Should Every Table Have a Primary Key? Exploring Data Uniqueness in Databases

2024-07-27

Uniqueness: They guarantee each record is distinct, preventing duplicate data. Imagine a customer table without a primary key - you might end up with multiple entries for the same customer.
Data Integrity: They act as an anchor for referencing data between tables. When tables are linked, the primary key is often used for efficient joins and lookups.
Efficiency: Database engines often use the primary key to optimize queries and data retrieval.

However, there are some rare cases where a table might not need a primary key:

Temporary Tables: Tables used for temporary calculations or reports might not require the overhead of a primary key.
Staging Tables: Tables used to hold data before processing might not need a permanent unique identifier.

-- Create a table "Customers" with "customer_id" as the primary key
CREATE TABLE Customers (
  customer_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  customer_name VARCHAR(255) NOT NULL,
  phone_number VARCHAR(20),
  email VARCHAR(255)
);

-- Create a table "Orders" with a composite primary key of "order_id" and "customer_id"
CREATE TABLE Orders (
  order_id INT NOT NULL,
  customer_id INT NOT NULL,
  order_date DATE NOT NULL,
  total_amount DECIMAL(10,2) NOT NULL,
  PRIMARY KEY (order_id, customer_id),
  FOREIGN KEY (customer_id) REFERENCES Customers(customer_id)
);

Python (using SQLAlchemy)

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship

engine = create_engine('sqlite:///mydatabase.db')
Base = declarative_base()

class Customer(Base):
  __tablename__ = 'customers'

  id = Column(Integer, primary_key=True)
  name = Column(String(255))
  phone_number = Column(String(20))
  email = Column(String(255))

class Order(Base):
  __tablename__ = 'orders'

  id = Column(Integer, primary_key=True)
  customer_id = Column(Integer, ForeignKey('customers.id'))
  order_date = Column(Date)
  total_amount = Column(Float)

  customer = relationship(Customer)

Base.metadata.create_all(engine)

Natural Key:

A natural key is a combination of existing columns in a table that uniquely identifies each row. For instance, in a "Customers" table, you might use a combination of "first_name", "last_name", and "date_of_birth" as a natural key.

Drawbacks:

Not always guaranteed to be unique. Names can be common, and date of birth might not be unique for identical twins.
Can be cumbersome to use in queries, especially if it involves multiple columns.

Unique Identifier (without auto-increment):

You can create a unique identifier column that isn't auto-incrementing. This could be a string of randomly generated characters (UUIDs) or a sequence generated by an external service.

Loses the benefit of auto-incrementing primary keys for efficient record insertion.
Randomly generated IDs might be less readable for humans.

Clustering by a Unique Column:

If you have a column that is already unique (like an email address), you can cluster the table by that column. This can improve query performance for searches based on that column. However, it doesn't enforce uniqueness for inserts or deletions.