Final answer:
Tables are joined when the primary key of one table matches the foreign key of another, ensuring referential integrity of the database.
Step-by-step explanation:
Tables are usually joined when the primary key from one table is equal to the foreign key from the other table. In relational database design, a primary key is a column or a set of columns that uniquely identifies each row in a table. A foreign key is a column or set of columns in one table that refers to the primary key in another table. The purpose of the foreign key is to ensure referential integrity of the data.
In other words, only values that are supposed to appear in the database are permitted. For example, in a database with tables for students and classes, a student's unique ID might be the primary key in the student table, and that same ID could be used as a foreign key in the classes table to indicate which students are enrolled in which classes.