A key is a column, or a combination of columns, whose values let you identify rows in a table or connect rows across tables. Without keys, a relational database is just a collection of lists: you cannot reliably say which row you mean, you cannot stop duplicates from creeping in, and you cannot tell which order belongs to which customer. Keys are what turn separate tables into a connected, trustworthy model of your data.
In this module you will work with several kinds of keys — primary, foreign, candidate, composite, and surrogate — and you will declare them in SQL as constraints using CREATE TABLE and ALTER TABLE. This first topic lays the groundwork: what a key is, why keys matter for organizing and identifying data, how they protect data integrity, and how the database engine enforces them automatically once you declare them.
As you watch the videos below, keep three questions in mind. First, what makes a set of column values suitable as an identifier (uniqueness, non-null, stability)? Second, how does a foreign key in one table point back to a key in another, and what happens when the referenced row changes or is deleted? Third, notice the difference between a key as a design idea and a key as a constraint you actually write in SQL — that distinction carries through the rest of the module.
An introductory overview of the relational model: tables made of rows and columns, how related data is split across multiple tables, and how those tables are linked together rather than duplicated. Based on its title and description, it should also touch on how SQL is used to query data across related tables and why this structure is useful compared with other data stores.
It sets the context for why keys exist at all — you cannot appreciate identifiers and relationships without first understanding the table-based model, and IBM Technology's explainers are short, accurate, and whiteboard-clear.
The second part of Lucid Software's ERD tutorial series, focused specifically on primary keys, foreign keys, and bridge (junction) tables. Expect to see how an entity gets a unique identifier, how a foreign key in one entity references the primary key of another, and how many-to-many relationships are resolved with an extra table that often uses a composite key.
It connects key concepts directly to data modeling diagrams, which previews the primary, foreign, and composite key types introduced in this topic and used throughout the module.
A short, focused explanation of what a foreign key constraint is, how it differs from a primary key, and why the constraint matters for keeping references valid between tables. The video is produced by a database vendor, so expect practical framing around what the database enforces when rows are inserted, changed, or removed.
It makes the shift from key-as-concept to key-as-constraint explicit, which is exactly the final subtopic here and the bridge to the SQL syntax and referential actions covered later in the module.
Together these videos build the vocabulary and mental model the module's outcomes depend on. The relational database overview grounds the idea that data lives in linked tables; the ERD tutorial introduces primary, foreign, and composite keys as design choices, supporting MO1 and giving you a way to reason about candidate and surrogate key decisions in MO3 and MO4; and the foreign key constraint video frames keys as rules the database enforces, previewing the constraint-writing work of MO2 and the ON DELETE/ON UPDATE referential actions of MO5. None of the videos replaces hands-on SQL practice, so treat them as conceptual preparation for the syntax-focused topics that follow.
These videos were chosen from YouTube on October 2, 2026. Each has captions; the player turns them on. Videos on YouTube can be changed or removed — if one no longer plays, tell your instructor. If a player does not appear, use its Watch on YouTube link.