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Databases

SQL fluency plus the theory that explains why SQL exists.

3,570+ MCQs17 topicsWeightage: 10%3 difficulty levels
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Overview

Databases is consistently one of the highest-scoring NSCT sections for students who prepared properly and one of the lowest for those who did not. Expect SQL questions on joins, GROUP BY with HAVING, subqueries, window functions at a basic level, and the normalization ladder up to BCNF. ER diagrams, ACID properties, indexing trade-offs, and NoSQL vocabulary round out the section. The NSCT favours questions that test your understanding of why a query returns what it does, not just whether it parses.

Why This Subject Matters

Every production application has a database behind it, and debugging a slow query or a locked row is a daily task for most software engineers. Strong NSCT database scores correlate tightly with ability to pass backend and full-stack interviews.

Topics in Databases

Theory and modelling

The mental model: relations, tuples, attributes, ER diagrams, and the algebra that underlies SQL.

Introduction to Database SystemsDatabase ArchitectureData ModelsRelational Database ConceptsRelational Algebra & Calculus

SQL and design

Writing, reading, and debugging SQL; plus schema design via normalization forms.

SQLAdvanced SQLDatabase Design & Normalization

Transactions, indexing, and modern databases

ACID, isolation levels, indexing strategies, query planners, and the vocabulary of NoSQL, data warehouses, and distributed databases.

Transaction ManagementConcurrency ControlRecovery ManagementIndexing & File OrganizationQuery Processing & OptimizationDatabase SecurityDistributed DatabasesNoSQL & Modern DatabasesData Warehousing & Data Mining

How to Study This Subject

Write SQL for an hour a day for two weeks on sample datasets. Focus on joins (especially left and full outer), aggregation with GROUP BY, and subqueries (correlated vs uncorrelated). Separately, memorize the normalization forms — 1NF removes repeating groups, 2NF removes partial dependencies, 3NF removes transitive dependencies, BCNF tightens 3NF. This map is worth 4–5 guaranteed marks.

Suggested time budget

8–10 hours. Half on SQL drills, a quarter on normalization, a quarter on transactions and indexing.

Common Mistakes to Avoid

  • 1Confusing WHERE and HAVING. WHERE filters rows before aggregation; HAVING filters groups after.
  • 2Mixing up INNER, LEFT, RIGHT, and FULL OUTER joins under time pressure.
  • 3Assuming indexing always speeds up queries. Writes, inserts, and updates pay a cost — and the question will exploit that.
  • 4Treating ACID as an acronym without understanding what each letter means in practice (isolation levels are a favourite NSCT trap).

Sample Questions

Two example MCQs from the Databases question bank, with full explanations. The live quiz draws from 3,570+ verified questions across three difficulty levels.

Database Design & NormalizationMedium

Q1. A table has columns (A, B, C). If A → B and B → C, which normalization form removes this dependency chain?

  1. A.1NF
  2. B.2NF
  3. C.3NF✓ Correct
  4. D.BCNF

Explanation

3NF specifically targets transitive dependencies — situations where a non-key column (B) determines another non-key column (C). Splitting the table so that C lives with its determinant B, separate from A, satisfies 3NF. 2NF removes partial dependencies on composite keys; BCNF tightens 3NF further for edge cases.

SQLEasy

Q2. Which SQL clause filters GROUPS after aggregation?

  1. A.WHERE
  2. B.HAVING✓ Correct
  3. C.FILTER
  4. D.GROUP BY

Explanation

HAVING is the post-aggregation filter. WHERE filters rows before they enter the aggregation. GROUP BY defines the grouping but does not filter. FILTER is a PostgreSQL extension used inside aggregate functions, not a top-level filtering clause.

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