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
Related
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.
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 have a products table link to a bill of materials table. See diagram
Each product can have more than one formula. For example:
Currently ProdIDNeed and ProdIDNeeded are my composite primary key, both fields also link as foreign keys to ProdID (Products). The only way I can think of preventing a key violation is to create a Formula field and have a 3 field composite primary key (ProdIDNeed, ProdIDNeeded, FormulaNumber).
However, I have to link the product_billmaterials table to a workorders table (bascially an order to make the product according to formula). Linking three fields to another table is a pain.
I guess I could also create a surrogate key on the product_billmaterials table which I am not too crazy about either.
Is there any other way I can organize this or must I choose one of the options I have thought of?
Is it possible, in MySQL, to create two unique indexes, together?
For example, there is a table that holds two IDs - customer ID and product ID.
I would like to create uniqueness for the pair only.
In simple words, combination of user and product can be present only once.
The same user with another products can be present as much as needed.
And the same product with another users can be present as much as needed.
But the pair itself to be unique. How?
This query actually has an OR relation. I am looking for an AND relation.
ALTER TABLE top ADD CONSTRAINT unique_pair UNIQUE (uid, pid)
This does what you describe:
ALTER TABLE top ADD CONSTRAINT unq_top_unique_pair UNIQUE (uid, pid);
As does:
CREATE UNIQUE INDEX unq_top_uid_pid ON TOP(uid, pid);
Both of these create a unique index, which is then used for enforcing the constraint.
in DB tables i've: Patient Table, PatientBasicInfomation Table, PatientImageFindings Table..
i've multiple Questions depend on this design..(note that i'm beginner in DB)
1) if i have for each Patient an ID.. so according to DB concepts both PatientBasicInfomation, PatientImageFindings should have this key (ID) as a foreign key?!
2) in the Patient Table i should reference to the PatientBasicInfomation, PatientImageFindings by using their private keys so they will be in Patient Table as a foreign keys?! am i thinking correct...
3) Now my big problem: i want to insert in PatientImageFindings Table a record but under condition ID + Date (where Date is an important Field in PatientImageFindings Table, i don't know if i should put it as a private key or not..), how could i do this insertion statement in my java class..(Insertion under conditions)
What you want to do is add a UNIQUE constraint across multiple columns.
This question provides an answer to do just that:
How do I specify unique constraint for multiple columns in MySQL?
alter table votes add unique index(user, email, address);
Unless you have another reason, you should enforce this at the database level and treat exceptions as they arise after attempted INSERTs.