As the demand for skilled data professionals continues to grow, SQL remains a critical and widely-used tool for managing and manipulating relational databases. Full form of SQL is Structured Query Language. In the fast-paced world of technology, SQL interview questions can be both challenging and competitive. Employers seek candidates who can demonstrate proficiency in SQL concepts, database design, query optimization, and data manipulation. Understanding the diverse range of SQL interview questions and answers that can arise during interviews is essential for success.
Whether you are a seasoned SQL expert or DBMS expert looking to brush up on your knowledge or a job seeker preparing for upcoming SQL Server or SQL DBMS or Oracle SQL , SQL Query interviews, this blog will cover an extensive collection of carefully curated SQL interview questions and their detailed answers. We have categorized these SQL interview questions to encompass a wide range of SQL topics, including database concepts, queries, joins, subqueries, indexes, transactions, and more. Each of the SQL interview questions is explained with clarity, enabling you to grasp the underlying concepts effectively.
1. What is SQL?
Ans: SQL stands for Structured Query Language and is a standard language used to manage and manipulate relational databases. It enables users to interact with the database by performing tasks such as querying data, inserting, updating, and deleting records.
2. What are the different types of SQL commands?
Ans: SQL commands can be broadly categorized into four types:
3. What is the difference between INNER JOIN and OUTER JOIN?
Ans: INNER JOIN returns only the matching rows from both tables based on the join condition. On the other hand, OUTER JOIN (LEFT JOIN, RIGHT JOIN, FULL JOIN) returns all matching rows as well as any unmatched rows from one or both tables.
4. Explain the difference between WHERE and HAVING clauses?
Ans: The WHERE clause is used with SELECT, UPDATE, and DELETE statements to filter rows based on specified conditions. The HAVING clause, however, is used with GROUP BY to filter group results based on specified conditions.
5. What is a primary key, and why is it important?
Ans: A primary key is a unique identifier for each row in a table. It ensures that each record can be uniquely identified and helps maintain data integrity. Primary keys are essential as they enforce uniqueness and provide the basis for relationships between tables.
SQL Basic Interview Questions
6. What are indexes in SQL, and how do they improve performance?
Ans: Indexes are database objects that speed up the data retrieval process by providing quick access to rows based on the indexed columns. When a query involves indexed columns, the database engine can use the index to find the required data faster, thus improving query performance.
7. What is the difference between UNION and UNION ALL?
Ans: Both UNION and UNION ALL are used to combine the results of two or more SELECT queries. The difference is that UNION removes duplicate rows from the combined result set, while UNION ALL retains all rows, including duplicates.
8. What is a subquery, and how is it used?
Ans: A subquery, also known as an inner query or nested query, is a query within another SQL statement. It is used to retrieve data that will be used as a condition or filter for the main query. Subqueries are enclosed within parentheses and can be used with SELECT, UPDATE, DELETE, or INSERT statements.
9. What is ACID in the context of database transactions?
Ans: ACID is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. It is a set of properties that guarantee database transactions are processed reliably and securely. Atomicity ensures that a transaction is treated as a single, indivisible unit of work. Consistency ensures that the database remains in a valid state before and after the transaction. Isolation prevents interference between concurrent transactions, and Durability ensures that committed changes are permanent and recoverable.
10. How do you handle NULL values in SQL?
Ans: NULL is used to represent missing or unknown data in SQL. To handle NULL values, you can use functions like IS NULL, IS NOT NULL, COALESCE, or NULLIF. Additionally, you can use the CASE statement to provide custom handling for NULL values in queries.
11. What is the difference between TRUNCATE and DELETE in SQL?
Ans: TRUNCATE is a DDL command used to remove all rows from a table, resulting in the release of storage space and a faster operation. It cannot be rolled back and does not trigger any individual row deletion triggers.
DELETE is a DML command used to remove specific rows from a table. It is slower than TRUNCATE and generates individual row deletion triggers if defined. DELETE can be rolled back within a transaction.
12. Explain the concept of normalization in databases?
Ans: Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves breaking down a table into smaller, related tables and establishing relationships between them. Normalization is achieved through a series of rules (normal forms) that ensure data is stored efficiently and without data anomalies.
13. What are stored procedures in SQL, and what are the benefits of using them?
Ans: Stored procedures are precompiled SQL code stored in the database and executed when called upon. Benefits of using stored procedures include:
14. Explain the concept of SQL injection and how to prevent it?
Ans: SQL injection is a security vulnerability where malicious SQL code is inserted into a query, allowing unauthorized access or manipulation of the database. To prevent SQL injection, use prepared statements or parameterized queries, which safely separate SQL code from user input, making it impossible for an attacker to alter the SQL statement’s structure.
