How to implement one-to-one, one-to-many and many-to-many relationships while designing tables?

One-to-one: Use a foreign key to the referenced table:

student: student_id, first_name, last_name, address_idaddress: 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" sideclasses:  class_id, class_name, teacher_id  # the "many" side

Many-to-many: Use a junction table (example):

student: student_id, first_name, last_nameclasses: class_id, name, teacher_idstudent_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

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 isrelated 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));

One-to-many

The one-to-many table relationship looks as follows:

In a relational database system, a one-to-many table relationship links two tables based on a Foreign Key column in the child which references the Primary Key of the parent table row.

In the table diagram above, the post_id column in the post_comment table has a Foreign Key relationship with the post table id Primary Key column:

ALTER TABLE    post_commentADD CONSTRAINT    fk_post_comment_post_idFOREIGN KEY (post_id) REFERENCES post

One-to-one

The one-to-one table relationship looks as follows:

In a relational database system, a one-to-one table relationship links two tables based on a Primary Key column in the child which is also a Foreign Key referencing the Primary Key of the parent table row.

Therefore, we can say that the child table shares the Primary Key with the parent table.

In the table diagram above, the id column in the post_details table has also a Foreign Key relationship with the post table id Primary Key column:

ALTER TABLE    post_detailsADD CONSTRAINT    fk_post_details_idFOREIGN KEY (id) REFERENCES post

Many-to-many

The many-to-many table relationship looks as follows:

In a relational database system, a many-to-many table relationship links two parent tables via a child table which contains two Foreign Key columns referencing the Primary Key columns of the two parent tables.

In the table diagram above, the post_id column in the post_tag table has also a Foreign Key relationship with the post table id Primary Key column:

ALTER TABLE    post_tagADD CONSTRAINT    fk_post_tag_post_idFOREIGN KEY (post_id) REFERENCES post

And, the tag_id column in the post_tag table has a Foreign Key relationship with the tag table id Primary Key column:

ALTER TABLE    post_tagADD CONSTRAINT    fk_post_tag_tag_idFOREIGN KEY (tag_id) REFERENCES tag