_8_1KG descr
1 laundry 1.5 2 5 acount 1, account 2, account 3
2 walk. 2.5 3 4 acount 1, account 2, account 3
How to do It?
Please take a look at the corresponding fiddle:
http://sqlfiddle.com/#!2/16f05/3

Answer 1

You can get your desired output with this query:
select ap.id_price
, p.service
, p.cost_1_1Kg
, p.cost_4_1Kg
, cost_8_1Kg
, group_concat(
concat('account ', ap.id_account)
order by ap.id_account
separator ', '
) as descr
from account_prices ap
inner join prices p using (id_price)
group by ap.id_price
Result:
| ID_PRICE | SERVICE | COST_1_1KG | COST_4_1KG | COST_8_1KG | DESCR |
|----------|---------|------------|------------|------------|---------------------------------|
| 1 | laundry | 1.5 | 2 | 5 | account 1, account 2, account 3 |
| 2 | walk. | 2.5 | 3 | 4 | account 1, account 2, account 3 |
Check updated SQL fiddle
How this works:
You only need to join account_prices with prices to get all the data you need.
To get the account x stuff, concatenate "account " with the value of id_account using the concat() function
Finally, to get a concatenated group of values, use the group_concat() function. It works like any other aggregate function, but instead of performing an operation (like sum() or count()), it concatenates the values of the column (or expression). You can define the order you want for the output and a custom separator (the default separator is ,.
Hope this helps you.