SQL Interview Questions
Introduction:
SQL was introduced in 1970, and from that time on, it has been working with the advantages of read-and-write APIs. You can access many records with one single-line command. Eliminates the need to reach a record with or without an index. however, This blog will help you reach out to the different but important SQL interview questions.
It’s a very important skill to learn nowadays because of its job security. People, before giving the interview, frequently asked what the SQL interview questions were. However, they did not check specifically for basic, intermediate, and advanced levels.
So let’s first understand what is SQL.
What is SQL?
SQL which stands for Structured Query Language was developed by IBM in the early 1970s. It was first standardized in 1986 and enhanced in 1989. SQL language is a tool that is used to retrieve, manage, and organize the data present in a computer database.
The abbreviation of SQL is ‘structured query language’. For historical reasons, it was also pronounced as sequel . SQL language is used to work with a computer database. In fact it works on a specific database called relational database. When you need to retrieve data from the database, you use SQL language to make the request.
The DBMS receives the SQL request, fetches the requested data, and returns it to the user. The process of requesting data from a database and retrieving it is called a database query.
SQL has control of all functions that a DBMS provides –
- Data retrieval
- Data manipulation
- Data definition
- Data control
- Data Sharing
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SQL Basics interview questions:
What is SQL?
Answer – SQL stands for a structured query language that is used to manage, retrieve, and organize the computer database. It is a domain-specific language that is pronounced as sequel but the alternate pronunciation is ‘S.Q.L’.
Distinguish between SQL and NoSQL databases.
Answer – The main difference between SQL and NoSQL databases are
| parameter | SQL databases | NoSQL databases |
| Type of database | Relational database | Non-relational database |
| Query language | Structured query language | No declarative query language |
| scheme | Scheme of the database is fixed | Scheme of the database is not fixed and is dynamic. |
| Ability to scale | Vertically scalable | Horizontally scalable |
| Model | Uses ACID model | Uses BASE model |
What is a Primary key?
Answer – Primary key, also known as a primary keyword contains a unique value for each row and column. It inserts, deletes, and manipulates the data from the database. A table can not have more than one primary key.
It uniquely distinguishes the rows and columns.
Also Read: Which SQL Keyword Is Used To Retrieve A Maximum Value
What is a Foreign key?
Answer – A foreign key is a place in a relational database table that is defined as the foreign key of another table. It represents the relationship between two tables based on the value of these keys. A Foreign key is a set of attributes that contain the primary key of another table.
What is normalization?
Answer – Normalization is a process of eliminating data redundancy and maintaining data integrity. Organizing data in the database by structuring rows and columns to ensure dependencies is also a use of normalization. It ensures that the database is well-organized and easy to maintain.
Intermediate SQL Interview Questions:
What is a subquery, and how is it different from a JOIN?
Answer – A Subquery is a nested query that is defined in SELECT, DELETE, INSERT, or inside a subquery. Retrieve data used for different queries through its use. Define subqueries in parentheses.
A simple example of a subquery in the WHERE clause:
Subqueries are different from JOIN as
Combining rows from multiple columns, ‘JOIN’ combines columns from two or more rows based on related columns.
The type of join is specified using ‘JOIN’ keywords.
Performance is increased with ‘JOIN’ depending on the type of join and index used.
Also Read: What Is A Magic Table In SQL?
What is the Difference Between UNION and UNION ALL.
Answer –
| UNION | UNION ALL |
| It removes duplicate rows. | It does not remove duplicate rows as it returns all from all queries. |
| It uses a distinct sort. | UNION ALL does not use a distinct sort, so the performance of UNION ALL is higher than UNION. |
| It cannot work with columns because it has a text datatype. | Working with all data type columns is possible. |
| It is slower than UNION ALL. | Faster than UNION. |
What is a stored procedure?
Answer – The stored procedure is used to collect the predefined SQL statements that can be used over and over again. If you are writing an SQL code again and again, save it in a stored procedure and call it whenever required. Stored procedures store SQL statements in a single unit, also known as encapsulation.
What is a trigger, and when would you use it?
Answer – Trigger is a SQL code that is stored in a database and automatically invokes when any event occurs. A trigger is invoked when inserting any row or updating any column. Events that are responsible for the occurrence of triggers are INSERT, UPDATE, and DELETE.
Trigger is used to
- Validate the incoming data and check that it passes some criteria.
- Apply complex business rules.
- Automate some data maintenance tasks.
- Implement certain security measures.
What is the purpose of the CASE statement in SQL?
Answer – Use a CASE statement to write a value based on a specified condition.
You can employ CASE statements in SELECT, WHERE, and ORDER BY clauses when you need conditional logic.
The basic syntax of the CASE statement is as follows:
Advanced SQL Interview Questions:
Explain the differences between OLAP and OLTP databases.
Answer –
| Parameter | OLAP | OLTP |
| Purpose | It is designed for complex queries and analysis of historical data. | It is designed for transactional operations. |
| Datatype | It deals with summarized and historical data. | It deals with transactional data. |
| Schema | It utilizes a star and snowflake schema. | It utilizes a normalized schema. |
| Operations | Involves complex queries, data mining, and advanced analytics. | Involves transactions like INSERT, UPDATE, DELETE, and simple SELECT operations. |
| Response time | Takes longer response time for complex queries. | Takes a low response time. |
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Explain the concept of window functions. Provide an example.
Answer – The window function is a calculation or aggregate function that performs calculations on the related table. Window functions operate on a ‘window’ defined by an OVER() clause, calculating values for analytical and reporting purposes.
Here is the basic window example:
What is the purpose of the OVER() clause in SQL window functions?
Answer – The OVER() clause is a function that defines a window within a window function. It performs calculations on a set of rows related to the current rows. Furthermore OVER() clause function also controls the scope of the window and specifies how the window function should consider rows for its calculations.
syntax of the OVER() clause is:
What is the purpose of the MERGE statement? Provide an example.
Answer – MERGE statement in SQL is used to handle insert, delete, and select functions in a single run without rewriting the logic. It combines multiple statements in a single statement and synchronizes data between tables.
Syntax of merge statement:
What is SQL injection, and how can it be prevented?
Answer – SQL injection is a cyber attack in which a malicious SQL code is inserted into the database. The goal of SQL injection is to gain unauthorized access to the database, manipulate its structure, and disclose sensitive information.
SQL injection attacks occur when the user inputs a malicious code without any validation, thereby allowing attackers to gain unauthorized access to the database.
Injecting a malicious code into the database triggers the SQL injection condition.
If the system does not validate the input then the attacker can exploit the vulnerabilities and gain unauthorized access.
A simple example of a vulnerable SQL query:
Conclusion:
In summary, this blog will provide you with three levels of questions and answers that you can use to enhance your SQL preparation and skills in your SQL interviews. SQL has a wide scope because it helps structure data and manage data stored in RDBMS.
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