Database - Am i doing this right? - sql

Question

I'm trying to make a simple database for a personal project, and I'm not sure whether i'm using Primary Keys properly.
Basically, the database contains users who have votes yes/no on many different items.
Example :
User "JOHN" voted YES on item_1 and item_2, but voted FALSE on item_3.
User "BOB" voted YES on item_1 and item_6.
User "PAUL" votes NO on item_55 and item_76 and item_45.
I want to use the following 3 tables (PK means Primary Key) :
1) table_users, which contains the columns "PK_userID" and "name"
2) table_items, which contains the columns "PK_itemID" and "item_name"
3) table_votes, which contains the columns "PK_userID", "PK_itemID", and "vote"
and the columns with the same name will be linked
Does it look like a proper way to use primary keys ? (so the table_votes will have two Primary Keys, being linked to the two other tables)
Thanks :)

Answer 1

Since user can vote for multiple items and multiple users can vote for a single item, you should not create following two primary keys in third table table_votes. Just create them as fields otherwise it will restrict you add only a userId or itemId only once. Yep, you should make them NOT NULL
"PK_userID", "PK_itemID",

Answer 2

No, that's not correct.
There can only be one primary key per table. You can have other columns with unique indexes that could have been candidate keys, but that's not the primary.
I think you'd have three tables:
create table PERSON (
PERSON_ID int not null identity,
primary key(PERSON_ID)
);
create table ITEM (
ITEM_ID int not null identity,
primary key(ITEM_ID)
);
create table VOTE (
PERSON_ID int,
ITEM_ID int,
primary key(PERSON_ID, ITEM_ID),
foreign key(PERSON_ID) references PERSON(PERSON_ID),
foreign key(ITEM_ID) references ITEM(ITEM_ID)
);
It's a matter of cardinality. A person can vote on many items; an item can be voted on by many persons.
select p.PERSON_ID, i.ITEM_ID, COUNT(*) as vote_count
from PERSON as p
join VOTE as v
on p.PERSON_ID = v.PERSON_ID
join ITEM as i
on i.ITEM_ID = v.PERSON_ID
group by p.PERSON_ID, i.ITEM_ID

Answer 3

This looks reasonable. However, I would not advise you to name your primary keys with a "PK_" prefix. This can be confusing, especially because I advise giving foreign keys and primary keys the same name (the relationship is then obvious). Instead, just name it after the table with Id as a suffix. I would recommend a table structure such as this:
create table Users (
UserId int not null auto_increment primary key,
Name varchar(255) -- Note: you probably want this to be unique
);
create table Items (
ItemId int not null auto_increment primary key,
ItemName varchar(255) -- Note: you probably want this to be unique
);
create table Votes (
UserId int not null references Users(UserId),
ItemId int not null references Items(ItemId),
Votes int,
constraint pk_UserId_ItemId primary key (UserId, ItemId)
);
Actually, I would be inclined to have an auto-incremented primary key in Votes, with UserId, ItemId declared as unique. However, there are good arguments for doing this either way, so that is more a matter of preference.