SQL Interview Questions and Answers in 2023
What Exactly is SQL?
At its core, SQL stands for Structured Query Language. This programming language plays an essential role in managing and working with relational databases, making it possible to organize and retrieve data effectively.
Unraveling the Database Mystery
A database is akin to an organized treasure trove of data that are electronically stored and accessed. This electronic organization facilitates seamless data storage, retrieval, and management of voluminous amounts of information.
The Significance of Primary Keys
A primary key is a column or a combination of columns that uniquely identifies each individual row within a table. In the realm of relational databases, primary keys enforce the entity integrity rule, adding a layer of data accuracy.
Understanding Foreign Keys
A foreign key is a column or a combination of columns that establishes a bridge between data in two separate tables. This linkage ensures referential integrity, maintaining the relationships between various tables.
CHAR vs. VARCHAR: Decoding the Difference
CHAR stores fixed-length character strings, while VARCHAR holds variable-length ones. The storage size of CHAR is constant, whereas VARCHAR dynamically adapts.
Embracing the Potential of Stored Procedures
Stored procedures are sets of SQL statements residing within databases. They facilitate code reusability and enhance performance by enabling repeated execution.
Unveiling Subqueries
Subqueries are nested queries embedded within others, leveraging the result of an inner query to retrieve data.
The Versatility of Views
Views serve as virtual tables generated from SQL statements’ outcomes. Users can retrieve and manipulate data as if dealing with conventional tables.
Cross Joins and Inner Joins: Differentiating Roles
Cross joins yield all possible combinations of rows from multiple tables. Inner joins, however, return solely matching rows based on specified join conditions.
Committing and Rolling Back: Transaction Management
The COMMIT statement solidifies permanent changes made during a transaction, while the ROLLBACK statement undoes such alterations.
Null Values: An Enigma to Solve
NULL denotes an absent or unknown value. It necessitates special consideration within SQL queries, distinct from zero or empty strings.
From Views to Materialized Views
Materialized views are tangible copies of view results stored in databases, periodically updated for performance enhancement, albeit at the expense of data freshness.
Correlated Subqueries: Unveiling the Complexity
Correlated subqueries refer to outer query columns, executing once per row processed in the outer query.
DISTINCT Keyword: A Unique Outlook
The DISTINCT keyword fetches distinct values from columns, ensuring a streamlined output free from duplicates.
CHAR vs. VARCHAR: Storage Saga
CHAR retains fixed-length character strings, while VARCHAR boasts variable-length ones. The former’s storage size remains constant, while the latter dynamically adapts.
IN vs. EXISTS: Making Comparisons
The IN operator scrutinizes values against a set or subquery’s result, while the EXISTS operator validates a subquery’s result’s existence.
TRIGGER: The Art of Automation
TRIGGER establishes a link between specific SQL statements and database events, automating action upon event occurrence.
Unique Constraint vs. Unique Index: Differentiating Roles
While a unique constraint ensures uniqueness across columns, a unique index enhances uniqueness and query performance.
Limiting Results: TOP or LIMIT Clause
The TOP clause (SQL Server) and LIMIT clause (MySQL) cap query result rows. Often paired with ORDER BY, these clauses facilitate result management.
UNION vs. JOIN: Merging Approaches
UNION vertically combines result sets, while JOIN horizontally unites columns based on join conditions.
Entering the World of Data Warehousing
A data warehouse functions as a centralized repository for data, meticulously curated from diverse sources for proficient reporting, analysis, and business intelligence endeavors.
Primary Keys vs. Candidate Keys
A primary key uniquely identifies a table’s row, whereas candidate keys potentially serve as primary keys.
Empowering through GRANT Statements
The GRANT statement empowers users and roles with specific database permissions or privileges.
Empowering through Correlated Updates
Correlated updates harness subquery results within the same table for row-based updates.
The Multifaceted CASE Statement
The CASE statement facilitates conditional logic in queries, allowing variable result returns based on specified conditions.
COALESCE: Nulls Beware
COALESCE, a powerful function, retrieves the first non-null expression from a list, serving as a remedy for null values.
Unveiling ROW_NUMBER()
ROW_NUMBER() assigns a unique, incremental number to each result set row, often employed for ranking or pagination.
A Natural Join and Inner Join: Making Comparisons
A natural join relies on matching columns with identical names, whereas an inner join follows specified join conditions.
