Top 30 Most Common Sql Queries Asked In Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Introduction
SQL (Structured Query Language) is the backbone of data management, fundamental for anyone working with databases. Whether you're a data analyst, developer, database administrator, or stepping into a data-centric role, a solid grasp of SQL is non-negotiable. Technical interviews frequently include questions designed to test your understanding of SQL concepts, syntax, and ability to manipulate data effectively. These range from basic definitions and commands to complex joins, subqueries, and performance optimization techniques. Preparing thoroughly for common sql queries asked in interview questions is crucial for demonstrating your database proficiency and problem-solving skills. This guide covers 30 essential SQL interview questions to help you build confidence and ace your next interview. Mastering these questions will provide a strong foundation in database principles and query writing, common areas assessed in sql queries asked in interview questions.
What Are SQL Queries Asked in Interview Questions?
SQL queries asked in interview questions refer to the practical SQL statements and theoretical concepts interviewers use to evaluate a candidate's database knowledge. These questions test your ability to interact with relational databases, retrieve specific data, modify records, structure tables, and understand underlying database principles. They go beyond simple SELECT statements, delving into topics like joins, subqueries, aggregate functions, indexing, normalization, and transaction management. Effectively answering sql queries asked in interview questions requires not just memorizing syntax but understanding why and when to use specific commands and techniques to efficiently manage and query data. These questions assess your comfort level with data manipulation and database design, critical skills for roles involving data.
Why Do Interviewers Ask SQL Queries Asked in Interview Questions?
Interviewers ask sql queries asked in interview questions to gauge a candidate's foundational understanding of databases and their practical ability to work with data. Data is central to many modern roles, and SQL is the universal language for interacting with structured data. These questions reveal if you can think logically about data relationships, write efficient queries to extract required information, and understand how data operations impact performance and integrity. Your ability to explain concepts like joins, indexing, or transactions shows your grasp of database design principles. Practical sql queries asked in interview questions demonstrate your problem-solving approach using SQL. It's a direct way to verify if you possess the necessary technical skills to handle data-related tasks and contribute effectively from day one.
Preview List
What is SQL?
What are the different types of SQL commands?
What is a Primary Key and Foreign Key?
What are joins? Explain types of joins.
Difference between WHERE and HAVING clauses?
What is the difference between DELETE, DROP, and TRUNCATE?
What is normalization? Explain different normal forms.
Explain aggregate functions with examples.
What is the difference between clustered and non-clustered index?
How to create an empty table similar to another table?
What is a subquery? Types of subqueries?
What is a view? Why is it used?
How can you find duplicate records in a table?
What is the difference between UNION and UNION ALL?
Explain the difference between an INNER JOIN and OUTER JOIN.
What is indexing? How does it improve performance?
What is a stored procedure?
What is the difference between NULL and empty string?
What is the use of the GROUP BY clause?
What are constraints? Name different types.
How to handle missing or NULL values in SQL?
What is a self join?
What is the difference between scalar and correlated subqueries?
What is a transaction? Explain ACID properties.
How to optimize a SQL query?
What is pattern matching in SQL?
How to delete duplicate rows without using a temporary table?
Difference between UNION and JOIN?
What is a recursive stored procedure?
How to find the second highest salary in a table?
1. What is SQL?
Why you might get asked this:
This is a foundational question to check your basic understanding and definition of SQL. It's a warm-up for more complex sql queries asked in interview questions.
How to answer:
Define the acronym and explain its purpose – managing and manipulating relational databases. Mention its standard status.
Example answer:
SQL stands for Structured Query Language. It's the standard language used for communicating with and managing relational databases, allowing you to query, insert, update, and delete data.
2. What are the different types of SQL commands?
Why you might get asked this:
Evaluates your knowledge of SQL's functional categories. Shows you understand the different operations possible, a common area in sql queries asked in interview questions.
How to answer:
List and briefly describe the main categories: DDL, DML, DCL, TCL, giving examples for each.
Example answer:
SQL commands are grouped into DDL (Create, Alter, Drop), DML (Select, Insert, Update, Delete), DCL (Grant, Revoke), and TCL (Commit, Rollback).
3. What is a Primary Key and Foreign Key?
Why you might get asked this:
Tests your understanding of core relational database concepts crucial for designing schemas and writing correct join sql queries asked in interview questions.
How to answer:
Define both keys, explaining their role in identifying records and establishing relationships between tables.
Example answer:
A Primary Key uniquely identifies each record in a table and cannot be null. A Foreign Key in one table references the Primary Key in another table, linking the two.
4. What are joins? Explain types of joins.
Why you might get asked this:
Joins are fundamental for combining data from multiple tables, a very frequent task when answering sql queries asked in interview questions.
How to answer:
Explain that joins combine rows based on related columns. Describe INNER, LEFT, RIGHT, and FULL OUTER JOINs and what results they yield.
