Top 30 Most Common Sql Query Related Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Introduction
Preparing for technical interviews often involves demonstrating proficiency in key database skills, and SQL (Structured Query Language) is fundamental. SQL query related interview questions are a staple in interviews for data analysts, database administrators, software engineers, and anyone working with data. Mastering SQL is crucial for managing, manipulating, and retrieving information efficiently from relational databases. This guide covers the top 30 most common sql query related interview questions you are likely to face. From basic syntax and commands to more complex topics like joins, subqueries, and database design principles, understanding these concepts and being able to answer sql query related interview questions effectively will significantly boost your confidence and performance in your next interview. Get ready to dive deep into the world of SQL interview preparation.
What Are SQL Query Related Interview Questions
SQL query related interview questions are designed to assess a candidate's understanding of database concepts and their ability to interact with data using SQL. These questions cover a wide range of topics, including fundamental SQL commands like SELECT, INSERT, UPDATE, and DELETE, as well as more advanced concepts like joins, subqueries, window functions, and transaction management. Interviewers use sql query related interview questions to evaluate your knowledge of database structure, data retrieval efficiency, data manipulation techniques, and problem-solving skills using the SQL language. Being able to articulate the purpose of different SQL statements and clauses, explain database design principles like normalization, and solve practical query problems demonstrates a solid foundation in data handling essential for many roles.
Why Do Interviewers Ask SQL Query Related Interview Questions
Interviewers ask sql query related interview questions for several critical reasons. Firstly, SQL is the universal language for interacting with relational databases, which are ubiquitous across industries. Proficiency in SQL is a direct measure of a candidate's ability to access and manage data, a core requirement for numerous positions. Secondly, these questions help gauge a candidate's problem-solving skills. Presenting a scenario and asking for the corresponding SQL query reveals how a candidate breaks down a problem and translates business logic into database operations. Lastly, complex sql query related interview questions can assess a candidate's understanding of performance implications, database design principles, and data integrity, showing their depth of knowledge beyond just basic syntax. Ultimately, strong SQL skills demonstrated through these questions signal readiness for roles involving data analysis, development, or database administration.
Preview List
What is SQL?
What are the different types of SQL commands?
What is a Primary Key?
What is a Foreign Key?
What are Joins? Explain different types.
How do you fetch unique values from a column?
What is the difference between DELETE and TRUNCATE?
What is the use of the WHERE clause?
How do you sort query results?
What are aggregate functions?
What is normalization? What are its types?
What is a subquery?
What is an Index? What is the difference between clustered and non-clustered index?
How do you update existing records in a table?
How to insert new data into a table?
What is the difference between UNION and UNION ALL?
What is a view?
What are constraints? Name different types.
What is a transaction? What are its properties?
What is NULL in SQL? How is it different from zero or empty string?
How do you handle NULL values in queries?
What is the difference between CHAR and VARCHAR data types?
How do you create an empty table with the same structure as an existing table?
What is pattern matching in SQL?
What is a recursive stored procedure or query?
What is the difference between DELETE, DROP, and TRUNCATE commands?
How do you restore a backup in PostgreSQL?
What is a clustered index?
What is the order of results shown by default if ASC or DESC is not specified?
Explain the usage of COMMIT and ROLLBACK.
1. What is SQL?
Why you might get asked this:
This is a fundamental question to check if you know the basics of SQL. It assesses your foundational knowledge before moving to more complex sql query related interview questions.
How to answer:
Define SQL, explain its purpose (database communication), and mention its standard nature and key operations (querying, manipulating data).
Example answer:
SQL stands for Structured Query Language. It is the standard language used for managing and manipulating relational databases. It allows users to create, modify, and query database structures, as well as insert, update, and delete data.
2. What are the different types of SQL commands?
Why you might get asked this:
Interviewers want to know if you understand the different categories of SQL operations and their functions in database management.
How to answer:
List and briefly explain the main categories: DDL, DML, DCL, and TCL, providing examples for each.
