50+ SQL Interview Questions Answers

Today, most businesses depend on data to manage their information. And SQL (Structured Query Language) is one of the essential database programming languages in the Information Technology (IT) industry. Top companies, MNCs, corporates, etc always have a penchant for professionals with exceptional skills in data management. SQL remains one of the fundamentals for building a career in database management. This essential list of SQL interview questions comprise different section with basic, mid-level, and experienced for professionals.

SQL has been an important part of the computer world ever since the Internet boomed in the last century. If you want to build your career in data science, data analysis, or simple database systems, you must consider SQL in your skillset. In addition, SQL is regularly used in conjunction with more popular programming languages such as Python or Java to scrutinize and manage data.

It’s simple and easy to learn, and people with SQL knowledge are in high demand. Getting SQL in your bag of tricks is a fantastic way to broaden your developer’s flexibility and relevance. Let us now focus on some of the interview questions of SQL Online Training to make them clearer.

Interviews are the first point of contact for getting attention from top recruiters. Professionals should prioritize going through these SQL interview questions with answers to be ready and have confidence in achieving their dream.

Here are the top SQL interview questions that you should know to have the confidence for moving ahead in your career.

Top 100+ SQL Interview Questions & Answers in 2022

1. What is exactly SQL?

SQL is Structured Query Language, a computer language needed to store, attempt to alter, and fetch data stored in a database system. Developed by IBM in the 1970s and afterward acquired by Oracle. Nowadays, most relational databases use SQL to write queries, availability, modify, and recover information.

2. What is a database?

A database is a structured collection of data that is digitally stored and retrieved from either a locally or remotely computer system. Databases can be large and complex, and they are created utilizing corrected design and modeling approaches. As opposed to a spreadsheet, a database can easily manage huge amounts of data whilst also enabling authorized users to share the same Database to run safe and self-reliant queries.

3. What is RDBMS?

Relational Database Management System also called RDBMS and it is the foundation of all present database systems such as ORACLE, SQL, MS SQL Server, IBM DB2, MySQL, and so on. It is primarily a database that employs a table from schema to organize multiple data elements that are related to one another. It allows you to define, generate, and retain relational databases while also offering constrained access to the data contained within.

4. What Distinguishes RDMS from DBMS?

A DBMS, or Database Management System, works in the same way as the RDBMS defined in the previous paragraph. On the other hand, DBMS data elements are stored as file systems instead of tabularly, so there is no connection between them. DBMS are divided into four types:

• Hierarchical Database
• Network Database
• Relational Database 
• Object-oriented Database

5. What is Normalization, and What are its Different Types?

Normalization is used to reduce data duplication and dependency by organizing fields and tables in databases. It entails creating tables and establishing relationships between them according to predefined rules. These rules can be used to eliminate redundancy and conflicting dependency, making normalization more flexible.

The various types of normalization are as follows:

• First Normal Form
• Second normal form 
• Third normal form
• Boyce Codd normal form

6. What Exactly is Denormalization?

Denormalization is the inverse of normalization; superfluous information is uploaded to expedite complex data that require the joining of multiple tables. Attempts are made to improve a database’s read performance. Additionally or combining duplicative copies of data.

7. Describe the Various Types of SQL Commands?

• DDL(Data definition language): Creates and restructures relational database objects like tables.
• DML(Data manipulation language): Used for data manipulation inside relational database things.
• DQL ( Data query language): DQL is often used to conduct queries inside a relational database with a single command.
• DCL (Data control language): Controls access to information inside a relational database.
• Data administration command: Analyzes database operational processes or performs audits.
• TCL ( transactional control language): A relational database has been used to start managing transactions.

8. What are SQL joins?

A join clause is used in a SQL database to combine records or rows from multiple, two or more, tables relevant to the research columns between the tables.

9. Describe the Various Types of SQL Joins.

SQL joins are classified into four types:

• Inner join: It can be used to extract records with matching values from both tables engaged in the join. The inner join is most commonly also used in join queries.
• (Left)Outer join: It is a type of join which fetches all records or rows from the left and the corresponding ones from the right. 
• (Right)Outer join: It is being used to fetch all of the record-keeping or rows from the right and the ones that are equalled from the left.
• (Full)Outer join: A full join is often used to fetch records that have a fit either in the left or right table.

