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Database / SQLite Interview questions

What is a composite primary key in SQLite?

A composite (or compound) primary key spans more than one column, with uniqueness enforced across the combination of those columns rather than any single one alone — useful for tables representing a many-to-many relationship, where the natural unique identifier really is a pair (or more) of values together.

CREATE TABLE enrollment (
    student_id INTEGER,
    course_id INTEGER,
    PRIMARY KEY (student_id, course_id)
);
-- the same student_id can appear multiple times (for different courses),
-- and the same course_id can appear multiple times (for different students),
-- but the (student_id, course_id) pair as a whole must be unique

Unlike a single-column INTEGER PRIMARY KEY, a composite primary key doesn't get the rowid-alias performance shortcut — lookups against it go through a normal B-tree index traversal on the combined key, still efficient, but not the absolute fastest single-integer lookup path.

What does a composite primary key enforce uniqueness on?
Does a composite primary key get the same rowid-alias performance shortcut as a single INTEGER PRIMARY KEY?

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