x-to-x-relationships

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).

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: 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

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: 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

Example

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)
);

Many to ones

https://stackoverflow.com/questions/4601703/difference-between-one-to-many-and-many-to-one-relationship

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