Database Notes

Why Concurrency Control Is Needed

A plain-language look at why databases need concurrency control, the four classic anomalies it prevents, and how isolation levels trade strictness for...

DBMS

File System versus DBMS

The file based approach fails on redundancy, inconsistency, isolation, atomicity, concurrency, security and integrity. Each failure maps to one specif...

DBMS

What a DBMS Is and Why It Exists

A DBMS is the software layer between users and stored data. It exists to give many users controlled, concurrent, recoverable and consistent access to...

DBMS

Data, Information and Databases

Data is raw recorded fact, information is data placed in context, and a database is an organised, shared, self describing collection of related data...

SQL

PostgreSQL, MySQL, SQL Server, Oracle and SQLite Compared

One reference table for the differences that actually break code: paging, auto increment, strings, dates, upserts and the features each product lacks.

SQL

Users, Roles, GRANT, REVOKE and Least Privilege

Controlling who can do what: creating users, granting the minimum needed, grouping privileges into roles, and the accounts a typical application actua...

SQL

SQL Injection and Parameterised Queries

Why string concatenation lets user input rewrite your query, and why prepared statements make it structurally impossible - with the cases parameters c...

SQL

ACID Properties Explained

Atomicity, consistency, isolation and durability - what each guarantee actually promises, how the database delivers it, and where each one can be weak...

SQL

BEGIN, COMMIT, ROLLBACK and Savepoints

Grouping statements into one unit of work that either fully applies or fully undoes. Learn transaction boundaries, savepoints, autocommit and what sil...

SQL

First, Second and Third Normal Form

Normalisation worked through on one table: remove repeating groups, then partial dependencies, then transitive ones - with the SQL at each step.

SQL

What NULL Really Means

NULL is not zero, not an empty string and not equal to itself. Learn what it represents, how it spreads through expressions and how to handle it delib...

SQL

What is SQL: Databases, DBMS and RDBMS

SQL is the declarative language used to define and query relational data. Start here to see what a database, a DBMS and an RDBMS actually are.