Example answer:
Joins combine data from tables. INNER returns matching rows. LEFT returns all left rows and matched right. RIGHT returns all right and matched left. FULL returns all rows where there's a match in either table.
5. Difference between WHERE and HAVING clauses?
Why you might get asked this:
Distinguishing these shows understanding of filtering data before vs. after aggregation, key for complex sql queries asked in interview questions.
How to answer:
State that WHERE filters individual rows before GROUP BY, and HAVING filters groups after GROUP BY and aggregation.
Example answer:
WHERE filters individual records before data is grouped. HAVING filters groups of records after the grouping has occurred, typically used with aggregate functions.
6. What is the difference between DELETE, DROP, and TRUNCATE?
Why you might get asked this:
Assesses your knowledge of different ways to remove data or objects, including performance and logging differences relevant for sql queries asked in interview questions.
How to answer:
Explain that DELETE removes specific rows, TRUNCATE removes all rows quickly, and DROP removes the entire table structure and data. Mention logging differences.
Example answer:
DELETE removes specific rows (with WHERE), logging each deletion. TRUNCATE removes all rows quickly without logging. DROP removes the table structure and all data.
7. What is normalization? Explain different normal forms.
Why you might get asked this:
Tests your grasp of database design principles aimed at reducing redundancy and improving data integrity, relevant for architecting systems using sql queries asked in interview questions.
How to answer:
Define normalization as organizing data to reduce redundancy. Explain 1NF, 2NF, and 3NF concisely.
Example answer:
Normalization is structuring a database to reduce redundancy and improve integrity. 1NF eliminates repeating groups, 2NF removes partial dependencies, 3NF removes transitive dependencies.
8. Explain aggregate functions with examples.
Why you might get asked this:
Aggregate functions are essential for summary reports and analysis, frequently used in practical sql queries asked in interview questions.
How to answer:
Explain they perform calculations on sets of rows, returning a single value. Give examples like COUNT, SUM, AVG, MAX, MIN.
Example answer:
Aggregate functions compute a single result from multiple rows. Examples: COUNT(*) finds the number of rows, SUM(column) calculates total values, AVG(column) finds the average.
9. What is the difference between clustered and non-clustered index?
Why you might get asked this:
Tests understanding of indexing mechanics and performance optimization in databases, important for efficient sql queries asked in interview questions.
How to answer:
Explain that a clustered index physically sorts data rows (one per table), while a non-clustered index is a separate structure pointing to data locations (multiple per table).
Example answer:
A clustered index determines the physical order of data rows in a table (only one). A non-clustered index is a separate sorted list that points to the data rows' locations.
10. How to create an empty table similar to another table?
Why you might get asked this:
A practical task testing knowledge of DDL commands used in various data manipulation scenarios for sql queries asked in interview questions.
How to answer:
Provide the syntax using CREATE TABLE ... LIKE ... or CREATE TABLE ... AS SELECT * FROM ... WHERE 1=0.
Example answer:
You can use CREATE TABLE newtable LIKE existingtable; or CREATE TABLE newtable AS SELECT * FROM existingtable WHERE 1=0; The latter copies structure but no data due to WHERE 1=0.
11. What is a subquery? Types of subqueries?
Why you might get asked this:
Subqueries are a powerful way to write complex logic; understanding them is key for advanced sql queries asked in interview questions.
How to answer:
Define a subquery as a query within another query. List common types like single-row, multiple-row, and correlated subqueries.
Example answer:
A subquery is a query nested inside another SQL query (SELECT, INSERT, UPDATE, DELETE). Types include scalar (single value), multi-row (list/table), and correlated (depends on outer query).
12. What is a view? Why is it used?
Why you might get asked this:
Views simplify complex sql queries asked in interview questions, enhance security, and abstract data structure. It's a common topic.
How to answer:
Define a view as a virtual table based on a query result. Explain its uses: simplifying complexity, security (limiting access), consistency.
Example answer:
A view is a virtual table created from a SQL query result. It's used to simplify complex joins, restrict data access for security, and provide consistent data presentation.
13. How can you find duplicate records in a table?
Why you might get asked this:
A common data cleansing task testing use of GROUP BY and HAVING, practical skills for sql queries asked in interview questions.
How to answer:
Explain using GROUP BY on the relevant column(s) and HAVING COUNT(*) > 1.
Example answer:
You can find duplicates using GROUP BY on the column(s) you suspect have duplicates and then using HAVING COUNT(*) > 1 to filter for groups with more than one entry.
14. What is the difference between UNION and UNION ALL?
Why you might get asked this:
Tests understanding of combining result sets and handling duplicates, relevant for complex reporting and analysis sql queries asked in interview questions.