The Commitment of COMMIT
The COMMIT statement ensures permanent change preservation, concluding a transaction and making alterations visible to other users.
COALESCE: A Null-Tolerant Solution
COALESCE adeptly retrieves the first non-null expression from a list, addressing the intricacies of null values.
The Impact of ROW_NUMBER()
ROW_NUMBER() assigns a unique, incremental value to each result set row, frequently employed for ranking or pagination.
Natural Joins and Inner Joins: A Comparative Study
Natural joins hinge on matching column names for row inclusion, while inner joins depend on specified join conditions.
Understanding the Commitment of COMMIT
The COMMIT statement assures the preservation of permanent changes, concluding transactions and rendering changes visible to other users.
ROLLBACK: Unraveling Its Essence
The ROLLBACK statement reverses transactional changes, restoring the database’s previous state before the transaction’s commencement.
Navigating NULL Values
NULL signifies the absence or uncertainty of a value, necessitating distinct treatment within SQL queries.
Grasping Views and Materialized Views
Views represent virtual tables derived from SQL statement outcomes, while materialized views offer tangible copies refreshed periodically for performance gains.
Unveiling Correlated Subqueries
Correlated subqueries rely on references to outer query columns, executing per outer query row.
Embracing the DISTINCT Keyword
The DISTINCT keyword extracts unique values from columns, ensuring an uncluttered output devoid of duplicates.
CHAR vs. VARCHAR: Storage Trade-offs
CHAR accommodates fixed-length character strings, while VARCHAR adapts to variable-length strings, adjusting storage dynamically.
Comparing IN and EXISTS Operators
The IN operator assesses values against a set or subquery result, whereas the EXISTS operator verifies the subquery result’s existence.
Unraveling ROLLBACK’s Purpose
The ROLLBACK statement undoes transaction changes, restoring the database’s prior state before the transaction’s initiation.
Grappling with NULL Values
NULL denotes the absence or uncertainty of a value, necessitating particular handling within SQL queries.
Views vs. Materialized Views: A Deep Dive
A view is a virtual table derived from SQL statements, whereas a materialized view is a tangible copy of view results stored in the database.
Embracing Correlated Subqueries
Correlated subqueries depend on outer query column references, executing per outer query row.
A Unique Perspective with the DISTINCT Keyword
The DISTINCT keyword retrieves distinct values from columns, ensuring a clean output devoid of duplicates.
CHAR vs. VARCHAR: The Storage Spectrum
CHAR contains fixed-length character strings, while VARCHAR accommodates variable-length strings, dynamically adjusting storage size.
IN vs. EXISTS: Unveiling the Contrast
The IN operator assesses values within a set or subquery result, while EXISTS ascertains subquery result existence.
TRIGGER: The Automation Enabler
TRIGGER binds SQL statements to database events, automating actions upon event occurrence.
Deciphering the Distinction: Primary Key vs. Unique Key
While a primary key’s purpose is to uniquely identify a row in a table by maintaining a distinct value, a unique key ensures that a specific column or a combination of columns bears a unique value, without necessarily identifying the entire row.
Diving into Normalization
Normalization is a strategic process aimed at optimizing database organization by minimizing redundancy and dependency. This technique involves dissecting a table into smaller fragments and establishing relationships between them.
Demystifying Normalization Levels
Different levels of normalization exist, each with its own distinct characteristics:
- First Normal Form (1NF)
- Second Normal Form (2NF)
- Third Normal Form (3NF)
- Boyce-Codd Normal Form (BCNF)
- Fourth Normal Form (4NF)
- Fifth Normal Form (5NF) or Project-Join Normal Form (PJNF)
Grasping the Concept of Joins
A join operation is a mechanism for combining rows from multiple tables based on related columns. This enables the simultaneous retrieval of data from various tables, enhancing data connectivity.
DELETE vs. TRUNCATE: A Closer Look
The DELETE statement is utilized to selectively remove specific rows based on conditions, enabling rollback and individual delete operations. On the other hand, TRUNCATE eradicates all rows within a table, irreversibly releasing data pages and excelling in efficiency.
The Union Dilemma: UNION vs. UNION ALL
UNION and UNION ALL play a pivotal role in amalgamating result sets from multiple SELECT statements. While UNION removes duplicate rows, UNION ALL includes all rows, including duplicates.
Navigating WHERE and HAVING Clauses
The WHERE clause filters rows based on pre-aggregation conditions, while the HAVING clause refines grouped rows post-aggregation. The latter operates in conjunction with the GROUP BY clause.
