Have Any Question?
+44 7452 122728
Back
Table Of Contents
44 7452 122728
Your dream job in Structured Query Language (SQL) Database Management, Data Analysis, and Backend Development is just one interview away! You need to be well-versed in the world of SQL to crack your next interview because one tricky query can make or break your chances. But don't worry! This blog compiles more than 60 must-know SQL Interview Questions with answers that break down even the trickiest concepts. So read on, crack the code to the perfect interview and leave a lasting impression on your future employer!
Table of Content
60+ Top SQL Interview Questions
What is SQL?
What are the Different Types of SQL Commands?
What are SQL Dialects? Give Some Examples
What is a Primary Key in SQL?
What is a Foreign Key?
What are the Main Uses of SQL?
What is a Database?
How Does DELETE Differ From TRUNCATE?
What is a Query in SQL?
Define a Unique Key in SQL
Conclusion
60+ Top SQL Interview Questions
SQL is a valuable skill for anyone handling data, including Database Administrators, Data Analysts, and Software Developers. Whether you're new to SQL or a pro, being prepared for diverse interview Questions is essential. The following SQL Interview Questions and Answers to help you ace your next interview.
1. What is SQL?
SQL, short for Structured Query Language, is a Programming Language for manage and manipulate relational databases. It helps users perform operations such as querying, updating, inserting and deleting data while also managing database structures like tables, indexes, and constraints.
2. What are the Types of SQL Commands?
SQL commands are categorised into five types:
Data Definition Language (DDL)
Data Manipulation Language (DML)
Data Query Language (DQL)
Transaction Control Language (TCL)
Data Control Language (DCL)
Each serves different purposes, such as defining structures, modifying data, retrieving records, or managing transactions.
3. What are SQL Dialects? Give Some Examples
SQL dialects are variations of SQL designed for specific database systems. While the core syntax stays similar, different databases offer proprietary extensions. Examples of these dialects include MySQL, PostgreSQL, Microsoft SQL Server (T-SQL), Oracle SQL (PL/SQL), and SQLite. Each has unique functions, optimisations, and procedural extensions.
4. What is a Primary Key in SQL?
A primary key refers to a column or a set of columns which uniquely identifies each record in a table. It ensures data integrity by preventing duplicate and NULL values. For example, EmployeeID in an Employees table is a primary key as it uniquely identifies employees.
5. What is a Foreign Key?
A foreign key is the column in one table which references the primary key in another table, establishing a relationship between them. It enforces referential integrity. For example, OrderID in an Orders table can be a foreign key referencing CustomerID in a Customers table.
6. What are the main uses of SQL?
SQL is used for data retrieval, insertion, updating, deletion and managing database structures. It also supports Data Analysis, reporting, security control and Transaction Management in relational databases. SQL enables businesses to store, process and retrieve large volumes of structured data.
7. What is a database?
A database refers to an organised collection of data stored electronically. It enables easy access, retrieval and management. It is structured using tables, indexes and relationships. Examples include relational databases like MySQL and PostgreSQL or NoSQL databases like MongoDB.
8. How does DELETE differ from TRUNCATE?
DELETE removes specific rows and allows rollback if inside a transaction but TRUNCATE removes all rows instantly and cannot be rolled back. DELETE logs each row deletion while TRUNCATE is more efficient as it resets the table’s storage without logging individual row deletions.
9. What is a query in SQL?
An SQL query refers to a statement used to retrieve, modify, insert, or delete data in a database. The most common query is the SELECT statement, which fetches records based on conditions, such as:
10. Define a Unique Key in SQL
A unique key ensures all the values in a column are distinct, similar to a primary key, but it allows NULL values. A table can have multiple unique keys, whereas it can only have one primary key.
11. What is the purpose of the SQL SELECT statement?
The SELECT statement us used to retrieve data from a database. It allows filtering, sorting, and aggregating records using clauses like WHERE, ORDER BY, and GROUP BY.
12. What does a NULL value represent in SQL?
NULL represents an unknown or missing value within a database. It differs from zero or an empty string, which are actual values. Queries using NULL require IS NULL instead of = for proper evaluation.
Learn to write complex queries, manage databases, and elevate your data skills in our comprehensive SQL Training - Sign up now!
13. What is a DBMS, and what are its types?
DBMS is the software for storing, retrieving and managing data. Types include Relational DBMS (RDBMS) like MySQL and PostgreSQL and NoSQL DBMS like MongoDB, which handle unstructured data.
14. What is a SQL join?
A JOIN retrieves data from multiple tables based on a related column. It combines records from two or more tables using conditions like matching keys.
15. What are the different types of joins in SQL?
These are the types of joins in SQL:
INNER JOIN: Matches records in both tables
LEFT JOIN: Returns all left-table records and matching right-table records
RIGHT JOIN: Returns all right-table records and matching left-table records
FULL JOIN: Returns every record from both tables
16. What is a table in SQL?
A table stores structured data in rows and columns within a database. Each column has a defined data type, and each row represents a record.
