Most Asked Database Topics

31 topics

• 1

  Joins (types and queries)

  SQLJoinsMedium

  Explain the different types of SQL joins (inner, left, right, full outer, self join) and write queries that combine data from two or more tables based on related columns.

  Sample query

  Asked in 34 companies

  

  

  

  

  

  +29
• 2

  Indexing (concept and query)

  PerformanceIndexesMedium

  What a database index is, how it speeds up data retrieval, and how to decide which column(s) to index.

  Sample query

  Asked in 30 companies

  

  

  

  

  

  +25
• 3

  Normalization / Normal Forms

  DesignNormalizationMedium

  The process of organizing database tables to reduce redundancy and dependency, explained through the normal forms (1NF, 2NF, 3NF).

  Asked in 23 companies

  

  

  

  

  

  +18
• 4

  GROUP BY / HAVING / WHERE clause

  SQLAggregationMedium

  How to filter and group query results — WHERE filters individual rows, GROUP BY groups rows sharing a value, and HAVING filters those grouped results.

  Sample query

  Asked in 18 companies

  

  

  

  

  

  +13
• 5

  Stored Procedures

  SQLProceduresMedium

  Precompiled SQL code stored in the database that can be executed repeatedly — why they're used and how they differ from plain queries in terms of efficiency.

  Sample query

  Asked in 14 companies

  

  

  

  

  

  +9
• 6

  Database schema/ERD design

  DesignERDMedium

  Designing the structure of a database — drawing an Entity-Relationship Diagram (ERD) to represent entities, attributes, and relationships, then translating it into actual tables.

  Asked in 13 companies

  

  

  

  

  

  +8
• 7

  Triggers

  SQLTriggersMedium

  Database objects that automatically execute a specified action in response to certain events (insert, update, delete) on a table.

  Sample query

  Asked in 12 companies

  

  

  

  

  

  +7
• 8

  Primary Key vs Foreign Key vs Unique Key

  KeysConstraintsEasy

  The different types of keys used to uniquely identify records and maintain relationships between tables.

  Sample query

  Asked in 12 companies

  

  

  

  

  

  +7
• 9

  Views

  SQLViewsEasy

  Virtual tables created from the result of a stored query, used to simplify complex queries or restrict data access.

  Sample query

  Asked in 11 companies

  

  

  

  

  

  +6
• 10

  Nth highest/lowest salary query

  SQLQueriesMedium

  A classic practical SQL task: write a query to find the 2nd, 3rd, or Nth highest (or lowest) salary from an employee/salary table.

  Sample query

  Asked in 10 companies

  

  

  

  

  

  +5
• 11

  ACID properties / Transactions

  TransactionsMedium

  The four properties (Atomicity, Consistency, Isolation, Durability) that guarantee reliable processing of database transactions.

  Asked in 9 companies

  

  

  

  

  

  +4
• 12

  Many-to-many relationship

  DesignRelationshipsMedium

  How to represent and implement a relationship where multiple records in one table relate to multiple records in another, typically using a junction/bridge table.

  Sample query

  Asked in 8 companies

  

  

  

  

  

  +3
• 13

  Denormalization

  DesignPerformanceMedium

  The process of intentionally introducing redundancy into a normalized database to improve read performance.

  Asked in 7 companies

  

  

  

  

  

  +2
• 14

  Subqueries / Nested queries

  SQLQueriesMedium

  A query written inside another query, used to filter or compute values needed by the outer query.

  Sample query

  Asked in 5 companies

  

  

  

  

  
• 15

  SQL vs NoSQL

  ConceptsEasy

  Comparing relational (SQL) databases with structured schemas against non-relational (NoSQL) databases with flexible schemas, and when to use each.

  Asked in 4 companies

  

  

  

  +1
• 16

  Constraints (CHECK, UNIQUE, NOT NULL)

  ConstraintsEasy

  Rules applied to table columns to enforce data integrity, such as preventing null values or duplicate entries.

  Sample query

  Asked in 5 companies

  

  

  

  

  
• 17

  Database locking / concurrency control

  ConcurrencyHard

  Mechanisms used to manage simultaneous access to data by multiple transactions, preventing conflicts or data corruption.

  Asked in 5 companies

  

  

  

  

  +1
• 18

  Query optimization / performance

  PerformanceHard

  Techniques for making a query run faster, especially on large datasets with many joins, subqueries, or indexes.

  Asked in 4 companies

  

  

  

  +1
• 19

  Aggregate functions (COUNT, SUM, AVG etc.)

  SQLAggregationEasy

  Built-in SQL functions that perform a calculation on a set of values and return a single summary value.

  Sample query

  Asked in 4 companies

  

  

  

  
• 20

  Bridge/junction table

  DesignRelationshipsMedium

  An intermediary table used to implement a many-to-many relationship between two other tables.

  Sample query

  Asked in 4 companies

  

  

  

  
• 21

  DROP vs DELETE vs TRUNCATE

  SQLEasy

  The differences between these three commands for removing data or structures — DROP removes the whole table, DELETE removes specific rows, and TRUNCATE removes all rows quickly without logging individually.

  Sample query

  Asked in 3 companies

  

  

  
• 22

  Self join

  SQLJoinsMedium

  A join where a table is joined with itself, typically used to compare rows within the same table.

  Sample query

  Asked in 3 companies

  

  

  
• 23

  SQL Injection

  SecurityMedium

  A security vulnerability where malicious SQL code is inserted into a query through unsanitized user input.

  Sample query

  Asked in 3 companies

  

  +2
• 24

  DDL vs DML vs DCL

  ConceptsEasy

  The three categories of SQL commands — Data Definition Language (structure), Data Manipulation Language (data), and Data Control Language (permissions).

  Asked in 3 companies

  

  

  
• 25

  One-to-one / one-to-many relationship

  DesignRelationshipsEasy

  How to model and implement relationships where one record relates to exactly one, or to multiple, records in another table.

  Sample query

  Asked in 3 companies

  

  

  +1
• 26

  Composite key

  KeysEasy

  A primary key made up of two or more columns combined to uniquely identify a row.

  Sample query

  Asked in 2 companies

  

  
• 27

  Database anomalies

  DesignNormalizationMedium

  Inconsistencies that can occur in poorly designed databases during insert, update, or delete operations — a key motivation for normalization.

  Asked in 2 companies

  

  
• 28

  Cascading delete/update

  ConstraintsMedium

  A foreign key behavior where deleting or updating a record automatically applies the same action to related records in other tables.

  Sample query

  Asked in 2 companies

  

  
• 29

  Referential integrity

  ConstraintsEasy

  A rule ensuring that foreign key values always correspond to an existing primary key in the related table.

  Asked in 2 companies

  

  
• 30

  OLTP vs OLAP

  ConceptsMedium

  Comparing Online Transaction Processing systems (fast, frequent small transactions) with Online Analytical Processing systems (complex analytical queries on large datasets).

  Asked in 1 companies

  
• 31

  Database backup/recovery

  OperationsEasy

  Strategies and methods for backing up a database and restoring it in case of data loss or failure.

  Asked in 1 companies