Example answer:
SQL commands are categorized into DDL (Data Definition Language) for schema management (CREATE, ALTER, DROP), DML (Data Manipulation Language) for data handling (SELECT, INSERT, UPDATE, DELETE), DCL (Data Control Language) for permissions (GRANT, REVOKE), and TCL (Transaction Control Language) for managing transactions (COMMIT, ROLLBACK).
3. What is a Primary Key?
Why you might get asked this:
This question tests your understanding of fundamental database integrity concepts, essential for proper database design and querying.
How to answer:
Explain its purpose: uniquely identifying rows. Mention its key properties: unique and non-NULL.
Example answer:
A Primary Key is a field or set of fields in a table that uniquely identifies each record. It ensures data integrity by enforcing that each row has a distinct identifier and that this identifier is never NULL. A table can only have one primary key.
4. What is a Foreign Key?
Why you might get asked this:
Tests your knowledge of how relationships between tables are established and maintained, which is crucial for joining data.
How to answer:
Define a foreign key as a column referencing a primary key in another table and explain its role in linking tables and enforcing referential integrity.
Example answer:
A Foreign Key is a column (or columns) in one table that refers to the Primary Key in another table. Its purpose is to link tables together and enforce referential integrity, ensuring that relationships between tables are consistent and valid.
5. What are Joins? Explain different types.
Why you might get asked this:
Joins are central to querying related data across multiple tables. This tests your ability to combine information effectively.
How to answer:
Explain that joins combine data from multiple tables. List and briefly describe INNER, LEFT, RIGHT, and FULL JOINs, explaining how they handle matching and non-matching rows.
Example answer:
Joins combine rows from two or more tables based on a related column. Common types include INNER JOIN (returns matched rows), LEFT JOIN (all left rows, matching right), RIGHT JOIN (all right rows, matching left), and FULL JOIN (all rows from both, filling NULLs where no match).
6. How do you fetch unique values from a column?
Why you might get asked this:
A basic but common task. Assesses your knowledge of foundational SELECT statement clauses.
How to answer:
Mention the DISTINCT keyword and show a simple SELECT query example.
Example answer:
You use the DISTINCT keyword in the SELECT statement. For example, SELECT DISTINCT columnname FROM tablename; will return only the unique values present in the specified column, removing any duplicates.
7. What is the difference between DELETE and TRUNCATE?
Why you might get asked this:
Tests understanding of data removal commands and their implications on logging, rollback, and performance.
How to answer:
Explain that DELETE removes rows one by one and logs the operation (allowing rollback), while TRUNCATE removes all rows quickly by deallocating space and is generally not rollbackable.
Example answer:
DELETE removes rows based on a condition or all rows, logging each deletion, making it slower but allowing rollback. TRUNCATE removes all rows by deallocating the table space, is much faster, but typically cannot be rolled back and doesn't log individual row operations.
8. What is the use of the WHERE clause?
Why you might get asked this:
Essential for filtering data. Tests your ability to retrieve specific information based on criteria.
How to answer:
Explain that the WHERE clause is used to filter records based on specified conditions in SELECT, UPDATE, or DELETE statements.
Example answer:
The WHERE clause is used to filter the results of a query. It specifies conditions that must be met for a row to be included in the result set of a SELECT statement, or for rows to be affected by UPDATE or DELETE statements.
9. How do you sort query results?
Why you might get asked this:
Common requirement for presenting data. Tests knowledge of the ORDER BY clause.
How to answer:
Mention the ORDER BY clause and explain how to specify ascending (ASC) or descending (DESC) order.
Example answer:
You sort query results using the ORDER BY clause. Specify the column(s) to sort by, followed by ASC for ascending order (default) or DESC for descending order. For example, SELECT * FROM tablename ORDER BY columnname DESC;.
10. What are aggregate functions?
Why you might get asked this:
Tests understanding of functions that operate on sets of rows, crucial for reporting and data summarization.
How to answer:
Define aggregate functions as operations performed on multiple input rows to produce a single output row, listing common examples.
Example answer:
Aggregate functions perform calculations on a set of rows and return a single value. Common examples include COUNT() (counts rows), SUM() (sums values), AVG() (calculates average), MAX() (finds maximum value), and MIN() (finds minimum value).
