Database Management System Questions and Answers

Answer: A

Correct answer: Total participation

Double lines from an entity to a relationship mean total participation: every entity in the set must take part in the relationship. A single line means participation is partial.

Answer: A

Correct answer: Both undo and redo

With immediate modification a transaction's writes reach the database before it commits, so an uncommitted transaction's changes must be undone after a crash, and a committed transaction's changes that had not yet reached disk must be redone. Deferred modification needs redo only, since nothing is written before commit.

Answer: B

Correct answer: Select * from R cross join S.

Listing two tables in the FROM clause with a comma and no join condition produces every combination of their rows - the Cartesian product, which is what CROSS JOIN means. A natural join would match on the common columns, which is a different result.

Answer: B

Correct answer: Update anomaly.

Lost updates, unrepeatable reads and dirty reads all arise from transactions running at the same time. An update anomaly is a consequence of poor normalisation - the same fact stored twice - and would occur even with one user.

Answer: D

Correct answer: All of the above.

Selection can be pushed through any of the three set operations, since deciding whether a tuple satisfies a condition does not depend on which relation it came from:

\[ \sigma_\theta(R \cup S) = \sigma_\theta(R) \cup \sigma_\theta(S) \]

and the same holds for intersection and difference. Doing the selection first is what makes such transformations worth applying.

Answer: C

Correct answer: DDL interpreter

Metadata - the description of tables, columns and constraints - is produced when a schema is defined, so it is the DDL processor that writes it into the data dictionary. DML compilers and query interpreters read that metadata but do not create it.

Answer: B

Correct answer: weak relationship sets

A weak entity is identified by its owner, so the relationship connecting the two carries no information beyond what the weak entity's own table already holds through the borrowed key. Giving that relationship a table of its own would therefore be redundant.

Answer: C

Correct answer: all of the above

Natural describes how the join columns are chosen - automatically, from the columns the two tables share - and that is independent of whether the join is inner, left, right or full. So it can be combined with any of them.

Answer: D

Correct answer: serializable

The SQL standard's default isolation level is serializable, the strictest, in which concurrent execution must be equivalent to running the transactions one after another. Individual products often default to something weaker for performance, but the standard's default is this.

Answer: C

Correct answer: both read and write Q

An exclusive lock gives the holder complete access - it may both read and write the item - and excludes every other transaction. A shared lock permits reading only, but allows other readers at the same time.