Database fundamentals

A database that stores data in tables and links those tables together with keys, so each fact is stored once in the table it belongs to and other tables point at it rather than copying it.

A primary key uniquely identifies each row in its table. A composite key does that using two or more columns together, when no single column is unique. A foreign key holds a primary key value from another table, which is how the link is made.

Reduced duplication, because each fact is stored once; data integrity, because the database enforces rules such as refusing an invalid foreign key; and consistency, because changing a fact in one place is seen by everyone.

A rigid schema that must be decided up front and is disruptive to change; poor handling of unstructured data such as documents and images; and difficulty scaling very large volumes across many machines.

The same fact gets written out on every row that mentions it. Changing it means finding every copy, and missing one leaves the data contradicting itself.