11. What is normalization? What are its types?
Why you might get asked this:
Tests understanding of database design principles aimed at reducing redundancy and improving data integrity.
How to answer:
Define normalization as organizing database tables to reduce redundancy and dependency. Mention common normal forms like 1NF, 2NF, 3NF.
Example answer:
Normalization is the process of organizing a database to reduce redundancy and dependency. Its goals are to eliminate redundant data and ensure data dependencies are logical. Common levels (Normal Forms) include 1NF, 2NF, and 3NF, each addressing specific types of anomalies.
12. What is a subquery?
Why you might get asked this:
Tests ability to write more complex queries where one query's result is used by another.
How to answer:
Define a subquery as a query nested inside another SQL query (SELECT, INSERT, UPDATE, DELETE). Explain its use in filtering data or providing values.
Example answer:
A subquery is a SELECT query nested within another SQL statement (like SELECT, INSERT, UPDATE, DELETE, or even another subquery). It's often used in the WHERE clause to filter data based on the results of the inner query, or in the FROM clause as a derived table.
13. What is an Index? What is the difference between clustered and non-clustered index?
Why you might get asked this:
Tests understanding of performance optimization techniques in databases.
How to answer:
Explain that indexes improve query performance. Differentiate clustered (sorts physical data, one per table) from non-clustered (separate structure pointing to data, multiple per table).
Example answer:
An index is a database structure that speeds up data retrieval operations by providing quick lookup paths. A clustered index determines the physical order of data rows in a table (only one per table). A non-clustered index stores pointers to the data location, keeping data separate from the index structure (multiple possible).
14. How do you update existing records in a table?
Why you might get asked this:
Fundamental DML operation. Tests knowledge of the UPDATE statement.
How to answer:
Provide the basic syntax for the UPDATE statement, emphasizing the SET clause and the importance of the WHERE clause.
Example answer:
You update records using the UPDATE statement. The syntax is UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition;. The WHERE clause is crucial to specify which rows to update; omitting it updates all rows.
15. How to insert new data into a table?
Why you might get asked this:
Fundamental DML operation. Tests knowledge of the INSERT statement.
How to answer:
Provide the basic syntax for the INSERT INTO statement, showing how to specify columns and values.
Example answer:
You insert new data using the INSERT INTO statement. The syntax is INSERT INTO table_name (column1, column2, ...) VALUES (value1, value2, ...);. You can omit the column list if inserting values for all columns in their default order.
16. What is the difference between UNION and UNION ALL?
Why you might get asked this:
Tests understanding of combining result sets from multiple SELECT statements and handling of duplicates.
How to answer:
Explain that both combine results. UNION removes duplicate rows from the combined result, while UNION ALL includes all rows from both results, including duplicates.
Example answer:
Both UNION and UNION ALL combine the result sets of two or more SELECT statements. The key difference is that UNION removes duplicate rows from the combined result, while UNION ALL returns all rows from all result sets, including duplicates. UNION performs a DISTINCT operation.
17. What is a view?
Why you might get asked this:
Tests understanding of database objects that simplify complex queries or restrict data access.
How to answer:
Define a view as a virtual table based on the result set of a SELECT query. Explain that it doesn't store data physically but provides a way to structure or limit data presentation.
Example answer:
A view is a virtual table based on the result of an SQL statement. It contains rows and columns like a real table, but it doesn't physically store data itself. It acts as a saved query that can be used to simplify complex SELECT statements, filter data, or provide a layer of abstraction.
18. What are constraints? Name different types.
Why you might get asked this:
Tests understanding of rules that enforce data integrity and business rules within the database schema.
How to answer:
Define constraints as rules on data columns. List common types like NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, and DEFAULT.
Example answer:
Constraints are rules applied to columns in a table to limit the type of data that can be inserted, ensuring data accuracy and integrity. Common types include NOT NULL (ensures a column cannot have NULL values), UNIQUE (ensures all values in a column are different), PRIMARY KEY, FOREIGN KEY, CHECK (ensures values meet a specific condition), and DEFAULT (sets a default value).
19. What is a transaction? What are its properties?
Why you might get asked this:
Tests understanding of how database operations are grouped and managed to ensure data consistency, especially in multi-user environments.
