Data Archive

Many-to-Many Relationships

Many-to-Many Relationships

Occurs when multiple records in one table are associated with multiple records in another table.

Need to use a junction table (also known as a bridge table or associative entity). This table will contain foreign keys that reference the primary keys of the two tables involved in the relationship.

Steps to Implement Many-to-Many Relationships

  1. Identify the Entities: Determine the two entities that will participate in the many-to-many relationship. For example, consider students and courses.

  2. Create the Junction Table: Create a new table that will serve as the junction table. This table will hold the foreign keys from both entities. In our example, we can create a table called enrollments.

  3. Define Foreign Keys: In the junction table, define foreign keys that reference the primary keys of the two related tables. This establishes the relationship between the entities.

  4. Add Additional Attributes (if necessary): If needed, you can also include additional attributes in the junction table that are relevant to the relationship itself. For instance, you might want to track the enrollment date.

Example Schema

-- Create the students table
CREATE TABLE students (
    "id" INTEGER PRIMARY KEY,
    "name" TEXT NOT NULL,
    "email" TEXT UNIQUE NOT NULL
);
 
-- Create the courses table
CREATE TABLE courses (
    "id" INTEGER PRIMARY KEY,
    "title" TEXT NOT NULL,
    "description" TEXT
);
 
-- Create the junction table for the many-to-many relationship
CREATE TABLE enrollments (
    "student_id" INTEGER,
    "course_id" INTEGER,
    "enrollment_date" DATE NOT NULL,
    PRIMARY KEY("student_id", "course_id"), -- Composite primary key
    FOREIGN KEY("student_id") REFERENCES "students"("id"),
    FOREIGN KEY("course_id") REFERENCES "courses"("id")
);

A composite primary key (student_id, course_id) ensures that each student can enroll in a course only once.

To retrieve data from a many-to-many relationship, you can use SQL JOIN statements. For example, to find all courses a specific student is enrolled in, you can run:

SELECT courses.title
FROM courses
JOIN enrollments ON courses.id = enrollments.course_id
WHERE enrollments.student_id = 1;  -- Replace 1 with the desired student ID

Many-to-Many Relationships

  • Records in Table A relate to multiple records in Table B, and vice versa.
  • Requires a junction table to manage the relationships.
  • Example: Students and Courses tables with a junction table Enrollments.

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