Top 30 Most Common Sql Query Interview Questions And Answers You Should Prepare For
Written by
James Miller, Career Coach
Navigating the complexities of technical interviews can be challenging, especially when specific skills like database management and data manipulation are key requirements. For roles involving data analysis, software development, or database administration, proficiency in SQL is often a prerequisite. Preparing for sql query interview questions and answers is crucial to demonstrate your technical capabilities and problem-solving skills. This guide provides a comprehensive list of 30 essential sql query interview questions and answers, covering foundational concepts, common operations, and advanced topics. Mastering these topics will significantly boost your confidence and performance in your next interview.
What Are sql query interview questions and answers?
sql query interview questions and answers focus on testing a candidate's understanding of SQL (Structured Query Language), the standard language for managing and manipulating relational databases. These questions assess your ability to write, interpret, and optimize SQL queries to retrieve, insert, update, and delete data. They range from basic syntax and commands to more complex concepts like joins, subqueries, window functions, indexing, and database normalization. The goal is to evaluate your practical skills in interacting with databases, your grasp of relational database theory, and your ability to solve data-related problems efficiently using SQL. Preparing thoroughly for sql query interview questions and answers ensures you can effectively communicate your technical knowledge.
Why Do Interviewers Ask sql query interview questions and answers?
Interviewers ask sql query interview questions and answers to gauge a candidate's foundational knowledge and practical experience with databases. SQL is a fundamental skill across many tech roles, and assessing proficiency helps employers determine if you can effectively work with data. These questions reveal your problem-solving approach, your understanding of data structures, and your ability to write efficient code. They also help differentiate candidates by testing their depth of knowledge, from basic data retrieval to complex query optimization and database design principles. Strong performance on sql query interview questions and answers indicates readiness for roles requiring significant data interaction.
What is SQL?
What are the different types of SQL statements?
What are SQL constraints?
What is normalization in database design?
What are SQL indexes and why are they important?
How do you use a subquery?
What are the different types of JOINs in SQL?
What is the difference between INNER JOIN and LEFT JOIN?
What is a clustered index?
What is a non-clustered index?
What is the main advantage of a clustered index over a non-clustered index?
How do you fetch unique values from a field?
What statement is used to update data in the database?
What statement is used for adding data to a database?
What is normalization that has neither composite values nor partial dependencies?
What allows us to define how various tables are related to each other formally in a database?
What is the order of results shown by default if the ASC or DESC parameter is not specified with the ORDER BY command?
How do you query to select all records with "bar" in their name?
What command is used to tell PostgreSQL to make all changes made to the database permanent?
What is a Query?
What is the difference between a PRIMARY KEY and a FOREIGN KEY?
How do you use window functions?
What is a subquery?
What is a Common Table Expression (CTE)?
How do you optimize SQL queries for performance?
What is UNION and INTERSECT?
What is a FULL OUTER JOIN?
What are text and date manipulation functions in SQL?
What is the purpose of a PRIMARY KEY constraint?
What is the purpose of a CHECK constraint?
Preview List
1. What is SQL?
Why you might get asked this:
This fundamental question assesses your basic understanding of SQL as a language and its purpose in database management. It's a standard starting point.
How to answer:
Define SQL and explain its primary use case in interacting with relational databases for data operations.
Example answer:
SQL (Structured Query Language) is a standard programming language used for managing and manipulating relational databases. It allows users to retrieve, insert, update, and delete data, as well as define database schemas.
2. What are the different types of SQL statements?
Why you might get asked this:
This tests your knowledge of the different categories of SQL commands and their functions (defining structures, manipulating data, controlling access, querying).
How to answer:
List and briefly describe the main categories like DDL, DML, DCL, and DQL.
Example answer:
SQL statements are broadly categorized into: DDL (Data Definition Language) for schema definition (CREATE, ALTER), DML (Data Manipulation Language) for data management (INSERT, UPDATE, DELETE), DCL (Data Control Language) for permissions (GRANT, REVOKE), and DQL (Data Query Language) for data retrieval (SELECT).
3. What are SQL constraints?
Why you might get asked this:
This question evaluates your understanding of how to enforce data integrity and business rules within a database.
How to answer:
Explain their purpose and list common examples like PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, and CHECK.
Example answer:
SQL constraints are rules enforced on columns or tables to limit the data that can be inserted. They ensure data accuracy and integrity. Examples include PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, and CHECK constraints.
4. What is normalization in database design?
Why you might get asked this:
This assesses your knowledge of database design principles aimed at reducing redundancy and improving data consistency.
How to answer:
Define normalization and explain its goal of organizing data to minimize redundancy and dependencies, typically involving breaking tables into smaller ones.
