Whenever you search for SQL interview questions, almost every website pops up with the same set of easy-level questions and answers. As a SQL developer, repeating the basics may not be beneficial. If you ask why, let me explain. Since we all know SQL developers are in great demand across various industries, as organizations rely on databases to store and manage their data, the same happens with competition—yes, it's booming.
I hope you realize that you will be competing with many advanced, talented developers for the position you're preparing for. Given the high competition, do you think practicing only basic SQL interview questions will be sufficient? It helps only to some extent. Basic SQL questions serve as an initial filter in the hiring process, as employers often use them to quickly assess whether candidates possess the minimum knowledge required for a particular role.
Since we realized that candidates are seeking difficult-level questions and answers (so you can give 100% competition), in this article, we will provide 50 SQL interview questions that will help you shift yourself to the advanced level. You can cover all these questions after practicing the basic ones.
SQL INTERVIEW QUESTIONSÂ
Difficulty Level = MediumÂ
1. Question: What is the difference between INNER JOIN and LEFT JOIN?
Answer: INNER JOIN retrieves matching records from both tables, while LEFT JOIN retrieves all records from the left table and matching records from the right table.
2. Question: Explain the purpose of the GROUP BY clause.
Answer: The GROUP BY clause serves the purpose of grouping rows in a SQL query based on the values of one or more specified columns. It is often used with aggregate functions like COUNT, SUM, AVG to perform operations on each group of rows.
3. Question: How do you remove duplicate records from a table?
Answer: Duplicate records can be removed using the DISTINCT keyword in a SELECT statement or by using the DELETE statement with a self-join.
4. Question: What is the difference between UNION and UNION ALL?
Answer: UNION combines and returns distinct rows from two or more SELECT statements, while UNION ALL returns all rows, including duplicates.
5. Question: Explain the purpose of the HAVING clause.
Answer: The HAVING clause is used in conjunction with the GROUP BY clause to filter the results of aggregate functions based on specified conditions.
6. Question: How can you update the values of a specific column in a table?
Answer: The UPDATE statement is what you use in SQL to change the values of existing records in a table. If you want to update a specific column with a new value under certain conditions, you'd use something like this: UPDATE table_name SET column1 = value1 WHERE condition;
7. Question: What is a subquery?
Answer: A subquery is a query nested within another query. It can be used to retrieve data that will be used by the main query as a condition or to perform calculations.
8. Question: Explain the purpose of the CASE statement.
Answer: The CASE statement is used to perform conditional logic within a SQL query. It can be used in SELECT, WHERE, and ORDER BY clauses to customize the output or conditions.
9. Question: What is normalization in the context of databases?
Answer: Normalization is a method used in database management to organize data systematically with the aim of minimizing redundancy and dependency. It includes the division of large tables into smaller, more manageable ones and the establishment of relationships between these tables.
10. Question: How do you find the second highest (or nth highest) value in a column?
Answer: The second highest value can be found using the LIMIT and OFFSET clauses in combination with the ORDER BY clause. For example: SELECT column FROM table ORDER BY column DESC LIMIT 1 OFFSET 1;
11. Question: Explain the purpose of the COALESCE function.
Answer: The COALESCE function is used to return the first non-null expression among its arguments. It is often used to replace null values with a default or alternative value.
12. Question: What is the purpose of the ROW_NUMBER() function?
Answer: ROW_NUMBER() is a window function that assigns a unique number to each row within a partition of a result set. It is commonly used for ranking and pagination.
13. Question: How can you calculate the total number of rows in a table?
Answer: The COUNT function is used to calculate the total number of rows in a table. For example: SELECT COUNT(*) FROM table;
14. Question: What is a foreign key?
Answer:Â A foreign key is a field that points to the primary key in a different table, creating a connection between the two tables and ensuring referential integrity.
15. Question: Explain the purpose of the TRUNCATE TABLE statement.
Answer: The TRUNCATE TABLE statement is used to quickly delete all rows from a table, but it does not log individual row deletions, making it faster than the DELETE statement.
16. Question: How do you retrieve the top N rows from a table?
Answer: The LIMIT clause is used to retrieve the top N rows from a table. For eg: SELECT * FROM table_name LIMIT N;
17. Question: What is the difference between a primary key and a unique key?
Answer: Both primary and unique keys enforce uniqueness, but a table can have only one primary key, while it can have multiple unique keys.
