I have a doubt about the way of relating some tables. I have these tables:
User table: username (primary key)
Team table: team_name(primary key), username (foreign key references User(username))
With this relationship, I get that an user can have more than one team.
Group table: group_name (primary key)
I want that a group can have many teams, but these teams have to be of different users, so two teams of a user cannot be in the same group.
I have thought to do a relationship with the three tables of this way:
Group_teams table: (group_name, username, team_name). This table would have a composite primary key (group_name and username), in this way I would be sure that an user cannot has more than one team in a same group.
In addition, I think that I should create a composite foreign key references User(username) and Team (team_name) to be able to control that the team of a user exists. Finally, I should create another foreign key references Group (group_name) to control that a group exists.
I'm not sure that it would be of this way because I have errors when I try to do it. Could yo help me and tell me your opinions?
If a user can be on (at most) only one team, then you have a 0/1 - many relationship.
The easiest approach is to have TeamId in the Users table. This would be a foreign key reference to Teams.
There is no need for a third table to represent this relationship.
There's no need to reproduce the username field in the Group_teams table, just have group_name & team_name.
Create a trigger on the Group_teams table to fire before a new row is inserted, checking for more other teams with the same username.
Have a look at the question "How do you check constraints from another table when entering a row into a table?", specifically this answer from Jim V describing such a setup.
Related
I have a CUSTOMERS table and a CONTACTS table the relation between them is one to many obviously.
also I have PROJECTS table and PROJECT_CUSTOMERS table with relation one to many and with relation one to one between CUSTOMERS and PROJECT_CUSTOMERS.
my problem is that I have a fifth table PROJECT_CONTACTS ....I can't figure which tables shall I refer to in this tables, currently I am refering to PROJECT_CUSTOMERS and CONTACTS table, is this correct or there is something better ?
Your title refers to "foreign keys" but your question just seems to be about what columns should go in what tables.
First, decide what situations can arise and what you want/need to say about them. (This will your tables, columns, candidate keys, foreign keys and constraints.)
Every table holds rows that make some predicate (statement template parameterized by column names) true. Your design seems to have:
CUSTOMERS(id, ...) -- ID identifies a customer and ...
CONTACTS(id, ...) -- ID identifies a contact and ...
PROJECTS(id, ...) -- ID identifies a project and ...
PROJECT_CUSTOMERS(pid, cust_id, ...) -- project PID has customer CUST_ID and ...
PROJECT_CONTACTS(pid, cont_id, cust_id)...)
-- project PID has contact CONT_ID and project pid has customer CUST_ID and ...
A foreign key has a table & column list referencing a table and column list that forms a candidate key. It says that lists of values in the first table appear as lists of values in the second table. Where that is so, declare a foreign key.
I want to create a database scheme and don't really know a solution for the following scenario:
I have users, teams and projects. I want to enable to create projects as a user, but also as a team. What I thought of, was to include two foreign keys in the projects table. One for 'userId' and one for 'teamId'. But in this case for each entitiy either userId or teamId would be null.
Is this a good solution or is there a better possibility to solve that?
You can create a constraint that would force userId != null OR teamId != null.
Alternatively, you can require each project to have a non null foreign key userId (some user in the team had to actively create the project, right?), and then create an associatedTeam foreign key (which can be optional).
Its just a quick question :
I have user table and it has fields name, address and books_bought. books_bought is foreign key and its value is some PK from other table. Now after 1st insert, I will fill out all of this fields , but after second INSERT I want only to add a additional books_bought, so that am creating array of books_bought values?
You're doing it the wrong way around - this is a one-to-many relationship i.e. many books bought to one user. You need to have the foreign key on the many side of the relationship, so instead of having a foreign key to books_bought on the users table, add a foreign key to users on the books_bought table.
If you have a books table and a users table, then this is a many-to-many relationship and you will need a link table to go between them to hold the foreign keys.
You should not have more than one book in the books_bought cell because it will violate the atomicity constraint fo the database tables. You have to have a separate entry for each book_bought. This would cause a lot of redundant information as name, address would be repeated for each book bought by a specific person.
To solve this, you have to split the table into something like this:
R1(primary_key , name , address) and R2(foreign_key , books_bought)
Here foreign_key refers to primary_key of R1
I'm very new to Databases and looking forward to develop an application with some advanced functionality.
Here I'm going to have a table with users just like the following one.
So, in the sports column, value will be the sports the user plays. They will be some ids from the records of sports table.
So my problem is how to store those multiple values in the same row same column ?
Thanks
PS: Please note that I'm a beginner...
The proper way to do this would be to maintain three different tables.
User: id, name, age
Sport: id, name
UserSport: user_id, sport_id
The UserSport table references the primary key (id) of both tables (User and Sport) and contains a separate entry for each sport the user participates in.
The primary key of UserSport should then be a composite key of both user_id and sport_id. This allows you to have multiple rows for each user and multiple rows for each sport, but a unique combination of both the user and sport.
I'm new to the world of MySQL and I'm having a bit of trouble. Here's the problem:
I have 2 tables called TEAMS and FIXTURES. Here's the structure I'm trying to create (only showing relevant fields):
TEAMS table:
team_name (Primary key)
FIXTURES table:
fixture_id (Primary key)
home_team (Foreign key)
away_team (Foreign key)
home_team and away_team both get a team name from the same source (team_name).
I've successfully created the relationship with the FIXTURES(home_team) and TEAMS(team_name), but I can't get the other relationship working - FIXTURES(away_team) and TEAMS(team_name).
I get the "#1452 - Cannot add or update a child row: a foreign key constraint fails" error message. I'm assuming could be because I have 2 foreign keys in the same table referencing the same primary key. Is that correct? If so, how would I fix it?
Hopefully I've explained this ok and that someone can help me, thanks :-)
That means you already have value that doesn't satisfy the constraint, i.e. having no corresponding entry in the teams table. So the problem is not with the schema, but with your data. Fix the issue there.
SELECT * FROM fixtures LEFT JOIN teams ON (away_team=team_name)
WHERE team_name IS NULL;
will fetch you the offending rows.