Example answer:
Normalization is a process of organizing data in a database to reduce redundancy and improve data integrity. It involves structuring tables and columns according to a series of "normal forms" (e.g., 1NF, 2NF, 3NF).
5. What are SQL indexes and why are they important?
Why you might get asked this:
This tests your understanding of performance optimization techniques in SQL. Indexes are crucial for faster data retrieval.
How to answer:
Define indexes as data structures that speed up data retrieval and explain their importance for query performance.
Example answer:
SQL indexes are data structures that help retrieve data from a database faster. They work similarly to an index in a book, allowing the database system to quickly locate rows without scanning the entire table. They are vital for query performance.
6. How do you use a subquery?
Why you might get asked this:
This evaluates your ability to write more complex queries where one query's result is used as input for another.
How to answer:
Explain that a subquery (or inner query) is nested within another SQL query (the outer query) and can be used in WHERE, FROM, or SELECT clauses.
Example answer:
A subquery is a query nested inside another query. It can be used in a WHERE clause to filter results based on a condition from another table, in a FROM clause as a derived table, or in a SELECT clause for scalar values.
7. What are the different types of JOINs in SQL?
Why you might get asked this:
JOINs are fundamental for combining data from multiple tables. This question checks your understanding of different join behaviors.
How to answer:
List and briefly describe the common types: INNER, LEFT (OUTER), RIGHT (OUTER), FULL (OUTER), and CROSS JOIN.
Example answer:
The main types of SQL JOINs are: INNER JOIN (matches only), LEFT JOIN (all left, matching right), RIGHT JOIN (all right, matching left), FULL OUTER JOIN (all from both), and CROSS JOIN (all combinations).
8. What is the difference between INNER JOIN and LEFT JOIN?
Why you might get asked this:
This common question specifically tests your understanding of how INNER and LEFT JOINs handle non-matching rows.
How to answer:
Clearly state that INNER JOIN returns only rows where the join condition is met in both tables, while LEFT JOIN returns all rows from the left table plus matching rows from the right (with NULLs for non-matches).
Example answer:
An INNER JOIN returns only rows where there's a match in both tables based on the join condition. A LEFT JOIN returns all rows from the left table and the matched rows from the right table; if no match, it returns NULLs for right table columns.
9. What is a clustered index?
Why you might get asked this:
This delves deeper into index types and their physical impact on data storage.
How to answer:
Explain that a clustered index determines the physical order of data in the table itself. A table can only have one.
Example answer:
A clustered index dictates the physical storage order of the data rows in a table. Because the data rows themselves are sorted according to the index key, a table can only have one clustered index.
10. What is a non-clustered index?
Why you might get asked this:
This contrasts with clustered indexes and tests your understanding of indexes that use separate structures.
How to answer:
Describe it as an index that does not affect the physical data order but creates a separate structure storing the index key and a pointer to the data location.
Example answer:
A non-clustered index creates a separate structure that contains the index key values and pointers to the location of the data rows in the table. It does not change the physical order of the data rows. A table can have multiple non-clustered indexes.
11. What is the main advantage of a clustered index over a non-clustered index?
Why you might get asked this:
This tests your ability to compare index types and understand their specific performance benefits.
How to answer:
Highlight its benefit for range scans and sequential access because the data is physically ordered according to the index.
Example answer:
Clustered indexes are generally faster for range scans or retrieving large sets of data because the data rows are stored contiguously in the order of the index key. This reduces physical I/O operations.
12. How do you fetch unique values from a field?
Why you might get asked this:
This tests knowledge of the DISTINCT keyword, a basic but frequently used SQL function.
How to answer:
State that the DISTINCT keyword is used in a SELECT statement.
Example answer:
You use the DISTINCT keyword in the SELECT statement before the column name to retrieve only the unique values from that field. For example: SELECT DISTINCT columnname FROM tablename;.
13. What statement is used to update data in the database?
Why you might get asked this:
This is a core DML command. Interviewers check if you know the correct syntax for modifying existing records.
How to answer:
Specify the UPDATE statement. Mention the WHERE clause is critical to avoid updating all rows.
Example answer:
The UPDATE statement is used to modify existing records in a table. You use a WHERE clause to specify which records to update. Example: UPDATE table_name SET column1 = value1 WHERE condition;.
14. What statement is used for adding data to a database?
Why you might get asked this:
This is another core DML command, essential for populating tables.
How to answer:
Specify the INSERT statement and mention its use with INTO.
Example answer:
The INSERT statement is used to add new rows of data into a table. Example syntax: INSERT INTO tablename (column1, column2) VALUES (value1, value2); or INSERT INTO tablename VALUES (value1, value2, ...);.
15. What is normalization that has neither composite values nor partial dependencies?
Why you might get asked this:
This tests knowledge of specific normal forms beyond the first two, showing deeper understanding of normalization principles.