15. What are aggregate functions in SQL? Provide some examples
Ans: Aggregate functions perform calculations on a set of rows and return a single value. Examples include:
16. How can you optimize a slow-performing SQL query?
Ans: To optimize a slow query, you can take various steps, including:
17. Explain the difference between a clustered and a non-clustered index?
Ans: A clustered index determines the physical order of data in a table and dictates how rows are stored on disk. Each table can have only one clustered index.
A non-clustered index is a separate data structure that contains a sorted list of key values, with a pointer to the actual data rows. A table can have multiple non-clustered indexes.
18. What is a self-join in SQL? Provide an example
Ans: A self-join is a technique where a table is joined with itself, treating it as two separate entities. It is often used to retrieve related data from the same table. For example:
SELECT e1.Name, e2.ManagerName
FROM Employees e1
JOIN Employees e2 ON e1.ManagerID = e2.EmployeeID;
19. Explain the difference between a view and a table in SQL?
Ans: A table is a physical entity that stores data persistently in a database.
A view is a virtual table that does not store data but is based on the result of a SELECT query. It provides a way to simplify complex queries and restrict access to specific data.
20. What is the purpose of the GROUP BY clause in SQL, and how does it work?
Ans: The GROUP BY clause is used in combination with aggregate functions to group rows based on the values in one or more columns. It allows you to perform calculations on groups of data rather than on individual rows. The result set will have one row for each group, with aggregated values based on the specified columns in the GROUP BY clause.
SQL Query Interview Questions
21. What is a foreign key in SQL?
Ans: A foreign key is a column or a set of columns in a table that establishes a link between data in two tables. It enforces referential integrity, ensuring that the values in the foreign key column(s) match the values of the primary key column(s) in the referenced table.
22. Explain the difference between a candidate key and a primary key?
Ans: A candidate key is a column or a combination of columns that can uniquely identify each row in a table.
The primary key is the selected candidate key that is chosen to uniquely identify the rows of the table. A table can have only one primary key, but it may have multiple candidate keys.
23. What is a self-contained subquery in SQL?
Ans: A self-contained subquery is a subquery that can run independently and does not depend on the outer query for its results. It can be executed on its own and will return a result set that can be used by the outer query.
24. Explain the difference between the UNION and UNION ALL operators?
Ans: UNION combines the results of two or more SELECT queries into a single result set, removing duplicate rows.
UNION ALL also combines the results of SELECT queries into a single result set but retains all rows, including duplicates.
25. How can you find the nth highest/lowest value in a column?
Ans: You can use a combination of ORDER BY and LIMIT (or FETCH FIRST) clauses to find the nth highest/lowest value. For example, to find the 3rd highest salary:
SELECT salary
FROM employees
ORDER BY salary DESC
LIMIT 1 OFFSET 2;
26. Explain the difference between a transaction and a savepoint?
Ans: A transaction is a sequence of one or more SQL statements that are executed as a single unit of work. It must be either fully completed or fully rolled back in case of an error.
A savepoint is a marker within a transaction that allows you to set a point to which the transaction can be rolled back later without affecting the rest of the transaction.
27. What are the ACID properties of a transaction?
Ans: Atomicity: Ensures that a transaction is treated as a single, indivisible unit of work. It must either be fully executed, or none of its changes should be applied.
Consistency: Ensures that a transaction takes the database from one consistent state to another. All data modifications must follow defined rules.
Isolation: Ensures that the intermediate state of a transaction is invisible to other transactions until it is committed.
Durability: Ensures that once a transaction is committed, its changes are permanent and survive any subsequent system failures.
28. Explain the concept of data integrity in databases?
Ans: Data integrity refers to the accuracy, consistency, and reliability of data stored in a database. It is maintained through constraints like primary keys, foreign keys, and check constraints that prevent invalid data from being entered into the database.
29. How can you perform conditional INSERT and UPDATE in a single SQL statement?
Ans: You can use the INSERT INTO… ON DUPLICATE KEY UPDATE statement (MySQL) or MERGE statement (e.g., in Oracle) to perform conditional INSERT and UPDATE operations in a single query. This allows you to insert a new row or update an existing one if a duplicate key is found.
Different Types Sql Interview Questions for Freshers
30. Explain the concept of data warehousing in SQL?
Ans: Data warehousing is the process of collecting, storing, and managing data from various sources to support business intelligence and decision-making activities. It involves extracting data from operational databases, transforming it into a consistent format, and loading it into a data warehouse for analytical processing and reporting.
