Primary Key in Databases

Direct Answer

A primary key is one column or a group of columns that uniquely identifies a row. Its columns are unique and cannot be NULL. A table can have at most one primary-key constraint, although that key may contain several columns. Most entity tables should have one, and its value should normally stay stable.

Code Example

import jakarta.persistence.*;

@Entity
@Table(name = "customers")
public class Customer {
    @Id  // Marks this as the primary key
    @GeneratedValue(strategy = GenerationType.IDENTITY)  // Auto-increment
    private Long id;
    
    @Column(nullable = false, unique = true)
    private String email;
    
    @Column(nullable = false)
    private String name;
    
    // Composite key example in a junction table
}

@Entity
@Table(name = "order_items")
public class OrderItem {
    @EmbeddedId  // Uses both columns together as primary key
    private OrderItemId id;
    
    private Integer quantity;
    private Double price;
}

Common Mistakes & Pitfalls

• Confusing a multi-column key with several keys: SQL allows one primary-key constraint, and that constraint may contain several columns.
• Using unstable contact data as identity: changing an email key also requires a policy for every referencing row.
• Forgetting the JPA identity requirement: a persistent entity needs an identifier mapping; use @Id, @EmbeddedId, or a valid @IdClass mapping.
• Treating the primary key as immutable automatically: SQL can allow an update unless permissions, application rules, or referential actions prevent it.