Databases

Databases store and query data reliably.

📖 Key ideas

Relational databases store tables with rows and columns, queried with SQL.
Primary keys identify rows; foreign keys link tables.
Indexes make lookups faster at some cost to writes.
Transactions follow ACID: atomic, consistent, isolated, durable.
NoSQL databases (document, key-value, graph) trade some features for flexibility or scale.

💡 Real-life example: Like a school office register: one table of students, one of classes, linked by class number.

Primary keyUnique id of a row

Foreign keyColumn that links to another table

IndexStructure that speeds up lookups

TransactionGroup of operations that succeed or fail together