How to implement one-to-one, one-to-many and many-to-many relationships while designing tables?
Can anyone explain how to implement one-to-one, one-to-many and many-to-many relationships while designing tables with some examples?
Solution 1:
One-to-one: Use a foreign key to the referenced table:
student: student_id, first_name, last_name, address_id
address: address_id, address, city, zipcode, student_id # you can have a
# "link back" if you need
You must also put a unique constraint on the foreign key column (addess.student_id
) to prevent multiple rows in the child table (address
) from relating to the same row in the referenced table (student
).
One-to-many: Use a foreign key on the many side of the relationship linking back to the "one" side:
teachers: teacher_id, first_name, last_name # the "one" side
classes: class_id, class_name, teacher_id # the "many" side
Many-to-many: Use a junction table (example):
student: student_id, first_name, last_name
classes: class_id, name, teacher_id
student_classes: class_id, student_id # the junction table
Example queries:
-- Getting all students for a class:
SELECT s.student_id, last_name
FROM student_classes sc
INNER JOIN students s ON s.student_id = sc.student_id
WHERE sc.class_id = X
-- Getting all classes for a student:
SELECT c.class_id, name
FROM student_classes sc
INNER JOIN classes c ON c.class_id = sc.class_id
WHERE sc.student_id = Y
Solution 2:
Here are some real-world examples of the types of relationships:
One-to-one (1:1)
A relationship is one-to-one if and only if one record from table A is related to a maximum of one record in table B.
To establish a one-to-one relationship, the primary key of table B (with no orphan record) must be the secondary key of table A (with orphan records).
For example:
CREATE TABLE Gov(
GID number(6) PRIMARY KEY,
Name varchar2(25),
Address varchar2(30),
TermBegin date,
TermEnd date
);
CREATE TABLE State(
SID number(3) PRIMARY KEY,
StateName varchar2(15),
Population number(10),
SGID Number(4) REFERENCES Gov(GID),
CONSTRAINT GOV_SDID UNIQUE (SGID)
);
INSERT INTO gov(GID, Name, Address, TermBegin)
values(110, 'Bob', '123 Any St', '1-Jan-2009');
INSERT INTO STATE values(111, 'Virginia', 2000000, 110);
One-to-many (1:M)
A relationship is one-to-many if and only if one record from table A is related to one or more records in table B. However, one record in table B cannot be related to more than one record in table A.
To establish a one-to-many relationship, the primary key of table A (the "one" table) must be the secondary key of table B (the "many" table).
For example:
CREATE TABLE Vendor(
VendorNumber number(4) PRIMARY KEY,
Name varchar2(20),
Address varchar2(20),
City varchar2(15),
Street varchar2(2),
ZipCode varchar2(10),
Contact varchar2(16),
PhoneNumber varchar2(12),
Status varchar2(8),
StampDate date
);
CREATE TABLE Inventory(
Item varchar2(6) PRIMARY KEY,
Description varchar2(30),
CurrentQuantity number(4) NOT NULL,
VendorNumber number(2) REFERENCES Vendor(VendorNumber),
ReorderQuantity number(3) NOT NULL
);
Many-to-many (M:M)
A relationship is many-to-many if and only if one record from table A is related to one or more records in table B and vice-versa.
To establish a many-to-many relationship, create a third table called "ClassStudentRelation" which will have the primary keys of both table A and table B.
CREATE TABLE Class(
ClassID varchar2(10) PRIMARY KEY,
Title varchar2(30),
Instructor varchar2(30),
Day varchar2(15),
Time varchar2(10)
);
CREATE TABLE Student(
StudentID varchar2(15) PRIMARY KEY,
Name varchar2(35),
Major varchar2(35),
ClassYear varchar2(10),
Status varchar2(10)
);
CREATE TABLE ClassStudentRelation(
StudentID varchar2(15) NOT NULL,
ClassID varchar2(14) NOT NULL,
FOREIGN KEY (StudentID) REFERENCES Student(StudentID),
FOREIGN KEY (ClassID) REFERENCES Class(ClassID),
UNIQUE (StudentID, ClassID)
);