17. What are the different types of constraints in SQL?
SQL constraints enforce data integrity and these are the main types:
PRIMARY KEY (unique + not null)
FOREIGN KEY (ensures referential integrity)
UNIQUE (prevents duplicate values)
NOT NULL (ensures non-empty values)
CHECK (validates conditions)
18. What is an index in SQL?
An index improves query performance by speeding up searches. It functions like a table of contents, allowing the database to find data faster.
19. What types of indexes do you know?
I know the following types of indexes:
Clustered Index: This reorders table data.
Non-clustered Index: This creates a separate structure.
Unique Index: This enforces unique values.
Composite Index: It covers multiple columns.
20. What is an SQL alias?
An alias provides a temporary name for a table or column in a query, improving readability. For example:
21. What is a trigger in SQL?
A trigger is a predefined database action that automatically executes when a specific event, such as INSERT, UPDATE, or DELETE, occurs in a table. It helps maintain data integrity, enforce business rules, and automate processes without manual intervention.
22. How does an index differ from a key in SQL?
A key primary or foreign ensures uniqueness or relationships, while an index improves query performance.
23. How do you update data in SQL?
The UPDATE command can be utilised to modify records. For example:
24. What is a sequence in SQL?
A sequence in SQL is a database object that automatically generates unique numeric values, typically used for primary keys. It ensures sequential and consistent numbering in tables without manual input.
25. What is the difference between UNION and UNION ALL?
The UNION command combines results from multiple SELECT statements while automatically removing duplicate rows. In contrast, UNION ALL also merges results but retains all duplicates, making it faster since it skips the duplicate-checking process.
26. What is a NULL value and how is it different from zero or a blank space?
A NULL value represents missing or unknown data, while zero is a numeric value, and a blank space is an empty string, both holding actual values.
Are you looking to upgrade your career in Data Management, analysis or development? Sign up for our Introduction to SQL Training now!
27. What is a clause in SQL?
A clause modifies SQL queries by filtering (WHERE), sorting (ORDER BY), grouping (GROUP BY), or limiting (LIMIT), enhancing query efficiency and structuring results based on conditions.
28. What are the pros and cons of using indexes in SQL databases?
Here are the pros and cons:
Pros: Indexes improve query performance by speeding up searches.
Cons: They require additional storage and can slow down data insertion, updates, and deletions due to indexing overhead.
29. How do clustered and non-clustered indexes differ?
A clustered index sorts and stores actual table records, whereas a non-clustered index creates a separate structure containing pointers to locate table rows efficiently.
30. What is the use of XML data type in SQL Server?
This data type in SQL Server enables storing, querying, and processing XML-formatted data, making it useful for handling hierarchical or semi-structured information within a relational database.
31. What is a SQL comment?
SQL comments annotate queries for readability and debugging. Single-line (-- comment) explains code, while multi-line (/* comment */) documents sections without affecting execution.
32. What is a SQL operator?
SQL operators perform calculations (+, -), comparisons (=, <, >), and logical evaluations (AND, OR, NOT), helping manipulate and filter data within SQL queries.
33. How does the CASE statement work in SQL?
It performs conditional logic, like IF-ELSE. Here’s an example:
34. How do you retrieve all columns from a table?
To retrieve all columns from a table, use the SELECT * FROM table_name; query, which fetches every record and field from the specified table.
Are you a Database Administrators or Data Analysts looking to advance your SQL proficiency? Our Advanced SQL Course will guide you - Sign up now!
35. D What is the difference between WHERE and HAVING in SQL?
The WHERE clause filters records before aggregation, while the HAVING clause filters results after aggregation, usually with GROUP BY.
36. Explain the SELECT statement
This statement retrieves data from tables:
37. What is the purpose of the COALESCE function?
This function replaces NULL with a default value. For example:
38. How do you rename a column in SQL?
A column in SQL can be renamed as follows:
39. How do you delete a column from a table?
A column in SQL can be deleted as follows:
40. What are some SQL query optimisation techniques?
To optimise SQL queries, we can use the following techniques: Use indexes for faster searches.
Avoid SELECT * to reduce data load.
Optimise joins by selecting necessary columns.
Use LIMIT to fetch only required records.
Analyse queries with EXPLAIN PLAN for performance insights.
41. What are the benefits of using sequences over identity columns?
Sequences generate unique numbers independent of tables, making them reusable across multiple tables. They offer greater flexibility, allowing customisation of increments, caching, and cycling, unlike identity columns, which are table-specific.
42. Explain the ACID properties in SQL
ACID ensures reliable transactions. Here’s what ACID stands for:
Atomicity guarantees all operations in a transaction succeed or fail together.
Consistency ensures data integrity.
Isolation prevents transactions from interfering.
Durability ensures changes persist even after system failures.
Learn how to design and deploy SSRS reports through our SQL Server Reporting Services (SSRS) Course - Sign up now!
