Why can a primary key not be NULL?

NULL means unknown, so a key that could be empty could not reliably identify the row.

How does WHERE differ from HAVING?

WHERE filters individual rows before grouping, while HAVING filters groups after grouping.

S6 ICT Databases and SQL, Paper 1: SELECT, WHERE and aggregates

2026-11-02

Paper 1 uses a four-row Student table to cover WHERE filtering, primary key rules, COUNT returning 4 and ORDER BY DESC, then two rows above 80, an average of 80.75, a sum of 323, a range of 27 and Lee first after sorting.

Reminders　WHERE filters before ORDER BY sorts, COUNT(*) counts rows, and averages need care with decimals.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S6 ICT: Databases and SQL
Level	S6

02　Questions (10 items)

FoundationWhich SQL clause is used to filter rows that meet a condition?
FoundationWhich two conditions must a primary key satisfy?
FoundationTable Student holds 4 rows. What does `SELECT COUNT(*) FROM Student;` return?
FoundationHow do you sort rows by Score from highest to lowest?
IntermediateScores are Chan 78, Lee 92, Wong 65 and Ho 88. How many rows does `WHERE Score > 80` return?
IntermediateFor the same scores, what does `SELECT AVG(Score) FROM Student;` return?
IntermediateFor the same scores, what does `SELECT SUM(Score) FROM Student;` return?
ChallengeFor the same scores, what does `SELECT MAX(Score) – MIN(Score) FROM Student;` return?
ChallengeWhich name appears first in `SELECT Name FROM Student WHERE Score >= 80 ORDER BY Score DESC;`?
Error redoError repair: a student wrote `SELECT * FROM Student ORDER BY Score WHERE Score > 80`. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	WHERE	WHERE tests each row, and only the rows that pass the test appear in the result.
2	Unique for every row and never empty	A primary key identifies each row, so its value must not repeat and must not be left empty.
3	4	COUNT(*) counts rows, so four rows return 4 regardless of the column contents.
4	ORDER BY Score DESC	ORDER BY sorts the rows and DESC means largest first, while ASC is smallest first.
5	2 rows	Only Lee at 92 and Ho at 88 are above 80, while Chan at 78 and Wong at 65 fail the test.
6	80.75	The four scores add to 323 and dividing by 4 gives 80.75.
7	323	SUM adds the four scores, and 78 plus 92 plus 65 plus 88 is 323.
8	27	The highest is 92 and the lowest 65, so the difference, the range, is 27.
9	Lee	Only Lee at 92 and Ho at 88 qualify, and sorted high to low Lee comes first.
10	The clause order is wrong; WHERE must come before ORDER BY	SQL filters with WHERE before it sorts with ORDER BY, so swapping them is a syntax error.

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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