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SQL

Joins, indexes, transactions, query optimization.

22 questions
JoinsIndexesTransactionsNormalizationWindow FunctionsOptimization

Sample Questions (22 live)

JoinsEasy

Explain INNER JOIN vs LEFT JOIN.

InfosysTCSDeloitteRead →
IndexesMedium

What is a database index and what is the trade-off?

AmazonDeloitteRead →
OptimizationEasy

WHERE vs HAVING — what's the difference, and which is faster?

InfosysTCSCognizantRead →
TransactionsEasy

DELETE vs TRUNCATE vs DROP — how do they differ, and which is transactional?

TCSInfosysWiproRead →
OptimizationEasy

UNION vs UNION ALL — why does the wrong one quietly slow your query?

InfosysCognizantAccentureRead →
NormalizationEasy

Primary key vs unique key vs foreign key — and how do they relate to indexes?

TCSInfosysDeloitteRead →
OptimizationMedium

How do you read an execution plan to find why a query is slow?

AmazonMicrosoftDeloitteRead →
IndexesMedium

How does a composite index work, and what is the leftmost-prefix rule?

AmazonMicrosoftGoogleRead →
IndexesMedium

What is a covering index, and how does it eliminate table lookups?

AmazonMicrosoftDeloitteRead →
TransactionsMedium

Explain ACID and the four transaction isolation levels — which is the default?

AmazonMicrosoftDeloitteRead →
TransactionsMedium

Dirty read, non-repeatable read and phantom read — what are they and how do you prevent each?

AmazonMicrosoftbankingRead →
OptimizationMedium

What is the N+1 query problem, and how do you fix it in an ORM?

AmazonDeloitteCognizantRead →
IndexesMedium

Clustered vs non-clustered index — how do they physically differ?

AmazonMicrosoftOracle-shopsRead →
Window FunctionsMedium

What problems do window functions solve that GROUP BY can't?

AmazonGoogleMicrosoftRead →
OptimizationMedium

Why is OFFSET pagination slow at scale, and how does keyset (cursor) pagination fix it?

AmazonGoogleMicrosoftRead →
TransactionsMedium

Optimistic vs pessimistic locking — which do you use for concurrent updates?

AmazonMicrosoftbankingRead →
OptimizationHard

An index exists but the query still does a full scan — what are the likely reasons?

AmazonMicrosoftOracle-shopsRead →
TransactionsHard

What causes database deadlocks, and how do you prevent them in a busy OLTP system?

AmazonMicrosoftbankingRead →
OptimizationHard

Walk me through diagnosing a slow query in production.

AmazonMicrosoftGoogleRead →
NormalizationHard

When do you deliberately denormalize, and what are the trade-offs?

AmazonMicrosoftGoogleRead →
OptimizationHard

What causes database connection-pool exhaustion, and how do you fix it?

AmazonMicrosoftDeloitteRead →
OptimizationHard

How do you keep queries fast on a table with billions of rows?

AmazonGoogleMicrosoftRead →

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