43. What is a transaction in SQL?
A transaction is a series of SQL operations that are executed as a single unit for data integrity. It follows ACID principles, meaning all operations succeed (COMMIT) or fail (ROLLBACK), preventing data corruption during concurrent database interactions.
44. How does a primary key differ from a unique key?
A primary key identifies each record uniquely and does not allow NULL values. A unique key also ensures uniqueness but allows one or more NULL values, making it more flexible in scenarios where a primary key isn't needed.
45. What are common challenges in working with SQL databases?
Challenges include slow queries due to inefficient indexing, deadlocks from concurrent transactions, schema changes affecting dependencies, storage constraints, and data inconsistency caused by poor database normalisation or incorrect transaction handling in multi-user environments.
46. What are the different data types in SQL?
SQL supports various data types, including:
Numeric (INT, FLOAT, DECIMAL)
String (VARCHAR, TEXT, CHAR)
Date/Time (DATE, DATETIME, TIMESTAMP)
Boolean (BOOLEAN)
Binary (BLOB, VARBINARY)
47. What is a spatial database?
A spatial database stores and processes geographic and spatial data like maps, locations, and geometric objects. It supports queries involving distance calculations, geospatial indexing, and GIS applications, commonly used in location-based services and mapping software.
Join our comprehensive Programming Training tailored for learners at all levels - Sign up now!
48. What are aggregate functions in SQL?
Aggregate functions perform calculations on data sets and return a single value. Examples include:
SUM(): Adds numeric values
AVG(): Computes average
COUNT(): Counts records
MAX() / MIN(): Finds highest/lowest values
49. What are scalar functions in SQL?
Scalar functions return a single value depending on input. Examples include:
LEN(): Returns string length
UPPER(): Converts text to uppercase
ROUND(): Rounds numeric values
GETDATE(): Retrieves current date/time
50. What is the MERGE statement used for?
The MERGE statement is used to combine INSERT, UPDATE, and DELETE operations based on conditions. It synchronises data between tables, commonly used for upserting records. Consider this example:
51. What is a database snapshot, and how is it used?
A database snapshot is a read-only, static copy of a database at a specific point in time. It’s used for reporting, backup or recovery by rolling back changes without restoring the entire database.
52. Explain the concepts of normalisation and denormalisation
Normalisation reduces data redundancy by dividing large tables into smaller ones and
denormalisation improves performance by combining tables to minimise joins.
53. What are Common Table Expressions (CTEs)?
A CTE is a temporary result set within a SQL query, improving readability. Here’s an example:
54. Which operator is used for pattern matching in SQL queries?
The LIKE operator is used with wildcards (%, _) for pattern matching. Here’s an example:
55. What are the differences between isolation levels in SQL?
Isolation levels which control transaction visibility have the following differences:
READ UNCOMMITTED: It Allows dirty reads
READ COMMITTED: It prevents dirty reads
REPEATABLE READ: It prevents non-repeatable reads
SERIALIZABLE: It fully isolates transactions
56. What is RDBMS? Give some examples of RDBMS
A RDBMS organises data into structured tables with relationships using SQL. It ensures data integrity, scalability, and efficient querying. Examples of RDBMS include MySQL, PostgreSQL, Oracle, SQL Server and IBM Db2, widely used for enterprise applications.
57. What is a self-join, and how would you use it?
A self-join joins a table with itself using aliases. Here's an example:
58. What is a composite primary key?
A composite primary key consists of multiple columns that uniquely identify a row. Here's an example:
59. Write a query to retrieve employees who earn more than the average salary
Here’s how the query is written:
60. How do you add a record to a table?
INSERT is used to add a record to a table:
61. What is a subquery? Provide an example
A subquery is a nested query within another SQL query, used to retrieve data dynamically based on conditions from an outer query. Here's an example:
62. How does a clustered index function and how does it differ from a non-clustered index?
A clustered index arranges and stores table data physically in sorted order based on the indexed column, meaning only one exists per table. A non-clustered index maintains a separate structure with pointers referencing table rows, allowing multiple indexes on different columns for faster lookups.
63. What are the differences between OLTP and OLAP systems?
Online Transaction Processing (OLTP) supports real-time transactional operations.
While Online Analytical Processing (OLAP) is optimised for complex queries and reporting.
64. What is a materialised view?
A materialised view is a database object for storing a query's results as a physical table, unlike a regular view, which dynamically retrieves data. It improves performance by reducing computational overhead, as queries don’t need to be re-executed each time. Materialised views are particularly useful for precomputed aggregations, summary reports, and complex joins, making data retrieval faster.
65. How to find the last id in a table?
The last id in a table can be found using Use MAX() or ORDER BY:
Or
Conclusion
Mastering SQL is essential for any database-related roles, and being well-prepared for interviews can set you apart. The frequently asked SQL Interview Questions outlined in this blog will help you confidently tackle any query thrown your way. It’s about practicing, refining your skills, brushing up on your SQL knowledge base and being confident.
Master MySQL queries for data retrieval in our comprehensive Introduction to MySQL Training - Register now!