How to answer:
Define a transaction as a sequence of operations treated as a single logical unit. List and explain the ACID properties (Atomicity, Consistency, Isolation, Durability).
Example answer:
A transaction is a single logical unit of work consisting of one or more operations. It must adhere to ACID properties: Atomicity (all or nothing), Consistency (transaction brings database from one valid state to another), Isolation (concurrent transactions don't interfere), and Durability (committed changes are permanent).
20. What is NULL in SQL? How is it different from zero or empty string?
Why you might get asked this:
Tests understanding of how missing or unknown data is represented and handled in SQL.
How to answer:
Explain that NULL represents missing or unknown data. Explicitly state it is distinct from numerical zero or an empty string character, as it signifies the absence of a value.
Example answer:
NULL represents a missing or unknown value in SQL. It is distinct from zero (which is a numerical value) and an empty string (which is a character string with zero length). NULL signifies that there is no value present, whereas zero or an empty string are actual, defined values.
21. How do you handle NULL values in queries?
Why you might get asked this:
Practical application of understanding NULL. Tests knowledge of specific SQL operators and functions for handling NULLs.
How to answer:
Mention using IS NULL and IS NOT NULL in the WHERE clause and functions like COALESCE() or IFNULL() (depending on the SQL dialect) to replace NULLs in results.
Example answer:
You handle NULL values using IS NULL or IS NOT NULL in conditions (e.g., WHERE columnname IS NULL). Functions like COALESCE(column, defaultvalue) can replace NULL values with a specified default in the output.
22. What is the difference between CHAR and VARCHAR data types?
Why you might get asked this:
Tests understanding of different string data types and their implications for storage and performance.
How to answer:
Explain that CHAR is fixed-length and padded with spaces, while VARCHAR is variable-length and stores only the characters provided. Discuss the storage implications.
Example answer:
CHAR is a fixed-length character data type; storage space is allocated regardless of the actual string length, padded with spaces. VARCHAR is variable-length; it only uses the space needed for the string plus a small overhead, making it more space-efficient for variable data.
23. How do you create an empty table with the same structure as an existing table?
Why you might get asked this:
Practical SQL task testing knowledge of CREATE TABLE and SELECT INTO/AS.
How to answer:
Provide the syntax using CREATE TABLE newtable AS SELECT * FROM oldtable WHERE 1=0; or equivalent, explaining that WHERE 1=0 ensures no rows are copied.
Example answer:
You can use CREATE TABLE newtablename AS SELECT * FROM existingtablename WHERE 1 = 0;. The WHERE 1 = 0 condition is always false, so the query returns zero rows, effectively copying only the structure (columns and data types) without any data.
24. What is pattern matching in SQL?
Why you might get asked this:
Tests ability to search for strings based on patterns, a common query requirement.
How to answer:
Mention the LIKE operator and the use of wildcard characters like % (any zero or more characters) and _ (any single character).
Example answer:
Pattern matching in SQL is done using the LIKE operator with wildcard characters in the WHERE clause. The % wildcard matches any sequence of characters (including empty), and _ matches any single character. For example, WHERE name LIKE 'J%' finds names starting with 'J'.
25. What is a recursive stored procedure or query?
Why you might get asked this:
Tests understanding of handling hierarchical or tree-like data structures in SQL.
How to answer:
Define it as a procedure/query that calls itself or references its own output to process hierarchical data, mentioning Common Table Expressions (CTEs) as a common implementation.
Example answer:
A recursive stored procedure or query is one that calls itself repeatedly until a base case is met, typically used to process hierarchical data or tree structures (like organizational charts or bill of materials). Recursive queries are often implemented using Recursive Common Table Expressions (CTEs).
26. What is the difference between DELETE, DROP, and TRUNCATE commands?
Why you might get asked this:
Often confused, this question tests precise understanding of DML and DDL commands for removing data and structures.
How to answer:
Reiterate DELETE (removes rows, rollbackable), TRUNCATE (removes all rows fast, not typically rollbackable), and add DROP (removes the entire table definition and data).