10. What’s the Distinction Between Both SQL and MySQL?

SQL is a programming language, whereas MySQL is a popular open-source relational database management system. SQL is being used to connect, edit, upgrade, and handle data in MySQL, whereas MySQL can be used to organize and store data.

11. What are SQL Constraints?

Constraints are often used to explicitly state the rules that apply to the data in the table. It can be applied to single or multiple fields in a SQL table during or after founding by using ALTER TABLE command. 

12. What are Different Types of Constraints?

• NOT NULL – Prevents the insertion of a NULL value into a column.
• CHECK – Checks to see whether most values inside a field fulfil a condition.
• DEFAULT – Assigns a default value to a field if no value has indeed been specified.
• UNIQUE – Ensures that only unique values are placed into the field.
• INDEX – Categorizes a field to allow for faster record retrieval.
• PRIMARY KEY – Recognizes each record inside a table uniquely.
• FOREIGN KEY – Maintains the referential integrity of a record in another table.

13. What is a Self Join?

When a table is joined to itself, every table row has been coupled with itself and each other row in the table. A JOIN clause joins rows from multiple tables bound by a common column. A self-join is similar to a regular join, except that the table has been joined with itself – this is very helpful for similarities inside a table.

14. What Exactly is a Cross Join? (Cartesian join)

The cross joins in SQL integrate each row from the first table with every row from the second table. It is also recognized as the Cartesian join because it gets back the Cartesian product of them ended up joining tables’ sets of rows.

15. What is Referential Integrity Constraint?

Retaining data integrity through some kind of system of regulations constrains the value systems of multiple columns of the tables depending on the principles of the referenced table’s primary key or unique key.

16. What Exactly is the Distinction Between a WHERE Clause and a HAVING Clause?

The WHERE clause would be used to specify the first condition, which categorizes and needs to return only the rows that satisfy that circumstance into a result set. After which, by using the HAVING clause, secondary circumstances can be applied to recover only the groups within which it sets that fulfil your new guidelines.

17. What Exactly is an Index?

A DBMS index is a data structure that allows for rapid retrieval of data in either a table’s column or columns. It speeds up operations that access data from a database table at the expense of additional writes and memory necessary to keep the index data structure.

18. Different Types of Indexes.

• Unique and Non-Unique Index: Unique indexes assist in maintaining information intended to ensure that no two rows of the database table have the same key value. When one table defines a unique index, uniqueness is imposed anytime keys are introduced or altered within the index.

Non-unique indexes are not used to impose limitations on the tables they are associated with

• Clustered and Non-Clustered Index: Clustered indexes are those in which the row order in the Database relates to the row order in the index. 

The only distinction between clustered and non-clustered indexes is that the database manager works hard to keep the information in the Database in much the same order as the correlating keys show up in the clustered index.

19. What’s the Difference Between a TRUNCATE Command and a DELETE Command?

• Delete: Removes particular rows or the tuples.
• Truncate: Delete all the rows and relations.

20. What is a Query?

A query is a sequence of commands crafted in a query language such as SQL that allows a person to access data stored in a database.

21. What do you Mean by Subquery?

A subquery, also known as the nested query, is actually a query inside a query.

22. How Many Types of a Subquery are There?

There are two types of a subquery:

• Correlated query: correspond to either a column in a table indicated by that of the main query’s FROM keyword
• Non-correlated: Subqueries are self-contained, and their outcome is used to supplement the main query.

23. What do you mean by Data Integrity?

Data Integrity is the affirmation of the data management system throughout its entire life cycle, and it is an important aspect of the configuration, execution, and use of any system that stores, mechanisms, or recovers information. This even identifies integrity constraints, which are used to implement business rules on data once it is reached into the application or Database.

24. What is UNION?

The UNION function joins the outcomes of two or more SELECT statements.

25. What is UNION ALL?

The UNION ALL performed to examine this very same operation as UNION, but it encompasses redundant rows inside its result set.

26. What is Intersect?

The INTERSECT functionality joins the outcome of two SELECT statements but gets back only the rows with the model can effectively in both sets.

27. What is MINUS?

The MINUS operation joins the output from the two SELECT statements but really only gives or returns rows with values from the first set of results.

28. What Exactly are Scalar Functions?

The user defines scalar functions, which return a single number (i.e., int, char, float, etc.) depending on the input data.

