Database Concepts
What a database is
A database is an organised collection of related data stored so that it can be retrieved, updated and managed efficiently. A database management system (DBMS) is the software that sits between the user and the stored data, handling storage, security, concurrency and backup. A relational database stores data as tables (relations) made of rows (records) and columns (fields). Avoid confusing the data itself with the software that manages it: the file on disk is not the DBMS.
Key concepts to know
Understand the difference between data and information: data are raw values, information is data processed into meaning. Know the terms table, record, field, primary key, foreign key, and data type. A primary key uniquely identifies each row and cannot be null or duplicated; a foreign key is a field in one table that references a primary key in another, enforcing referential integrity. Know what a data dictionary or schema describes (structure, not content). Understand validation versus verification, and the purpose of backup and access rights.
Traps
Questions often ask whether a table can have more than one primary key (no, but it may have a composite key of several fields). Another trap is confusing a foreign key with a duplicate of the primary key's values across tables: the foreign key need not be unique. Watch for questions that treat a spreadsheet as a full DBMS.
Rule of thumb
If a value must be unique per row and never blank, it is a primary key candidate; if it points to another table's row, it is a foreign key.