18. Question: Explain the purpose of the IS NULL and IS NOT NULL conditions.
Answer: The conditions IS NULL and IS NOT NULL are used to check whether a column contains null values or is not null, respectively.
19. Question: How can you perform a case-insensitive search in SQL?
Answer: To perform a case-insensitive search, you can use the LOWER() or UPPER() function. For example: SELECT * FROM table WHERE LOWER(column) = 'value';
20. Question: What is the purpose of the INDEX in a database?
Answer: An INDEX is used to improve the speed of data retrieval operations on a database table by providing a quick lookup mechanism.
21. Question: How do you calculate the average of a column?
Answer: The AVG() function is used to calculate the average of a column. For example: SELECT AVG(column) FROM table;
22. Question: Explain the purpose of the EXISTS condition.
Answer: The EXISTS condition is used to test for the existence of any records in a subquery. It returns true: if the subquery returns one or more rows.
23. Question: What is a self-join?
Answer: A self-join is a regular join, but the table is joined with itself. It is often used when a table has a foreign key that references its own primary key.
24. Question: How can you add a new column to an existing table?
Answer: The ALTER TABLE statement is used to add a new column to an existing table. For example: ALTER TABLE table_name ADD COLUMN new_column datatype;
25. Question: Explain the purpose of the CASCADE option in a foreign key constraint.
Answer: The CASCADE option, when used in a foreign key constraint, automatically deletes or updates the corresponding rows in the child table when the referenced rows in the parent table are deleted or updated.
SQL INTERVIEW QUESTIONS (26-50)
Difficulty Level = High
26. Question: Explain the differences between UNION and UNION ALL with examples.
Answer: UNION combines and returns distinct rows, while UNION ALL returns all rows, including duplicates. For instance, if you have two SELECT statements, SELECT column FROM table1 UNION SELECT column FROM table2; would eliminate duplicates, whereas SELECT column FROM table1 UNION ALL SELECT column FROM table2; would include all rows.
27. Question: What is a correlated subquery, and how is it different from a regular subquery?
Answer: A correlated subquery is one that makes references to columns from the outer query. This is in contrast to a regular subquery, which is executed only once and the result is used for the outer query.
28. Question: Explain the purpose and usage of the RANK() window function.
Answer: The RANK() window function assigns a unique rank to each distinct row within the result set. It handles ties by assigning the same rank to rows with equal values. For example, SELECT column, RANK() OVER (ORDER BY column DESC) AS ranking FROM table;
29. Question: How can you pivot data in SQL, and provide an example.
Answer: Pivoting involves transforming rows into columns. The PIVOT operator is used for this purpose. For instance, SELECT * FROM (SELECT column1, column2 FROM table) AS SourceTable PIVOT (MAX(column2) FOR column1 IN ([Value1], [Value2], [Value3])) AS PivotTable;
30. Question: Explain the concept of window functions in SQL.
Answer: Window functions perform calculations across a specified range of rows related to the current row. Examples include ROW_NUMBER(), RANK(), DENSE_RANK(), LEAD(), and LAG(). These functions are applied to a "window" of rows defined by an OVER clause.
31. Question: What are CTEs (Common Table Expressions) and why are they used?
Answer: CTEs are temporary result sets defined within the context of a SELECT, INSERT, UPDATE, or DELETE statement. They enhance readability and simplify complex queries. For example, WITH MyCTE AS (SELECT column FROM table) SELECT * FROM MyCTE;
32. Question: How do you handle errors in SQL transactions?
Answer: Errors in SQL transactions can be managed using the TRY...CATCH block. This allows you to handle exceptions and execute specific actions in case of errors.
33. Question: Explain the purpose and usage of the LEAD() and LAG() window functions.
Answer: The LEAD() function retrieves data from the next row, while LAG() retrieves data from the previous row within the result set. They are useful for comparing values in the current row with values in subsequent or preceding rows.
34. Question: How can you optimize a SQL query's performance?
Answer: Query performance can be optimized by creating indexes on columns used in WHERE clauses, avoiding the use of SELECT * and retrieving only necessary columns, and optimizing JOIN and WHERE conditions.
