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How Do One-to-One, One-to-Many, and Many-to-Many Database Relationships Work?
How Do One-to-One, One-to-Many, and Many-to-Many Database Relationships Work?
Jan 21, 2025 am 06:27 AM
Mastering Database Relationships for Efficient Data Management
Effective database design hinges on properly structuring tables and defining relationships between them. Three fundamental relationship types govern how records connect: one-to-one, one-to-many, and many-to-many. Let's explore each type and their implementation.
One-to-One Relationships
A one-to-one relationship signifies a one-to-one correspondence between records in two tables. This is achieved using a foreign key in the dependent table referencing the primary key of the parent table.
Example:
-- Table 1: Student
CREATE TABLE Student (
student_id INT PRIMARY KEY,
first_name VARCHAR(255),
last_name VARCHAR(255),
address_id INT
);
-- Table 2: Address
CREATE TABLE Address (
address_id INT PRIMARY KEY,
address VARCHAR(255),
city VARCHAR(255),
zipcode VARCHAR(10),
student_id INT UNIQUE
);
Each student_id uniquely maps to an address_id, illustrating a one-to-one link. Note the UNIQUE constraint on student_id in the Address table.
One-to-Many Relationships
In a one-to-many relationship, a single record in one table can relate to multiple records in another. The dependent table uses a foreign key referencing the parent table's primary key.
Example:
-- Table 1: Teacher
CREATE TABLE Teacher (
teacher_id INT PRIMARY KEY,
first_name VARCHAR(255),
last_name VARCHAR(255)
);
-- Table 2: Class
CREATE TABLE Class (
class_id INT PRIMARY KEY,
class_name VARCHAR(255),
teacher_id INT
);
One teacher can teach multiple classes, but each class has only one teacher.
Many-to-Many Relationships
A many-to-many relationship connects multiple records in one table to multiple records in another. This necessitates a junction table containing foreign keys from both tables.
Example:
-- Table 1: Student
CREATE TABLE Student (
student_id INT PRIMARY KEY,
first_name VARCHAR(255),
last_name VARCHAR(255)
);
-- Table 2: Class
CREATE TABLE Class (
class_id INT PRIMARY KEY,
name VARCHAR(255)
);
-- Junction Table: Student_Class
CREATE TABLE Student_Class (
class_id INT,
student_id INT,
PRIMARY KEY (class_id, student_id)
);
The Student_Class table allows for multiple students in multiple classes.
Data Integrity: Enforcing UNIQUE constraints on foreign keys in one-to-one and one-to-many relationships is crucial for maintaining data consistency.
Illustrative Queries:
-- Students in a specific class: SELECT s.student_id, s.last_name FROM Student_Class sc JOIN Student s ON s.student_id = sc.student_id WHERE sc.class_id = 1; -- Classes attended by a specific student: SELECT c.class_id, c.name FROM Student_Class sc JOIN Class c ON c.class_id = sc.class_id WHERE sc.student_id = 2;
The above is the detailed content of How Do One-to-One, One-to-Many, and Many-to-Many Database Relationships Work?. For more information, please follow other related articles on the PHP Chinese website!
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