You now understand SQL databases.
SQL databases provide ACID: Atomicity, Consistency, Isolation, Durability.
Index columns used in WHERE, JOIN, and ORDER BY. Don't over-index.
Primary keys identify rows. Foreign keys enforce relationships.
Start normalized. Denormalize for performance when needed.
Read replicas scale reads. Accept replication lag as a trade-off.
Choose SQL when you need transactions, complex queries, and data integrity. When scale demands it, the next sections cover sharding and NoSQL alternatives.