35. Question: What is the purpose of the MERGE statement, and how is it used?
Answer: The MERGE statement is used to perform multiple DML operations (INSERT, UPDATE, DELETE) in a single statement based on a condition. It simplifies complex operations involving data synchronization.
36. Question: Explain the differences between a clustered and non-clustered index.
Answer: A clustered index determines the physical order of data in a table, while a non-clustered index creates a separate structure to store the index data, keeping the actual data in the table unchanged.
37. Question: How do you implement row-level security in SQL?
Answer: Row-level security can be implemented using views, stored procedures, or security policies. Views and stored procedures can filter data based on user roles, while security policies enable more fine-grained control.
38. Question: What is the purpose of the XML data type in SQL Server, and how is it used?
Answer: The XML data type allows storage and manipulation of XML data. It is used to store XML documents, and various built-in functions enable querying and modification of XML data.
39. Question: Explain the differences between a heap table and a clustered table.
Answer: A heap table is a table without a clustered index, while a clustered table is one where data is physically ordered based on the clustered index. Heap tables may have no defined order.
40. Question: How can you implement full-text search in SQL Server?
Answer: Full-text search can be implemented using the CONTAINS() or FREETEXT() functions along with the CONTAINSTABLE() or FREETEXTTABLE() functions, depending on the complexity of the search.
41. Question: Explain the purpose and usage of the CROSS APPLY and OUTER APPLY operators.
Answer: CROSS APPLY and OUTER APPLY are used to invoke a table-valued function for each row in the outer table expression. CROSS APPLY returns only rows that produce a result from the table-valued function, while OUTER APPLY returns all rows, filling in NULLs for rows without a result.
42. Question: How can you create a recursive query in SQL?
Answer: Recursive queries can be created using the Common Table Expressions (CTEs) and the WITH RECURSIVE clause. This allows a query to refer to its own output, enabling hierarchical queries.
43. Question: Explain the purpose and usage of the PERSISTED keyword.
Answer: The PERSISTED keyword is used in computed columns. It stores the computed column's value physically on the disk, making it persistent and allowing for indexing.
44. Question: What is the purpose of the NOLOCK hint in SQL Server?
Answer: The NOLOCK hint is used to read uncommitted data in a SELECT statement. It is often used to prevent locking and improve query performance but may lead to reading uncommitted or dirty data.
45. Question: How can you pivot dynamic columns in SQL?
Answer: Dynamic column pivoting involves using dynamic SQL to generate a pivot query with unknown columns. It typically involves constructing a dynamic SQL statement based on the available data.
46. Question: Explain the differences between a database-level trigger and a table-level trigger.
Answer: A database-level trigger fires in response to database events, such as login events, while a table-level trigger fires in response to specific actions on a particular table, such as INSERT, UPDATE, or DELETE operations.
47. Question: What is the purpose of the APPLY operator in SQL Server?
Answer: The APPLY operator is used to invoke a table-valued function for each row returned by the outer table expression. It is similar to a join but allows for more flexibility, especially with table-valued functions.
48. Question: Explain the concept of database sharding.
Answer: Database sharding involves dividing a large database into smaller, more manageable parts called shards. Each shard is an independent database with its schema, and this approach is often used to distribute data horizontally across multiple servers.
49. Question: How do you handle transactions across multiple databases in SQL?
Answer: Handling transactions across multiple databases can be achieved using distributed transactions or by manually coordinating commits and rollbacks between the databases involved.
50. Question: What are the considerations for implementing a secure SQL Server environment?
Answer: Implementing a secure SQL Server environment involves considerations such as managing user permissions, encrypting sensitive data, applying the principle of least privilege, regularly auditing activities, and keeping the server and database software up-to-date.
I've grouped the interview questions and answers into two sections. The first set of 25 questions and answers is under the "Medium Difficulty Level" category, while the next set of 25 falls into the "High Difficulty Level" category.Â
Do any of the questions sound new to you? I can guess it's a yes. Practice makes perfect, and in this case, mastering these SQL interview questions will benefit you in the long run. let's ensure we bring our A-game and stand out as strong contenders among other candidates.