Unveiling the Realm of Transactions
Transactions involve sequences of SQL statements that function as a cohesive unit. They assure data consistency and integrity by committing all changes or reversing them in case of errors.
Clustered vs. Non-Clustered Indexes
Clustered indexes dictate the physical arrangement of data within a table, while non-clustered indexes remain detached, pointing to data locations. The former is singular, while the latter can be multiple within a table.
ACID: A Pillar of Transaction Assurance
ACID, embodying Atomicity, Consistency, Isolation, and Durability, constitutes a set of principles that guarantee steadfast database transaction processing.
Navigating Deadlocks
Deadlocks entail a scenario where transactions await each other to release resources, resulting in a standstill. Consequently, transactions cannot proceed, potentially rendering the system unresponsive.
Database and Schema: Distinguishing Features
A database functions as a container for tables, views, indexes, and more, grouping related data. Conversely, a schema serves as an organizational unit within a database, defining object ownership and offering efficient management.
Temporary Tables vs. Table Variables
Temporary tables are ephemeral entities that exist for a session or a transaction’s duration. In contrast, table variables store tabular structures in memory, automatically deallocating upon scope conclusion.
Harnessing the Power of GROUP BY
The GROUP BY clause groups rows based on specified columns, often used alongside aggregate functions like SUM, AVG, COUNT, and more, to perform computations on grouped data.
Unique Constraint vs. Unique Index: Discerning Roles
Unique constraints ensure column uniqueness, whereas unique indexes enhance both uniqueness and query performance.
Navigating Result Limits: TOP or LIMIT Clause
The TOP clause (SQL Server) and LIMIT clause (MySQL) limit result rows, often accompanying ORDER BY for efficient result control.
UNION vs. JOIN: A Fusion of Results
UNION vertically merges result sets, while JOIN horizontally combines columns through join conditions.
The World of Data Warehousing
A data warehouse centralizes diverse data sources, aiding efficient reporting, analysis, and business intelligence endeavors.
Decoding Primary Keys vs. Candidate Keys
Primary keys uniquely identify table rows, whereas candidate keys might eventually become primary keys.
Bestowing Authority with GRANT Statements
The GRANT statement confers specific database permissions or privileges upon users or roles.
Empowering through Correlated Updates
Correlated updates leverage subquery results within the same table for row-based updates.
The Many Facets of the CASE Statement
The CASE statement enables conditional logic within queries, delivering varied results based on predefined conditions.
COALESCE: Nulls Beware
COALESCE efficiently retrieves the first non-null expression from a list, addressing the intricacies of null values.
Unraveling ROW_NUMBER()
ROW_NUMBER() allocates a unique, incremental number to each result set row, often employed for ranking or pagination.
A Natural Join and Inner Join: A Comparative Analysis
A natural join utilizes matching column names for row selection, while an inner join depends on specified join conditions.
The Commitment of COMMIT
The COMMIT statement guarantees lasting change preservation, concluding a transaction and rendering changes visible to other users.
Unveiling ROLLBACK’s Purpose
The ROLLBACK statement undoes transactional changes, restoring the database’s previous state before the transaction initiation.
Wrangling NULL Values
NULL signifies absent or unknown values, necessitating specialized handling in SQL queries.
Views vs. Materialized Views: A Deep Dive
Views represent virtual tables derived from SQL statement outcomes, while materialized views store tangible copies of view results, updated at intervals.
Unearthing Correlated Subqueries
Correlated subqueries hinge on references to outer query columns, executed for each outer query row.
Embracing the DISTINCT Keyword’s Essence
The DISTINCT keyword retrieves distinct values from columns, ensuring a clean output devoid of duplicates.
CHAR vs. VARCHAR: The Storage Dilemma
CHAR accommodates fixed-length character strings, while VARCHAR adapts to variable-length strings, adjusting storage dynamically.
IN vs. EXISTS: A Contrast in Approach
The IN operator assesses values against a set or subquery result, whereas EXISTS verifies the existence of subquery results.
The Power of TRIGGER Statements
TRIGGER statements automate specified SQL actions upon database events, streamlining processes.
Differentiating Unique Constraints and Unique Indexes
Unique constraints ensure column distinctiveness, whereas unique indexes enhance both uniqueness and query performance.
Limiting Results with TOP or LIMIT Clauses
The TOP clause (SQL Server) and LIMIT clause (MySQL) restrict result row counts, often complemented by ORDER BY for effective result management.
