What is a primary key (PK) and a foreign key (FK) in SQL?

Primary Key (PK) — is a column or a set of columns in a table that uniquely identifies each record (row).

• Properties:
  • Every table must have a primary key.
  • Values in the primary key must be unique.
  • Values in the primary key cannot contain NULL.

Foreign Key (FK) — is a column or a set of columns in one table (child table) that references the primary key of another table (parent table).

• Purpose:
  • Creates a relationship between two tables.
  • Ensures referential integrity, guaranteeing that values in the foreign key exist in the related primary key.

CREATE TABLE parents (
    parent_id INT PRIMARY KEY, -- Primary Key
    name VARCHAR(255)
);

CREATE TABLE children (
    child_id INT PRIMARY KEY,
    parent_id INT, -- Foreign key referencing parents.parent_id
    FOREIGN KEY (parent_id) REFERENCES parents (parent_id)
);