31. What is the difference between a correlated subquery and a regular subquery?
Ans: A correlated subquery is a subquery that refers to a column from the outer query, creating a dependency between the two queries. It executes once for each row in the outer query, making it less efficient than a regular subquery.
A regular subquery, also known as a non-correlated subquery, can execute independently and does not reference columns from the outer query.
32. Explain the purpose of the GROUPING SETS clause in SQL?
Ans: The GROUPING SETS clause is used to specify multiple groupings within a single query, allowing you to aggregate data at different levels. It helps produce a result set with subtotals and grand totals for various combinations of grouping columns.
33. What is the difference between the CHAR and VARCHAR data types in SQL?
Ans: CHAR is a fixed-length character data type that stores a specific number of characters. It pads spaces to the right to fill the entire length.
VARCHAR is a variable-length character data type that stores data with a varying number of characters, using only the actual space required without padding.
34. Explain the concept of a deadlock in SQL and how to prevent it?
Ans: A deadlock occurs when two or more transactions are waiting for each other to release resources, preventing them from proceeding. To prevent deadlocks, database systems use various techniques, such as locking mechanisms, timeouts, and deadlock detection algorithms.
35. What is the purpose of the COALESCE function in SQL?
Ans: The COALESCE function returns the first non-null expression from a list of expressions. It is often used to handle NULL values and provide a default value if the first expression evaluates to NULL.
36. Explain the use of the ROLLUP operator in SQL?
Ans: The ROLLUP operator is used with the GROUP BY clause to generate subtotals and grand totals for groups of data. It creates a hierarchy of grouping levels and calculates aggregates for each level, leading up to the grand total.
37. How can you enforce a unique constraint on multiple columns in a table?
Ans: To enforce a unique constraint on multiple columns, you can use the UNIQUE keyword with the combination of columns you want to be unique. For example:
ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2);
38. What is a recursive common table expression (CTE) in SQL?
Ans: A recursive CTE is a CTE that references itself in its definition. It is used to perform recursive operations, such as traversing hierarchical data or calculating recursive sequences.
39. Explain the purpose of the LAG and LEAD functions in SQL?
Ans: LAG: The LAG function allows you to access the value of a specific column from the previous row within the result set.
LEAD: The LEAD function allows you to access the value of a specific column from the next row within the result set. These functions are often used to compare values between adjacent rows.
SQL Server Interview Questions
40. What is the SQL CASE statement, and how can you use it?
Ans: The CASE statement allows you to perform conditional logic in SQL. It can be used in SELECT, UPDATE, and other SQL statements. It has two forms, simple and searched. In the simple form, you specify an expression, compare it with several values, and return a result. In the searched form, you provide multiple conditions and return a result based on the first matching condition.
41. What is a materialized view in SQL?
Ans: A materialized view is a database object that stores the results of a query and refreshes its data periodically. It allows for faster data retrieval by precomputing and storing the query results, reducing the need for complex joins and calculations.
42. Explain the purpose of the HAVING clause in SQL?
Ans: The HAVING clause is used in conjunction with the GROUP BY clause to filter the results of a grouped query based on specified conditions. It acts as a filter for aggregated data, allowing you to filter out groups based on aggregate functions like SUM, AVG, COUNT, etc.
43. What are SQL window functions, and how are they different from aggregate functions?
Ans: SQL window functions, also known as analytic functions, perform calculations across a set of rows related to the current row. Unlike aggregate functions that produce a single result per group, window functions provide a result for each row without reducing the number of rows in the result set.
44. Explain the difference between UNION and UNION ALL in terms of performance and result set?
Ans: UNION: Performs a distinct operation and removes duplicate rows from the combined result set of two or more SELECT queries. It may have a slightly higher performance overhead due to duplicate removal.
UNION ALL: Does not remove duplicates and retains all rows from the combined result set, making it potentially faster than UNION because it avoids the duplicate removal step.
45. How can you handle errors and exceptions in SQL?
Ans: In SQL, you can use the TRY…CATCH construct to handle errors and exceptions. Within the TRY block, you place the code that might cause an error, and in the CATCH block, you specify the action to be taken if an error occurs, such as logging the error or rolling back the transaction.
46. Explain the purpose of the EXISTS operator in SQL and how it works?
Ans: The EXISTS operator is used to test the existence of rows returned by a subquery. It returns true if the subquery returns any rows; otherwise, it returns false. It is often used in conjunction with correlated subqueries to check for the existence of related data.