Example answer:
DELETE is a DML command that removes rows based on a condition and is rollbackable. TRUNCATE is a DDL command that removes all rows from a table efficiently by deallocating space and is usually not rollbackable. DROP is a DDL command that deletes the entire table structure (schema and data).
27. How do you restore a backup in PostgreSQL?
Why you might get asked this:
Tests practical database administration knowledge for a specific popular database system.
How to answer:
Mention using the pg_restore utility for custom backups or the psql command for plain SQL dumps, specifying the database name and backup file.
Example answer:
In PostgreSQL, you typically use the pgrestore command for custom format backups, like pgrestore -d databasename backupfile. For plain SQL dumps, you use psql, such as psql -d databasename -f backupfile.sql.
28. What is a clustered index?
Why you might get asked this:
Repeat from Q13, reinforcing its importance and your retention of key indexing concepts.
How to answer:
Explain that it physically sorts and stores the data rows of a table based on the index key, limiting a table to only one clustered index.
Example answer:
A clustered index determines the physical storage order of data rows in a table. The data rows themselves are stored in the order of the clustered index key. Because the physical data can only be stored in one order, a table can have only one clustered index.
29. What is the order of results shown by default if ASC or DESC is not specified?
Why you might get asked this:
Tests knowledge of default behavior in common SQL clauses.
How to answer:
State clearly that the default sort order for ORDER BY is ascending (ASC).
Example answer:
If you use the ORDER BY clause without explicitly specifying ASC or DESC, the results are sorted in ascending order by default. ORDER BY columnname; is equivalent to ORDER BY columnname ASC;.
30. Explain the usage of COMMIT and ROLLBACK.
Why you might get asked this:
Tests understanding of transaction control, vital for data integrity and concurrent operations.
How to answer:
Explain that COMMIT makes transaction changes permanent, while ROLLBACK undoes all changes made since the last COMMIT (or transaction start).
Example answer:
COMMIT is a TCL command that saves all changes made during the current transaction permanently to the database. ROLLBACK is a TCL command that undoes all changes made during the current transaction, restoring the database to the state it was in before the transaction started.
Other Tips to Prepare for a SQL Query Related Interview Questions
Beyond memorizing answers to common sql query related interview questions, practical application is key. Practice writing actual SQL queries on a database (like PostgreSQL, MySQL, or SQLite). LeetCode, HackerRank, and SQLZoo offer excellent SQL coding challenges that simulate real-world scenarios. As SQL veteran Joe Smith says, "You don't truly know SQL until you've wrestled with a tricky join or an optimization problem." Be prepared to explain your thought process when writing queries, especially for complex problems involving joins or subqueries. Understand database design principles like normalization and indexing, as these often inform optimal query writing. Consider using the Verve AI Interview Copilot (https://vervecopilot.com) for mock interviews focused on technical skills like SQL. The Verve AI Interview Copilot can provide instant feedback on your answers and query explanations. Jane Doe, a hiring manager, notes, "Candidates who can not only write queries but explain why they chose a certain approach using tools like Verve AI Interview Copilot often stand out." Leverage the Verve AI Interview Copilot to refine your articulation and problem-solving narrative for sql query related interview questions.
Frequently Asked Questions
Q1: Is knowing just SELECT enough? A1: No, interviews cover DML, DDL, DCL, TCL, and concepts like joins, indexes, and transactions for a complete picture.
Q2: Should I know a specific SQL dialect? A2: Focus on standard SQL, but be aware of minor differences in popular systems like MySQL, PostgreSQL, or SQL Server.
Q3: How important are query optimization questions? A3: Very important for roles involving performance. Understand indexes, execution plans, and efficient query writing.
Q4: Will I be asked to write queries by hand? A4: Possibly on a whiteboard or in a shared editor. Practice writing clean, readable code without an IDE.
Q5: What if I don't know an answer? A5: Be honest. Explain your thought process and how you would approach finding the answer (e.g., checking documentation).
Q6: Are analytical functions covered? A6: For data-focused roles, yes. Be familiar with functions like RANK(), ROW_NUMBER(), and aggregate functions with PARTITION BY.
