Choose a subtopic
Databases
Understand how data is organised, queried and protected. Start with tables and SQL, then explore joins, indexes, transactions and reliable recovery.
10 published guides · Free access · Examples & explained answers
10 guides to explore
Relational tables and keys: give each record a clear identity
A relational design separates concepts into tables and connects them through keys. Begin by deciding what one row represents before choosing column names or writing queries.
Read guideSQL SELECT and WHERE: ask for the rows and columns you need
A SELECT query describes a result. Learn to separate column selection, row filtering and ordering so a query returns a deliberate answer instead of a coincidental-looking sample.
Read guideSQL joins: combine relationships without losing unmatched rows
A join combines rows according to a condition. The important decision is whether your result should contain only matches or should also preserve rows without a match.
Read guideSQL GROUP BY and aggregates: measure the right unit
An aggregate summarises several rows, but a useful summary starts with a clear denominator. Decide whether you are counting records, non-missing values, people or distinct events.
Read guideDatabase normalization: store each fact in the right place
Normalization helps avoid contradictory copies of the same fact. Begin with the dependencies in your data, then separate entities and relationships so updates have a clear home.
Read guideDatabase indexes: inspect the query before adding an index
An index can help a database find rows without scanning all of them, but it also has storage and write costs. Choose one to support a real query and verify its effect with a query plan and representative data.
Read guideDatabase transactions: keep a multi-step change consistent
A transaction groups related operations into a unit that can commit or roll back. Understanding that boundary is essential when partial updates or simultaneous requests could leave conflicting state.
Read guideDatabase constraints and NULL: make valid data a stored rule
An application form is not the only way data reaches a database. Constraints provide another line of defence by expressing required values, uniqueness, references and allowed conditions where records are stored.
Read guideParameterized queries: keep user values out of SQL structure
A query should describe its structure separately from the values supplied by a visitor. Parameter binding lets a database driver treat input as data rather than as part of a command.
Read guideDatabase backups: prove recovery before you need it
A backup is useful only if it can support a recovery you understand. Define what must be recovered, protect the stored copy and test restoration in an isolated environment.
Read guide