Following are some of the scalar functions-

• CONCAT() joins multiple character strings together.
• FORMAT() specifies the format for displaying a set of values.
• LEN() returns the total length of a column.
• MID() helps in extracting substrings from a string value bundle.
• ROUND() is used to round the integer value of a numeric field.
• The existing date and time are returned by NOW().
• RAND() generates a random sequence of numbers of a given length.

29. How do You Deal With Expectations in SQL Server?

SQL Server’s TRY and CATCH blocks control exceptions. To control expectations, kept the SQL statement in the TRY block and the code in the CATCH block. Whether there’s an error in the code in the TRY block, the control will move to the CATCH block automatically.

30. Entities and Relationships: What Are They?

Entity: An entity can be a tangible or intangible real-world object that is easily identifiable.

Relationships: Relationships or links among both entities which have more in common

31. What Exactly are Aggregate Functions?

Also referred to as group functions are applied to a wide variety of values (or all values) in order to compute and return a single number.

Types of aggregate functions:

• AVG computes the mean of all values in a collection.
• COUNT returns the total number of rows in a group, including those with NULL values.
• MIN and MAX are functions that return the minimum and maximum value in a group, in both.
• SUM, the sum of all non-NULL values in a group, is returned by SUM.
• STDDEV is a function that computes the standard deviation.
• VARIANCE computes variance.

32. What do you Mean by Auto-Increment:

Autoincrement is basically used when a new record is created. It creates a unique number in the SQL table. As we know, that primary key is unique. It is added as an auto-increment field, and a new record is added.

33. What is Exactly “TRIGGER” in SQL?

A trigger is a fully automated process that occurs whenever an event happens in the database server. It contributes to the table’s integrity. When commands such as insert, update, and delete are issued, the trigger is triggered.

34. What Do You Mean by Alias?

An alias is a SQL function that is accepted by the majority, though not all, RDBMSs. It is a temporary name given to a table or table column for the purposes of a SQL query. Furthermore, aliasing can be used as a misinformation technique to protect the true names of database fields. A correlation name is another name for a table alias.

35. In SQL Server, How Many Different Authentication Modes are There? And what Exactly are They?

SQL Server supports two authentication methods. They are as follows:

 Windows Authentication Mode: It enables Windows authentication and not SQL Server authentication.

Mixed Mode: It supports both Windows and SQL Server authentication.

36. What are Your Thoughts On the Stuff() Function?

Basically, stuff() is trimming or deleting a part of a string and further adding it to another string part starting from a specific position.

37. What Exactly is a Stored procedure?

Rather than repeating the same SQL query, you could save this as a stored procedure and call it whenever it is needed to execute it.

38. What Exactly is OLTP?

OLTP, the stands for Online Transaction Processing, is software that can support transaction-oriented programs. The ability of an OLTP system to manage concurrency is a critical feature. OLTP systems are frequently decentralized to prevent vulnerabilities. These systems are typically designed for a large number of participants who perform brief transactions. Database queries are typically simple, with sub-second response times and a small number of records returned.

39. What are The Functions of Manipulating Characters?

It is generally the functions that can perform edit, alter or modifier, or reformation of a strings character. 

40. What are the Various Types of Replication in SQL Server?

There are three types of replications available in SQL Server:

• Replication of snapshots
• Replication of transactions
• Replication of merges

Conclusion:

It is the age of digitization, and new technologies are being introduced daily for various purposes, such as application development or web app development. However, SQL has been a fantastic piece of technology for a decade. A SQL Developer’s future scope is not limited to Computer Science but also includes Retail, Finance, Healthcare, Science & Technology, Public Sector, etc. Every company needs a database to store its information. There are numerous reasons why you should pursue a career in SQL.

Q. How would you like to define SQL in general?

A1. Structured Query Language or simple SQL has become a synonym for database management in the IT industry. SQL provides a platform for querying, storing, retrieving, and manipulating data from the relational databases management system (RDBMS).

SQL follows ANSI (American National Standards Institute) and remains as the standard language with different versions. Most of the systems today such as MS Access, MySQL, Postgres, Sybase, Oracle, SQL Server, etc use SQL as the main query and database language.

Q2. When did SQL come into existence?

A2. Here are some of the most essential snippets in the history of SQL.
1970 – Popularly known as the Father of relational databases, Dr. Edgar F. Ted Codd from IBM built the relational model for databases.
1974 – Structured Query Language came into existence.
1986 – IBM developed the prototype and then SQL was adopted as the standard for American National Standards Institute (ANSI)
1987 – And thereafter as the standard for International Organization for Standardization (ISO).