How to answer:
Explain that UNION combines results and removes duplicates, while UNION ALL combines results and keeps all rows, including duplicates. Mention performance implications.
Example answer:
UNION combines results from two SELECT statements and removes duplicate rows. UNION ALL combines results but includes all rows, including duplicates, making it faster if duplicates are acceptable.
15. Explain the difference between an INNER JOIN and OUTER JOIN.
Why you might get asked this:
A core concept in relational databases, essential for correctly combining data in sql queries asked in interview questions.
How to answer:
Reiterate that INNER JOIN returns only matching rows, while OUTER JOIN (LEFT, RIGHT, FULL) returns matching rows plus unmatched rows from one or both tables.
Example answer:
INNER JOIN returns only rows where there's a match in both tables. OUTER JOIN (like LEFT or RIGHT) returns matching rows plus unmatched rows from one or both tables.
16. What is indexing? How does it improve performance?
Why you might get asked this:
Indexing is a key performance optimization technique. Understanding it is vital for writing efficient sql queries asked in interview questions.
How to answer:
Define indexing as creating data structures (like B-trees) on columns to speed up data retrieval. Explain how it reduces disk I/O by avoiding full table scans.
Example answer:
Indexing creates a lookup structure on columns that database systems use to quickly locate rows, similar to a book's index. It improves query speed by reducing the amount of data scanned.
17. What is a stored procedure?
Why you might get asked this:
Stored procedures are pre-compiled database objects used for reusability and performance, a common topic around database operations beyond simple sql queries asked in interview questions.
How to answer:
Define it as a precompiled set of SQL statements stored in the database. Mention benefits like performance, reusability, and reduced network traffic.
Example answer:
A stored procedure is a precompiled collection of SQL statements and logic stored in the database. They enhance performance, reusability, security, and reduce network load.
18. What is the difference between NULL and empty string?
Why you might get asked this:
Tests understanding of data states and how they are handled in SQL, important for accurate data manipulation using sql queries asked in interview questions.
How to answer:
Explain that NULL signifies the absence of a value (unknown or missing), while an empty string ('') is a known value with zero length.
Example answer:
NULL means there is no value or the value is unknown. An empty string is a value that exists but has zero characters. They are treated differently in comparisons.
19. What is the use of the GROUP BY clause?
Why you might get asked this:
Essential for data aggregation and reporting, frequently combined with aggregate functions in sql queries asked in interview questions.
How to answer:
Explain that GROUP BY groups rows that have the same values in specified columns, allowing aggregate functions to operate on each group.
Example answer:
The GROUP BY clause groups rows with the same value in specified columns into a summary row, often used with aggregate functions to perform calculations on each group.
20. What are constraints? Name different types.
Why you might get asked this:
Constraints enforce data integrity rules, critical for maintaining database health, a key area in understanding database structure for sql queries asked in interview questions.
How to answer:
Define constraints as rules enforced on columns. List common types: NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, DEFAULT.
Example answer:
Constraints are rules enforced on data columns to maintain accuracy and integrity. Types include NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, and DEFAULT.
21. How to handle missing or NULL values in SQL?
Why you might get asked this:
Practical skill for dealing with incomplete data, a common scenario when writing sql queries asked in interview questions.
How to answer:
Mention using IS NULL or IS NOT NULL in WHERE clauses and functions like COALESCE() or IFNULL() to replace NULLs with default values.
Example answer:
You handle NULLs using IS NULL or IS NOT NULL in WHERE clauses. Functions like COALESCE() or IFNULL() can replace NULL values in the output with a specified alternative.
22. What is a self join?
Why you might get asked this:
Tests ability to join a table to itself for hierarchical data or comparisons within the same entity, a useful technique for certain sql queries asked in interview questions.
How to answer:
Explain that it's a join where a table is joined to itself, typically using aliases, to compare rows within the same table.
Example answer:
A self join is when a table is joined to itself using aliases. It's useful for comparing rows within the same table, such as finding employees who report to the same manager.
23. What is the difference between scalar and correlated subqueries?
Why you might get asked this:
Distinguishing these shows a deeper understanding of subquery execution context, important for optimizing and writing correct complex sql queries asked in interview questions.
How to answer:
Explain that a scalar subquery returns a single value independently of the outer query, while a correlated subquery depends on the outer query and executes for each row of the outer query.
Example answer:
A scalar subquery returns one value and runs once for the outer query. A correlated subquery runs once for each row processed by the outer query and depends on the outer query's data.
24. What is a transaction? Explain ACID properties.
Why you might get asked this:
Tests knowledge of transaction management and database reliability guarantees, important for data integrity in applications using sql queries asked in interview questions.
How to answer:
Define a transaction as a single unit of work. Explain the ACID properties: Atomicity (all or nothing), Consistency (valid state), Isolation (independent), Durability (persistent changes).
