Can anyone explain how to implement one-to-one, one-to-many and many-to-many relationships while designing tables with some examples?
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~ Asked on 2011-09-04 01:17:36
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 (
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
-- 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
~ Answered on 2011-09-04 01:24:07
Here are some real-world examples of the types of relationships:
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).
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);
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).
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 );
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) );
~ Answered on 2016-04-14 18:34:43