Q3. Define what is a database!

A. Electronic information about a person or things is collected in the form of data. And A database represents an organized form of data. This database is maintained inside a database management system with the ability to data access, data storage, data retrieval, and data manipulation in every aspect.

Almost all of the organizations today have some electronic data in the form of databases to secure information about their operations. Databases are Structured to ease analysis and perform tasks precisely.

4. Define DBMS And their types in general!

DBMS or Database Management systems are the programs or platform that allows the creation and maintenance of a database. DBMS acts as a file manager for controlling data inside the databases.

DMBS allows a user to build and create new databases, access data and fetch queries from present databases. Thus acting as an interface between the user and database to control data precisely.

There are four general types of database:

• Relational Databases (RDBMS)
• Hierarchical Databases (DBMS)
• Network Databases (IDMS)
• Object-oriented databases

5. Define RDBMS!

A. RDBMS is the short form for Relational Database Management System. RDMBS relies on a relational model for building and maintenance of databases. RDBMS uses tables to store data and allows the linking of tables via relational operators for smooth communication. Some of the RDBMS systems are MySQL, MS Access, Oracle database, etc. RDMBS ranks among the most popular form of databases in the industry owing to their ease in accessibility and allowing the complex queries structure.

6. What are tables and field in SQL?

In databases, information or data is stored in the form of tables. A simple table may consists of multiple rows and column. Column represents the vertical form and Rows are the horizontal form inside a table.

7. What is a database?

8. Define the scope and uses of SQL in general!

9. How SQL supports programming?

10. Define the subsets of the SQL!

11. What is DDL in SQL?

12. What is DML in SQL?

13. What is DCL in SQL?

14. Define a primary key in SQL!

15. Define a unique key in SQL!

16. Define a foreign key in SQL!

17. Define a view in SQL!

18. Define an index in SQL!

19. What are different type of indexes in SQL?

20. Define unique index in SQL!

21. Define Clustered index in SQL!

22. Define Non-Clustered index in SQL!

23. Define the Normalization process in a Database!

24. Define the denormalization process inside a Database!

25. What are the main uses of Normalization?

26. How would you deduce the pitfalls or disadvantages of not performing Database Normalization?

27. Define the inconsistent dependency

28. What are the main types of operators in SQL that you should know?

29. What is a join in SQL and the most common used SQL joins?

30. Explain the different types of join in general.

31. Define inner join in SQL.

32. Define left join in SQL.

33. Define right join in SQL.

34. Define full join in SQL.

35. Define self-join and its use in general.

36. Define TRIGGER in SQL.

37. Define constraint in SQL and its level.

38. Define the ACID property in SQL.

39. Define case manipulations functions in SQL.

40. Define the multiple case manipulations functions in SQL.

41. Define character manipulation functions in SQL!

42. Explain the multiple character manipulations functions in SQL!

43. Define use and syntax for COALESCE function!

44. Define NVL () function and its usage!

45. Define the use of DISTINCT keyword.

SQL interview questions for experienced

46. What are the main differences between SQL, MySQL, and SQL server?

47. Define the difference between SQL and PL/SQL!

48. How would you define the main difference between primary key and unique key?

49. How can you define the differences between the clustered and non-clustered index in SQL?

50. Define the main difference between the IN and BETWEEN condition operators!

51. What are the main differences between the DELETE and TRUNCATE statements in SQL?

52. How would you describe the main difference between NULL value, zero, and blank space in SQL?

53. What are the main differences between CHAR and VARCHAR2 datatypes in SQL?

54. Define the difference between DROP and TRUNCATE commands!

55. Define the difference between natural join and cross join!

56. Define the difference between WHERE Clause and HAVING clause!

57. Define stored procedure in SQL.

58. What are the advantages and disadvantages of stored procedure in SQL?

59. Define the collation in SQL.

60. Define Auto increment in SQL.

SQL queries for interview

61. How can you write a query to find names of employee from a given table that starts with alphabet ‘B’?

62. How you can display the current data using SQL query?

63. What is a subquery in SQL?

64. Define the types of subquery in SQL.

65. What are the different ways to fetch the count of records inside a table?

66. How you can fetch the second-highest salary for an employee from the company table?

67. Define the operators used for query in pattern matching!