Example answer:
A transaction is a sequence of operations treated as a single logical unit. ACID properties ensure reliability: Atomicity (all changes happen or none), Consistency (data remains valid), Isolation (transactions don't interfere), Durability (committed changes survive failures).
25. How to optimize a SQL query?
Why you might get asked this:
Performance is critical in database systems. This question assesses practical skills beyond just getting the right result, a key part of real-world sql queries asked in interview questions.
How to answer:
Provide actionable tips: Use indexes, select specific columns (avoid SELECT *), filter early with WHERE, optimize JOINs, avoid cursors, analyze query execution plans.
Example answer:
Optimize by using indexes, selecting only needed columns, filtering early with WHERE, using appropriate JOIN types, and analyzing the query execution plan to identify bottlenecks.
26. What is pattern matching in SQL?
Why you might get asked this:
Tests knowledge of the LIKE operator and wildcards for flexible string searches, useful for various sql queries asked in interview questions.
How to answer:
Explain using the LIKE operator with wildcards: '%' (zero or more characters) and '_' (single character).
Example answer:
Pattern matching uses the LIKE operator with wildcards: % matches any sequence of zero or more characters, and _ matches any single character. Example: WHERE name LIKE 'J%n'.
27. How to delete duplicate rows without using a temporary table?
Why you might get asked this:
A common, slightly more complex data cleaning task testing window functions or self-join techniques, demonstrating advanced practical skills for sql queries asked in interview questions.
How to answer:
Describe using a CTE with ROW_NUMBER() partitioned by the columns that define duplicates, deleting rows where the row number is greater than 1.
Example answer:
You can use a Common Table Expression (CTE) with ROW_NUMBER() partitioned by the columns identifying duplicates. Delete rows from the CTE where the generated row number for a partition is greater than 1.
28. Difference between UNION and JOIN?
Why you might get asked this:
Ensures you understand how these fundamental operations combine data differently – vertically (UNION) vs. horizontally (JOIN).
How to answer:
Explain that UNION combines the result sets of two SELECT queries vertically (adding rows), while JOIN combines columns from two or more tables horizontally based on a related column.
Example answer:
UNION combines the result sets of SELECT statements by appending rows. JOIN combines columns from different tables into a single result set based on related column values.
29. What is a recursive stored procedure?
Why you might get asked this:
Tests understanding of advanced procedure logic, specifically handling hierarchical or tree-like data structures using sql queries asked in interview questions or related code.
How to answer:
Define it as a stored procedure that calls itself, directly or indirectly. Mention its use for querying hierarchical data (e.g., organizational charts, bill of materials).
Example answer:
A recursive stored procedure is one that calls itself. This is typically used to traverse and process hierarchical data structures, like finding all subordinates in an organizational tree.
30. How to find the second highest salary in a table?
Why you might get asked this:
A classic problem-solving question testing various SQL techniques: subqueries, LIMIT/OFFSET, or window functions.
How to answer:
Provide a method using a subquery to find the max salary less than the overall max, or explain using ranking functions like ROWNUMBER() or DENSERANK().
Example answer:
One way is SELECT MAX(salary) FROM employees WHERE salary < (SELECT MAX(salary) FROM employees);. Another is using a window function like DENSE_RANK() to rank salaries and select the one with rank 2.
Other Tips to Prepare for a SQL Interview
Preparing for sql queries asked in interview questions involves more than just memorizing answers. Practice writing queries on sample datasets to solidify your understanding. Familiarize yourself with different SQL dialects (e.g., MySQL, PostgreSQL, SQL Server) as syntax can vary slightly. Be prepared to explain your thought process when solving a problem; interviewers want to see how you approach challenges. As the saying goes, "Practice makes perfect." Don't just read the answers; try to code them. Consider using tools like the Verve AI Interview Copilot (https://vervecopilot.com) for mock interview practice specifically tailored to technical roles, including those heavy in sql queries asked in interview questions. The Verve AI Interview Copilot provides feedback on your answers, helping you refine your explanations and approach to common database scenarios presented as sql queries asked in interview questions. Use resources like Verve AI Interview Copilot to simulate the interview environment.
Frequently Asked Questions
Q1: What is the difference between SQL and MySQL? A1: SQL is the standard language; MySQL is a specific relational database management system that uses SQL.
Q2: What are Window functions? A2: Functions that perform calculations across a set of table rows related to the current row, like ranking or aggregation.
Q3: When would you use a CTE? A3: For complex queries to improve readability, break down logic, or perform recursion.
Q4: Is SQL case-sensitive? A4: Generally, SQL commands are not, but data stored in tables may be, depending on the database system and column collation.
Q5: What's the difference between primary key and unique key? A5: A primary key uniquely identifies a row and cannot be null. A unique key also ensures uniqueness but allows one null value.
