P-6.1 The Relational Model
You can describe data as tables with keys and relationships, explain what a primary and a foreign key enforce, and say what a database gives you that a file does not.
Before any query syntax, the model itself is worth understanding: data as tables, rows identified by keys, and relationships expressed by referring to those keys. What makes it powerful is that the database enforces the rules rather than the application, so a defect in one program cannot leave the data in a state no program expects. That guarantee is the whole reason to use a database rather than a folder of files.
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Tables, rows, columns and types
A table is a set of rows with the same named, typed columns, and the type is enforced on write. That is already more than most file formats promise.
NPTEL: Data Base Management System · CoursePrimary keys: what makes a row identifiable
A primary key is the column or columns that uniquely identify a row, and the database refuses duplicates and missing values. Choosing it well is the first design decision you make.
Foreign keys: relationships the database itself enforces
Referring to a key in another table lets the database refuse rows that point at nothing. This is what stops orphaned records accumulating quietly for years.
One-to-many and many-to-many, and the join table
One side holding the other's key handles one-to-many; a many-to-many relationship needs a table of its own holding both keys. Recognising which one you have prevents most schema mistakes.
What a database gives you that a file does not
Concurrent access, enforced constraints, recovery after a crash and a query language you did not have to write. Each of those is months of work to reproduce badly.
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