How to answer:
Identify this as Third Normal Form (3NF).
Example answer:
This description matches the Third Normal Form (3NF). In 3NF, a table must be in 2NF, and all non-key attributes must be dependent only on the primary key, not on any other non-key attributes (transitive dependency).
16. What allows us to define how various tables are related to each other formally in a database?
Why you might get asked this:
This question focuses on how relationships are enforced in a relational database schema using keys.
How to answer:
Identify FOREIGN KEYs as the mechanism for defining and enforcing relationships between tables based on their primary keys.
Example answer:
Foreign keys allow us to formally define and enforce relationships between tables. A foreign key in one table references the primary key in another table, creating a link between the two and ensuring referential integrity.
17. What is the order of results shown by default if the ASC or DESC parameter is not specified with the ORDER BY command?
Why you might get asked this:
This is a simple syntax detail that indicates attention to default behavior in SQL.
How to answer:
State that the default order is ascending (ASC).
Example answer:
If you use the ORDER BY clause without specifying ASC or DESC, the results are sorted in ascending order by default.
18. How do you query to select all records with "bar" in their name?
Why you might get asked this:
This tests the use of the LIKE operator and wildcard characters for pattern matching in a WHERE clause.
How to answer:
Explain the use of LIKE and the % wildcard character before and after the pattern.
Example answer:
You use the LIKE operator with the wildcard % to perform pattern matching. The query would be: SELECT * FROM table_name WHERE name LIKE '%bar%';. The % matches any sequence of characters.
19. What command is used to tell PostgreSQL to make all changes made to the database permanent?
Why you might get asked this:
This tests knowledge of transaction control commands, specifically committing changes in systems like PostgreSQL or others using explicit transactions.
How to answer:
Identify the COMMIT command.
Example answer:
In PostgreSQL, and many other SQL database systems operating within a transaction, the COMMIT command is used to save all changes made during the transaction permanently to the database.
20. What is a Query?
Why you might get asked this:
A very basic definition question to ensure you understand the core concept of interacting with a database.
How to answer:
Define a query as a request to a database for data or to perform an action.
Example answer:
A query is a request made to a database to retrieve specific information or to perform an action, such as inserting, updating, or deleting data. Queries are written using SQL statements.
21. What is the difference between a PRIMARY KEY and a FOREIGN KEY?
Why you might get asked this:
This is a fundamental concept in relational database design, testing your understanding of how keys function to identify rows and relate tables.
How to answer:
Explain that a PRIMARY KEY uniquely identifies rows within its own table, while a FOREIGN KEY is a column(s) in one table that refers to the PRIMARY KEY in another table, establishing a link.
Example answer:
A PRIMARY KEY uniquely identifies each row in a table and cannot contain NULL values. A FOREIGN KEY is a column or set of columns in one table that references the PRIMARY KEY in another table, used to link the tables and enforce referential integrity.
22. How do you use window functions?
Why you might get asked this:
This tests knowledge of more advanced SQL features useful for analytical queries, calculating values across related rows without aggregation.
How to answer:
Explain that they perform calculations across a set of table rows that are somehow related to the current row, using the OVER() clause. Mention examples like ranking or aggregation over partitions.
Example answer:
Window functions perform calculations across a set of table rows related to the current row, defined by the OVER() clause. They don't collapse rows like GROUP BY. Examples include RANK(), ROW_NUMBER(), or aggregate functions like SUM() used with OVER().
23. What is a subquery?
Why you might get asked this:
This is a repeat question from the list provided but good to have it listed again as it's a common concept.
How to answer:
Reiterate that it's a query nested inside another SQL statement.
Example answer:
A subquery is a SELECT query embedded within another SQL statement. It can be used to filter data in a WHERE clause, provide a derived table in a FROM clause, or return a scalar value in a SELECT clause.
24. What is a Common Table Expression (CTE)?
Why you might get asked this:
CTEs are used for improving query readability and manageability, especially for complex or recursive queries. This tests knowledge of this feature.
How to answer:
Define a CTE as a temporary, named result set defined within a single SQL statement (SELECT, INSERT, UPDATE, DELETE).
Example answer:
A CTE is a temporary, named result set that you can reference within a single execution of a SELECT, INSERT, UPDATE, or DELETE statement. It helps break down complex queries into more readable parts and can be recursive.
25. How do you optimize SQL queries for performance?
Why you might get asked this:
This assesses your practical skills in making queries run faster, a critical skill for database roles.
How to answer:
Mention using indexes, limiting columns/rows selected, avoiding SELECT *, minimizing joins, optimizing WHERE clauses, and considering database-specific features.
