Why can NULL not be compared with an equals sign?

NULL means unknown, so an equality test returns unknown; use IS NULL or IS NOT NULL.

How does a primary key differ from a candidate key?

Candidate keys are all column sets that can identify a row uniquely; the primary key is the one chosen from them.

DSE ICT | Relational Databases and SQL Practice Paper 1 (Basic Queries)

2026-11-04

Starting from one twelve-row student table, count rows and columns, then work through LIKE prefixes, sorting and the NULL comparison trap.

Attempt first, mark afterwards　Allow about thirty minutes; write down the expected row and column counts before typing any SQL.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 items
Suggested time	About 30 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: DSE ICT \| Relational Databases and SQL
Level	S5 (Secondary 5) ICT

02　Questions (10 items)

FoundationStudents has 12 rows and 5 columns. `SELECT name FROM Students WHERE form = 4;` returns 5 students. How many rows and columns come back?
FoundationWhat two conditions must a primary key satisfy, and can a primary key column hold NULL?
FoundationWhat is a foreign key for, and may its value repeat across rows?
FoundationA Books table has 30 rows and 6 columns. How many data values does `SELECT * FROM Books;` return?
IntermediateWith the same data (form 4: S01 72, S02 85, S03 64, S04 91, S05 58), which rows does `SELECT name FROM Students WHERE form = 4 AND score >= 85;` return?
IntermediateStudent ids run from S01 to S12. How many rows does `WHERE id LIKE ‘S0%’;` return, and why is S10 excluded?
IntermediateThe twelve students total 902 marks. What are the two results of `SELECT COUNT(*), AVG(score) FROM Students;`?
ChallengeScores are 72, 85, 64, 91, 58, 77, 83, 69, 95, 88, 54 and 66. What are the three scores from `ORDER BY score DESC LIMIT 3;`, and what is the third highest?
ChallengeHow many rows does `SELECT DISTINCT form FROM Students;` return for forms 4, 5 and 6, and how many without DISTINCT?
Error redoError drill: a student writes `WHERE score = NULL;` to find students with no score. What is the correct form and why?

03　Answers and one-line notes

#	Answer	One-line note
1	5 rows and 1 column	WHERE filters rows (tuples); naming only name in SELECT keeps a single attribute, so the column count is one.
2	It must be unique and not null; NULL is not allowed	A primary key identifies each row, so a duplicate or a NULL would make row identity undecidable.
3	It references the primary key of another table to create a relationship; values may repeat, and may be NULL where allowed	A foreign key only enforces that the referenced row exists; repetition is normal, since many students can sit in one class.
4	180	Thirty rows times six columns gives 180 cells; SELECT * removes neither rows nor columns.
5	S02 (85) and S04 (91), two rows	AND requires both conditions, so only 85 and 91 pass the threshold.
6	Nine rows (S01 to S09); S10, S11 and S12 have 1 as the second character, so they do not match the prefix S0	The percent sign stands for any string, but S0 is a literal prefix that must match character by character.
7	COUNT(*) = 12 and AVG = 75.17, since 902 divided by 12 is 75.166…	AVG skips NULLs, but there are none here, so the total is simply divided by the row count.
8	95, 91 and 88; the third highest is 88	DESC sorts largest first and LIMIT 3 keeps three rows; 88 sits only three marks above 85, so the ordering must be exact.
9	Three rows with DISTINCT, twelve rows without it, one per student	DISTINCT removes duplicate values; without it every row is emitted, duplicates included.
10	Write `WHERE score IS NULL;`; NULL is not a value, so equality always evaluates to unknown and returns nothing	NULL means unknown, so any equality test against it fails; IS NULL and IS NOT NULL are the only correct tests.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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