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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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What is SQLite? What are the key features of SQLite? How is SQLite different from client-server databases like MySQL or PostgreSQL? What is the file format of a SQLite database? What are the supported data types in SQLite? What is dynamic typing (type affinity) in SQLite? How do you create a table in SQLite? How do you insert data into a SQLite table? How do you query data from a SQLite table? How do you update a record in SQLite? How do you delete a record in SQLite? What is a PRIMARY KEY in SQLite? What is AUTOINCREMENT in SQLite? What is a UNIQUE constraint in SQLite? What is a FOREIGN KEY in SQLite and how do you enable it? What is the sqlite3 command-line shell used for? How do you create an index in SQLite? What is a NULL value in SQLite? What is the difference between CHAR, VARCHAR, and TEXT in SQLite? What is the ROWID in a SQLite table? What is an in-memory SQLite database? How do you back up a SQLite database? What is the PRAGMA statement used for? What is a VIEW in SQLite? What is a TRIGGER in SQLite? What is the difference between DELETE, TRUNCATE, and DROP in SQLite? What is a transaction in SQLite and how do you use BEGIN/COMMIT/ROLLBACK? What is the difference between WAL mode and rollback journal mode? Why does SQLite recommend enabling foreign keys explicitly, given they're off by default? What is the difference between INTEGER PRIMARY KEY and a regular PRIMARY KEY in SQLite? How does SQLite handle concurrent writes? What is the difference between a SQLite VIEW and a TABLE? How do you perform a JOIN in SQLite? What is an UPSERT in SQLite (INSERT... ON CONFLICT)? How do you use the EXPLAIN QUERY PLAN statement? What is a composite primary key in SQLite? How does SQLite handle database locking? What is the difference between VACUUM and ANALYZE in SQLite? Explain the internal working of SQLite's B-tree storage structure? How does WAL mode improve concurrency compared to the default rollback journal? Explain the lifecycle of a SQLite transaction from BEGIN to COMMIT? Why might full-text search (FTS5) be needed instead of a LIKE query? How do you optimize a slow SQLite query? What are the limitations of SQLite for large-scale, high-concurrency applications? How does SQLite ensure ACID compliance without a separate server process? Explain how SQLite's write-ahead logging (WAL) checkpoint process works? What isolation level does SQLite provide, compared to other RDBMS? What is the STRICT keyword used for in SQLite table definitions?
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