Example answer:
Optimization methods include creating appropriate indexes, selecting only necessary columns and rows, avoiding SELECT *, optimizing WHERE and JOIN conditions, and using efficient structures like CTEs or window functions. Analyzing query execution plans is also key.
26. What is UNION and INTERSECT?
Why you might get asked this:
These set operations are used to combine or find common results from multiple SELECT statements.
How to answer:
Explain UNION combines results and removes duplicates (UNION ALL includes duplicates), while INTERSECT returns only rows present in both result sets.
Example answer:
UNION combines the result sets of two or more SELECT statements, removing duplicate rows (unless UNION ALL is used). INTERSECT returns only the rows that are common to all combined SELECT statements.
27. What is a FULL OUTER JOIN?
Why you might get asked this:
This tests understanding of joins that return all data from both tables, indicating NULLs where no match exists.
How to answer:
Explain that it returns all rows when there is a match in either the left or the right table. Rows without a match on either side will still appear, with NULLs for the columns of the table without a match.
Example answer:
A FULL OUTER JOIN (or FULL JOIN) returns all rows from both the left and the right tables. If a row from one table doesn't have a match in the other, the columns from the non-matching table will have NULL values.
28. What are text and date manipulation functions in SQL?
Why you might get asked this:
This assesses knowledge of built-in functions for common data transformations, essential for data cleaning and formatting.
How to answer:
Give examples of functions used for manipulating strings (like CONCAT, SUBSTR, TRIM) and dates (like DATEFORMAT, EXTRACT, DATEADD/SUB).
Example answer:
SQL provides functions for manipulating text and dates. Text functions include CONCAT() for joining strings, SUBSTR()/SUBSTRING() for extracting parts, and TRIM() for removing spaces. Date functions include DATE_FORMAT(), EXTRACT() (part of a date), and functions for adding/subtracting time units.
29. What is the purpose of a PRIMARY KEY constraint?
Why you might get asked this:
This is a rephrased fundamental concept question, ensuring you understand the role of primary keys in table structure.
How to answer:
State its purpose is to uniquely identify each record in a table and enforce data integrity by ensuring no duplicates and no NULLs.
Example answer:
The purpose of a PRIMARY KEY constraint is to uniquely identify every single row within a table. It ensures that each row has a distinct identifier and enforces data integrity by not allowing duplicate or NULL values for the primary key columns.
30. What is the purpose of a CHECK constraint?
Why you might get asked this:
This tests knowledge of constraints used to enforce domain integrity by limiting the range of values a column can accept.
How to answer:
Explain that it ensures values in a column satisfy a specific condition or range, enforcing business rules.
Example answer:
A CHECK constraint is used to limit the range of values that can be placed in a column. It enforces domain integrity by ensuring that the data inserted or updated in that column satisfies a specified boolean expression or condition.
Other Tips to Prepare for a sql query interview questions and answers
Preparation is key to acing your sql query interview questions and answers. Beyond memorizing syntax, focus on understanding why certain approaches are preferred. As database expert Joe Celko advises, "Understand the relational model, not just the SQL syntax." Practice writing queries for various scenarios, from simple SELECT statements to complex joins and aggregations. Work through online SQL challenges or use dummy datasets to simulate real-world problems. Improve your problem-solving skills by breaking down complex requests into smaller, manageable steps. Consider using tools like Verve AI Interview Copilot (https://vervecopilot.com), which offers practice simulations to sharpen your responses to technical questions, including those focused on sql query interview questions and answers. Getting comfortable explaining your thought process is as important as getting the query right. As another common piece of wisdom suggests, "The best code is readable code," and explaining your query logic clearly demonstrates your ability to collaborate effectively. Utilize resources and practice platforms, including mock interviews, to build confidence before the actual interview. Verve AI Interview Copilot can be a valuable resource in this preparation journey, offering targeted practice for sql query interview questions and answers.
Frequently Asked Questions
Q1: What is NULL in SQL?
A1: NULL represents a missing or unknown value in a database field. It's not the same as zero or an empty string.
Q2: What is a schema in SQL?
A2: A schema is a collection of database objects (like tables, views, indexes, etc.) associated with a particular database user.
Q3: What is the difference between DELETE and TRUNCATE?
A3: DELETE removes rows based on a WHERE clause and logs each deletion. TRUNCATE removes all rows from a table faster and doesn't log individual rows.
Q4: What is a View in SQL?
A4: A View is a virtual table based on the result-set of an SQL query. It contains rows and columns like a real table but doesn't store data itself.
Q5: What is the difference between COUNT(*) and COUNT(column_name)?
A5: COUNT(*) counts all rows, including those with NULL values. COUNT(column_name) counts only rows where the specified column is not NULL.
Q6: What is an Alias in SQL?
A6: An Alias is a temporary name given to a table or a column in a SQL query to make it more readable or shorter.
