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SQL and Relational Databases interview questions
Answer aloud before opening each explanation. Focus on the reasoning, not memorized wording.
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01What is a primary key?FoundationsBeginner
Model answer
A column or column set that uniquely identifies each row.
Why
Stable identity supports references, updates, and relationships.
02What is the logical role of WHERE?QueriesBeginner
Model answer
It filters rows before later result operations.
Why
WHERE limits which source rows participate in a query.
03How do INNER JOIN and LEFT JOIN differ?RelationshipsIntermediate
Model answer
INNER keeps matches; LEFT also keeps unmatched left rows.
Why
The required treatment of missing relationships determines the join.
04Why must selected non-aggregate columns usually appear in GROUP BY?AggregationIntermediate
Model answer
The database needs one defined value for each output group.
Why
Grouping determines the granularity of aggregate output.
05What does transaction atomicity mean?ReliabilityIntermediate
Model answer
All changes commit together or none do.
Why
Atomicity prevents partially completed business operations.
06Why not index every column?PerformanceAdvanced
Model answer
Indexes consume storage and slow writes and maintenance.
Why
Indexes should support measured query patterns.
07How do parameterized queries prevent injection?SecurityAdvanced
Model answer
They send values separately from SQL syntax.
Why
The database parses user values as data rather than executable structure.
08What problem does normalization address?DesignAdvanced
Model answer
Duplicated facts and update anomalies.
Why
Normalized relations keep each fact in a controlled place.