47. What is a surrogate key in SQL, and when should it be used?
Ans: A surrogate key is an artificial, system-generated primary key used to uniquely identify rows in a table. It is often an integer value that has no real-world meaning and is used when there is no natural candidate key or when natural keys might change over time.
48. Explain the purpose of the OFFSET-FETCH clause in SQL?
Ans: The OFFSET-FETCH clause is used to implement pagination in SQL. The OFFSET value specifies the number of rows to skip from the beginning of the result set, and the FETCH value indicates the number of rows to return after the OFFSET.
49. What is a correlated update in SQL, and how does it work?
Ans: A correlated update is an UPDATE statement that references another table in the WHERE clause, updating the rows in the target table based on the values retrieved from the correlated table.
50. Explain the difference between a primary key and a unique key constraint?
Ans: A primary key is a special type of unique key that uniquely identifies each row in a table and does not allow NULL values.
A unique key constraint, on the other hand, enforces the uniqueness of values in a column or a combination of columns but can allow NULL values (except in cases where the unique key is applied to a single column).
SQL Interview Questions for Experienced
51. What is the difference between a natural join and an equi join in SQL?
Ans: A natural join is an implicit join based on columns with the same names in both tables. It automatically matches and combines rows where the column names match.
An equi join is an explicit join that uses the equality operator (=) to combine rows from two tables based on a specified condition.
52. Explain the purpose of the TRIGGER in SQL?
Ans: A trigger is a database object that automatically executes a set of SQL statements when a specific event (INSERT, UPDATE, DELETE) occurs on a table. Triggers are used to enforce business rules, maintain data integrity, or perform actions in response to data changes.
53. What is a SQL Injection attack, and how can you prevent it?
Ans: SQL Injection is a type of cyber attack where malicious SQL code is inserted into input fields to manipulate the database. To prevent SQL Injection, use parameterized queries or prepared statements, which separate SQL code from user input, making it immune to injection.
54. Explain the difference between the OUTER APPLY and CROSS APPLY operators?
OUTER APPLY returns all rows from the left table, along with matching rows from the right table. If no match is found, it returns NULL values for the right table columns.
CROSS APPLY returns only rows that have matching rows in both tables. It filters out rows without matches, making it similar to an INNER JOIN.
55. What are Common Table Expressions (CTEs) in SQL, and when should you use them?
Ans: CTEs are temporary result sets that can be used within a SQL statement. They help improve query readability, simplify complex queries, and avoid redundant subqueries. Use CTEs when you need to reference the same subquery multiple times in a query.
56. Explain the difference between a clustered and non-clustered index?
Ans: A clustered index determines the physical order of data in a table. Each table can have only one clustered index, and it directly affects how data is stored on the disk.
A non-clustered index is a separate data structure containing a sorted list of key values, with a pointer to the actual data rows. A table can have multiple non-clustered indexes.
57. What is the purpose of the MERGE statement in SQL?
Ans: The MERGE statement allows you to perform both INSERT and UPDATE operations in a single SQL statement. It matches rows between the source and target tables and executes the specified actions based on whether a match is found or not.
58. Explain the concept of data denormalization in SQL?
Ans: Data denormalization is the process of intentionally introducing redundancy in a database by combining tables or duplicating data. It is done to improve query performance, reduce joins, and simplify data retrieval in certain scenarios, especially for reporting and analytical purposes.
59. How can you find the second highest/lowest value in a column without using the LIMIT clause?
Ans: You can use the OFFSET clause along with ORDER BY to find the second highest/lowest value in a column. For example, to find the second highest salary:
SELECT salary
FROM employees
ORDER BY salary DESC
OFFSET 1 LIMIT 1;
60. Explain the purpose of the SESSION_USER and CURRENT_USER functions in SQL?
Ans: SESSION_USER returns the username of the current database session.
CURRENT_USER returns the username of the currently executing user within the context of the SQL statement. It can be different from SESSION_USER if the statement is executed within a definer’s rights stored procedure or a function.
Summing up – SQL Interview Questions and Answers
SQL is a powerful language that lies at the core of database management, and mastering its intricacies can open doors to exciting career opportunities.
Throughout this blog, we covered a wide array of SQL topics, ranging from fundamental concepts to more advanced techniques. By now, you should have a solid understanding of data normalization, join types, window functions, subqueries, indexes, transactions, and much more. Our detailed explanations and example queries have aimed to provide you with a holistic view of SQL and its applications in real-world scenarios.
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