1. What is a primary key for?
- Storing every field twice
- Uniquely identifying a record
- Encrypting a table automatically
Check the answer and reasoning
Uniquely identifying a record
Uniqueness allows a record to be referenced unambiguously.
CS / IGCSE
Databases, Boolean logic and queries
A relational table contains records and fields. A primary key identifies records; queries select and organise data using conditions rather than changing its meaning.
Learning objectives
- Explain databases, boolean logic and queries using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
A relational table contains records and fields. A primary key identifies records; queries select and organise data using conditions rather than changing its meaning.
Logic gates implement Boolean operations. Construct a truth table by enumerating every input combination; AND requires both inputs, OR requires at least one and NOT reverses a truth value.
Equations and relationships
NOT(A AND B) = (NOT A) OR (NOT B)
One of De Morgan’s laws.
Worked example
What is 1 XOR 1?
Follow the solution
Exclusive OR is true only when the two input bits differ.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Use the prediction activity below; a local model appears only where it supports this topic.
Check your understanding
2. What is 1 XOR 1?
- 1
- 2
- 0
Check the answer and reasoning
0
Exclusive OR is true only when the two input bits differ.
3. Does SQL WHERE change the stored rows in a SELECT query?
- No
- Yes
- Only for numbers
Check the answer and reasoning
No
WHERE filters returned results; SELECT does not itself modify the source table.