SQL Schema Design & Data Integrity

Normalization — 1NF Through 3NF

1NF — every column holds a single atomic value. No arrays, no comma-separated lists.

-- BAD: multi-valued column
CREATE TABLE students (id INT PRIMARY KEY, phone_numbers VARCHAR(500));  -- '555-1234,555-5678'

-- GOOD: one fact per row
CREATE TABLE student_phones (
    student_id INT REFERENCES students(id),
    phone VARCHAR(20) NOT NULL,
    PRIMARY KEY (student_id, phone)
);

2NF — with composite keys, every non-key column depends on the entire key, not just part.