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EXAM TRAPS

5 DBMS traps that cost marks

Each one looks right in the exam hall. Each one is wrong. Swipe to check all five.
TRAP 1 / 5

Does marks = NULL find rows where marks is unknown?

NO
NULL is not a value, so comparing anything with = is never true. Write marks IS NULL instead.
TRAP 2 / 5

Does a NATURAL JOIN follow your foreign keys?

NO
It matches only same-named columns. If no names match, you silently get every row pair. Prefer an explicit JOIN ... ON.
TRAP 3 / 5

Can WHERE filter groups after GROUP BY?

NO
WHERE filters rows before grouping. To filter the grouped result - for example departments with COUNT(*) > 5 - use HAVING.
TRAP 4 / 5

Is a non-key column depending on another non-key column a 2NF violation?

NO
That is a 3NF (transitive dependency) problem. 2NF fails only when a non-key column depends on part of a composite key.
TRAP 5 / 5

Does UNION keep duplicate rows?

NO
UNION removes duplicates; only UNION ALL keeps them. If an exam answer needs every row, the keyword is UNION ALL.
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