Top 20 Core SQL Interview Questions and Answers in 2022

A relational database management system is the name of the programme used to store, administer, query, and retrieve data from a relational database (RDBMS). In this article, we have provided some SQL interview questions for experienced candidates. There is a tremendous need for SQL developers from several leading financial and technical heavyweights, like Paypal, Morgan Stanley, JP Morgan, Walmart Labs, and even startups like Nearbuy. With an average yearly pay of 4.6 Lakhs, SQL Server Developer salaries in India range from 2.1 Lakhs to 10.0 Lakhs. This page's content has been exclusively curated to cover the constraints in SQL interview questions and answers.

 

 

1. What are the various SQL subsets?

DDL, DML and DCL are the various subsets of SQL.

DDL (Data Definition Language) is the language used to define the structure of a database so that you may create, alter, and delete objects.

You can access the data and modify it in the Data Manipulation Language (DML). One can use Database Management Software for a variety of tasks.

Data Control Language (DCL) can restrict access to a database. Permissions can be granted or revoked.

 

2. Why is String immutable in Java?

Security, synchronisation and concurrency, caching, and class loading make the String immutable in Java. It is necessary to make the string final to prevent others from extending the string's immutability. There is no way to modify a series cached in the String pool.

 

3. Why are OOPs required?

OOPs enable you to simulate actual objects, decouple interfaces and implementation specifics, and persistently store object-oriented data in the database.

•       OOPs make it easier for consumers to grasp the software, even if they don't know how to implement it.
•       The code becomes much easier to read, comprehend, and maintain when written using OOPs.
•       Using OOPs, even the most complex software can be built and managed quickly and easily.

 

4. What is the Graphical based interface in SQL?

A diagrammatic form that presents a schema to the user is included in a graphical user interface. By adjusting the diagram, the user poses a question. In these interfaces, the mouse is used as a pointing tool to select certain areas of the diagram.

The users of electronic devices like touch screens and mobile phones use these types of interfaces.

 

5. Explain RDBMS?

IT professionals and others can use a relational database management system (RDBMS) to create, modify, administer, or interact with relational databases. Structured Query Language (SQL) is the query language most commonly used by commercial relational database management systems to access data contained in tables.

 

 

6. What do you mean by self-join?

Self joins, also known as unary relationships, represent a method where a table is joined to itself, particularly when the table has a foreign key that refers to its own primary key. When a table is joined, each row is merged with every other row in the table as well as with itself.

 

7. What is the SELECT statement?

A SELECT command can return zero or more rows in one or more database tables or views. SELECT is the most used command in data manipulation languages (DML). SELECT queries define a result set but do not specify how to calculate it, as SQL is a declarative programming language, 

 

8. Explain PostgreSQL?

Postgres was built in 1986 by a group led by Professor of Computer Science Michael Stonebraker. By providing data integrity and fault tolerance in systems, it was designed to help developers develop enterprise-level applications. PostgreSQL is an open-source object-relational database management system designed for large organisations. The international developer community has always supported it.

 

9. Give a brief description of SQL comments?

To prevent SQL statements from being run, SQL Comments can be used to explain specific parts of the SQL statement. Many programming languages place a high value on comments. A Microsoft Access database does not back up the comments. Consequently, both Firefox and Edge demonstrate using the Microsoft Access database.

 

10. What is schema?

It is the visual depiction of the database that is logical. It creates and specifies the database's various relationships. It is a term used to describe the different SQL data types of database restrictions. Tables and views can benefit from this feature.

 

11. What are the types of databases?

Numerous types of databases exist, some of them are: 

• Hierarchical databases 
• Object-oriented databases 
• Relational databases
• Network databases 

Complex and extensive databases are constructed using the same design and modelling principles. Large databases are stored on computer clusters or in the cloud, unlike file systems, which are better suited for smaller databases.

 

12. Explain normalisation

Normalisation is a process to reduce data redundancy and dependency by arranging fields and tables in databases in a normalised manner. In this process, tables are built, and linkages between tables are established using rules. These principles make normalisation more flexible by removing redundant and inconsistent dependencies.

 

13. What is the primary key?

All records in a table can be uniquely identified using a primary key. It can't contain any NULL values and must have only one data set. A composite key can have single or numerous fields, but only one can exist in a given table.

 

14. Are there any programming language features supported by SQL?

The Standard Query Language is referred to as SQL. As a result, while SQL is technically a language, it does not provide any programming assistance. This common language has no loops, conditional statements, or logical operations. It can only be used to manipulate data and nothing else.

 

 

15. What is the significance of the DDL language?

Data definition language is referred to as DDL. DDL commands in SQL are the part of a database that specifies the data structure of the database when it is first set up. – Tables can be added, removed, or modified using DDL commands.

 

16. Explain tables

Some rows and columns make up a table. It provides the ability to store and display data in a structured fashion. Similar to spreadsheet worksheets, it is a type of document. The tuples are represented by rows, while columns represent the attributes of the data items in a specific row. It's possible to think of rows as horizontal and columns as vertical.

 

17. State the difference between primary and unique keys

• To uniquely identify records in a database, a primary key might be one or more table fields. A unique key, on the other hand, stops two rows from having identical items in a column.
• In a relational database, a table can have numerous unique keys, but it can only have one primary key.
• A unique key can have NULL values, however only one NULL is permitted in a table, whereas a main key column cannot have NULL values.
• Although uniqueness is preferred for the main key, it's not a requirement that it serves as the primary key.

 

18. What is a foreign key?

A foreign key is an attribute or feature group that refers to a table's primary key in a different database. A foreign key is a data structure that connects two databases.

 

19. What are operators in SQL?

In SQL queries, specific operations can be performed using SQL operators, keywords, or characters. The WHERE clause of SQL statements can employ these operators. SQL operators filter data based on the condition.

 

20. Explain data integrity

It guarantees data accuracy and consistency throughout its lifecycle. Systems that store, process, or retrieve data necessitate this feature in their design, implementation, and use.

 

Conclusion

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Also, SQL Query Interview Questions are discussed in the live sessions where students clarify their doubts regarding DBMS languages from professional database admins.