UNION vs. JOIN: Merging Data Realms
UNION vertically merges result sets, while JOIN horizontally amalgamates columns based on specified join conditions.
Unveiling the Realm of Data Warehousing
A data warehouse serves as a centralized repository, aggregating diverse data sources for insightful reporting, analysis, and business intelligence applications.
Primary Keys vs. Candidate Keys: A Distinctive Perspective
Primary keys uniquely identify table rows, while candidate keys may potentially evolve into primary keys.
Bestowing Privileges with GRANT Statements
The GRANT statement confers specific database privileges upon users or roles, enabling fine-tuned access control.
Empowering via Correlated Updates
Correlated updates exploit subquery results within the same table for targeted row updates.
The Multifaceted CASE Statement
The CASE statement enables conditional logic within queries, facilitating diverse result outcomes based on specified conditions.
COALESCE: A Solution for Null Values
COALESCE efficiently retrieves the first non-null expression from a list, bridging the gap for handling null values.
The Role of ROW_NUMBER()
ROW_NUMBER() assigns a unique, incremental value to each result set row, commonly utilized for ranking or pagination purposes.
Natural Joins and Inner Joins: A Comparative Insight
A natural join leverages matching column names for row selection, whereas an inner join relies on specified join conditions.
Unveiling the Commitment of COMMIT
The COMMIT statement solidifies permanent changes, finalizing transactions and exposing changes to other users.
ROLLBACK: Unraveling Its Purpose
The ROLLBACK statement reverses transactional changes, reinstating the database’s prior state before the transaction’s commencement.
Navigating NULL Values
NULL signifies the absence or uncertainty of a value, necessitating distinct treatment within SQL queries.
Embracing the World of Views and Materialized Views
Views present virtual tables derived from SQL statement outcomes, while materialized views offer tangible copies, periodically refreshed for performance enhancement.
Decoding Correlated Subqueries
Correlated subqueries rely on references to outer query columns, executed per outer query row.
Unveiling the DISTINCT Keyword
The DISTINCT keyword extracts unique values from columns, ensuring an uncluttered output void of duplicates.
CHAR vs. VARCHAR: A Storage Dilemma
CHAR accommodates fixed-length character strings, while VARCHAR adapts to variable-length strings, adjusting storage size dynamically.
Comparing IN and EXISTS Operators
The IN operator assesses values against a set or subquery result, while the EXISTS operator validates subquery result existence.
The Potential of TRIGGER Statements
TRIGGER statements connect specified SQL actions with database events, automating processes.
The Difference between Unique Constraints and Unique Indexes
Unique constraints ensure column distinctiveness, while unique indexes enhance both distinctiveness and query performance.
LIMITing Results: The TOP and LIMIT Clauses
The TOP clause (SQL Server) and LIMIT clause (MySQL) curtail result row counts, often combined with ORDER BY for effective result management.
UNION vs. JOIN: The Merging Dynamic
UNION vertically merges result sets, while JOIN horizontally combines columns using defined join conditions.
Data Warehousing Unveiled
A data warehouse centralizes diverse data sources, facilitating effective reporting, analysis, and business intelligence applications.
Discerning Primary Keys from Candidate Keys
Primary keys uniquely identify table rows, while candidate keys harbor the potential to become primary keys.
Empowering via GRANT Statements
The GRANT statement endows users or roles with specific database privileges, fine-tuning access control.
Leveraging Correlated Updates
Correlated updates harness subquery results within the same table for focused row updates.
Unleashing the Power of the CASE Statement
The CASE statement introduces conditional logic to queries, delivering diverse result outcomes based on predefined conditions.
TRIGGER Statements: Automating Action
TRIGGER statements link specific SQL actions to database events, streamlining processes.
The Distinctive Roles of Unique Constraints and Unique Indexes
Unique constraints ensure column distinctiveness, while unique indexes amplify both distinctiveness and query performance.
Limiting Results: The TOP and LIMIT Clauses
The TOP clause (SQL Server) and LIMIT clause (MySQL) restrict result row counts, often combined with ORDER BY for effective result management.
UNION vs. JOIN: The Symbiotic Merge
UNION vertically merges result sets, while JOIN horizontally merges columns through specified join conditions.
The Essence of Data Warehousing
A data warehouse unites diverse data sources, enabling efficient reporting, analysis, and business intelligence endeavors.
