In a relational database, if there is a Student table and Employee table, and a third table StudentAsst who indicates a multiple inheritance from both Student and Employee tables.
My Question: in SQL how to create the StudentAsst, and what is its primary key?
Since a StudentAsst is-a Student and is-an Employee, your table could have a primary key consisting of EmployeeID and StudentID:
CREATE TABLE StudentAsst
(StudentID int,
EmployeeID int,
...,
CONSTRAINT PK_StudentAsst PRIMARY KEY (StudentID, EmployeeID),
CONSTRAINT FK_Employee FOREIGN KEY (EmployeeId)
REFERENCES Employee(EmployeeID),
CONSTRAINT FK_Student FOREIGN KEY (StudentID)
REFERENCES Student(StudentID)
)
Here is an example of a similar situation with Student, Teachers and Parents.
Related
I have following question: let's say we have a Chen-Notation 1:n and m:n.
So 1 has a primary key and n also, where do I type the foreign key ? in the n ?
And the second question is about m:n, both have a primary key, and I need 1 more table because it's m:n, do I type the both primary keys as foreign keys in the 3rd table?
Example of a 1:n relationship : customers and orders
One customer may have several orders. In this situation, you want a column in the orders table with a foreign key that references the primary key of the customers table.
Sample DDL:
create table customers (
id int primary key,
name varchar(50),
email varchar(50)
);
create table orders (
id int primary key
price float,
customer_id int foreign key references customer(id)
);
Example of a n:m relationship : books and authors
A book may be written by more than one author. An author may have written more than one book. You create a bridge table, also called junction table, called books_authors, to represent that relationship, and that contains foreign keys to the two other tables.
Sample:
create table books (
id int primary key,
name varchar(50)
);
create table authors (
id int primary key,
name varchar(50)
);
create table books_authors(
book_id int foreign key references books(id),
author_id int foreign key references authors(id),
constraint pk_books_authors primary key(book_id, author_id)
);
So 1 has a primary key and n also, where do I type the foreign key ? in the n ?
The foreign key lives in the n, because 1 can have many keys, but you can't have a field that holds multiple values, you need to have one value per field, so it's in the n.
And the second question is about m:n, both have a primary key, and I need 1 more table because it's m:n, do I type the both primary keys as foreign keys in the 3rd table?
Yes, because that 3rd table in effect has a many-to-one relationship to both tables.
I need to construnct a junction table for many-to-many relations between two parent tables. The design is set. How is it possible to constraint my junction table as a child table when FK's will become duplicates by default?
create table FactInternetSalesReason
( SalesOrderNumber int,
SalesOrderLineNumber int,
SalesReasonKey int,
CONSTRAINT FK_SalesOrder FOREIGN KEY (SalesOrderNumber, SalesOrderLineNumber) REFERENCES FactInternetSales (SalesOrderNumber, SalesOrderLineNumber),
CONSTRAINT FK_SalesReason FOREIGN KEY (SalesReasonKey) REFERENCES DimSalesReason (SalesReasonKey)
);
A Foreign Key can contain duplicates (try it). A Primary Key cannot.
I have a table in my database that models a m-to-n relation. The primary key of this relation is of course the combined primary keys of the 2 entities involved in this relation.
The relation goes like this: m customers have n orders
create table customer(
cid SERIAL PRIMARY KEY,
...
);
create table order(
oid SERIAL PRIMARY KEY,
...
);
create table has(
oid INTEGER REFERENCES order(oid) ON DELETE CASCADE,
cid INTEGER REFERENCES customer(cid) ON DELETE CASCADE,
FOREIGN KEY (oid,cid) or PRIMARY KEY (oid,cid)
);
I'm a bit confused on what to use here: primary key or foreign key to put them in relation?
thanks in advance for any help.
Jaiel
You should be using PRIMARY KEY for the composite key in table has. The combination of the order and customer IDs in the has table is a primary key in that table, because it allows the identification of a unique single record.
Note that both oid and cid in table has are foreign keys, pointing to the order and customer tables, respectively.
For instance, if I have a students table:
CREATE TABLE student (
student_id NUMBER(8),
student_name VARCHAR2(30),
CONSTRAINT pk_student PRIMARY KEY (student_id)
);
And a subject table:
CREATE TABLE subject (
subject_id NUMBER(8),
subject_name VARCHAR2(30),
CONSTRAINT pk_subject PRIMARY KEY (subject_id)
);
And then I create a third table of student's favorites subjects:
CREATE TABLE fav_sub (
student_id NUMBER(8),
subject_it NUMBER(8),
CONSTRAINT pk_fav_sub PRIMARY KEY (student_id, subject_id),
CONSTRAINT fk_1_fav_sub FOREIGN KEY (student_id) REFERENCES student(student_id),
CONSTRAINT fk_2_fav_sub FOREIGN KEY (subject_id) REFERENCES subject(subject_id)
);
Do I then need to manually create indexes for the foreign keys in the fav_sub table such as:
CREATE INDEX in_1_fav_sub ON fav_sub(student_id);
CREATE INDEX in_2_fav_sub ON fav_sub(subject_id);
Or are the indexes for the foreign keys automatically created by the database, since they're part of the composite key?
Edit: Just to clarify, I'm not asking if an index for a foreign key is automatically created, I'm asking if an index for a foreign key is created WHEN it's part of a composite key, since Oracle automatically indexes primary keys.
The creation of the primary key indexes the combination of [student_id, subject_id]. Adding foreign key constraints to each individual column does not create an index on them, and if you want it (which is probably a good idea), you'd have to manually create it.
We have two tables ManagementPlan which tells what type of product model has to be used.Based on this model number, a particular product of that type is assigned to a patient during therapy session.How can we model the many-many relation between Product table and ManagementPlan table
MangamentPlan(
PlanID(PK),
DiagnosisID(FK),
PhysicianID(FK),
PMCModelID,
Objective,
Description,
DateTime
)
Product(
PMCProductID(PK),
ManuProductID(FK),
ManufacturerID(FK),
PMCModelID,
Manufacturer model,
Features description,
PurchaseDate,
Storage Location
)
Add a junction table:
ManagementPlanProduct(PlanID(PK, FK(ManagementPlan)), PMCProductID(PK, FK(Product)))
You need a junction table
ManagementPlanProduct (
PlanID (PK, FK)
PMCProductID (PK, FK)
)
CREATE TABLE ManagementPlanProduct (
PlanID int NOT NULL,
PMCProductID int NOT NULL,
CONSTRAINT PK_mpp PRIMARY KEY CLUSTERED (PlanID, PMCProductID)
);
ALTER TABLE ManagementPlanProduct
ADD CONSTRAINT FK_mpp_mp
FOREIGN KEY (PlanID) REFERENCES ManagementPlan (PlanID)
ON DELETE CASCADE;
ALTER TABLE ManagementPlanProduct
ADD CONSTRAINT FK_mpp_p
FOREIGN KEY (PMCProductID) REFERENCES Product (PMCProductID)
ON DELETE CASCADE;
You can also add other columns to the junction table like quantity, date added, sort order and so on.