Top 40+ SQL Interview Questions and Answers for 2024 2024

SQL is an effective tool for managing and working with relational databases. It enables users to obtain, insert, update, and delete data by interacting with databases. Since SQL is utilized in various fields, including database administration, data analysis, data science and software development, it’s an essential skill for anyone working with data. Having strong preparation for SQL query-based interview questions is crucial, regardless of experience level.

This article covers more than 40 SQL interview questions and answers, which are frequently asked during high-paying SQL developer interviews. 

1. What is SQL?

Structured query language (SQL) is a programming language that stores and processes data in relational databases. A relational database stores data in tabular format, with rows and columns reflecting different data attributes and the relationships between them. In SQL, we can update, insert, delete, store and retrieve data. SQL can also be used to monitor and improve database performance.

2. What is a primary key in SQL?

It is a unique identifier assigned to each record in a table. It assures that each row in the table has a unique and non-null value in the main key field. It develops linkages among the tables and make sure that data is reliable and true.

3. What is a foreign key in SQL?

Foreign key refers to the key that exists in a table and refers to another table. It creates a link between the two tables, maintaining data consistency and allowing data retrieval across them.

4. How SQL and NoSQL are different?

SQL databases are distinguished by the use of organized tables and strict adherence to a specified schema, making them excellent for maintaining structured data while emphasizing data consistency and transaction support. In contrast, NoSQL databases are non-relational and excel at processing unstructured or semi-structured data; they are widely used for scalable, distributed, and adaptive data storage solutions.

5. What is a unique key?

A unique key, also known as a unique constraint, ensures that each value in a column (or combination of columns) is unique and cannot be copied within a table. Unlike a primary key, a table can have several unique keys.

6.What are the various types of SQL commands?

SELECT: extracts data from a database.

INSERT: Adds new records in a table.

UPDATE: Update the existing records in a table.

DELETE: removes records from a table.

CREATE: Sets up a new database, table, or view.

ALTER: Modifies the current database object structure.

DROP: deletes an existing database object. 

7. What are tables and fields?

A table has rows and columns, which are referred to as records and fields, respectively. A column is made up of data values of a specified type, such as numbers or alphabets, and each row in the database contains one value for that column.

8. What is a JOIN in SQL, and what are the different types?

A JOIN operation combines information from two or more tables by using a common column to connect them. There are various forms of JOINs, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN. These JOIN variations determine how data from the relevant tables is paired and obtained.

9. What is a query?

An SQL query is used to obtain the necessary data from the database. An ineffective query might deplete database resources, slow down database performance, or cause service interruptions for other users. As a result, query optimization is critical for achieving optimal database performance.

10. What is a subquery?

A subquery is a query that exists within another query. It retrieves data from the primary query as a filter or condition.

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11. What do you mean by a null value in SQL?

A null value means that there is no data in the column. It is not equivalent to an empty string or zero; it indicates that the data is missing or unknown. NULL values may be used in columns with optional data or when actual data is unavailable.

12. Explain the SELECT statement.

The SELECT statement retrieves data from one or more tables, allowing you to select which columns to obtain, apply filters via the WHERE clause, and sort the results using the ORDER BY clause.

13. How should you use the WHERE clause?

The WHERE clause in SQL queries selectively filters rows based on defined conditions, allowing you to retrieve just the rows that meet the criteria you provide. For example:

SELECT * FROM TABLE 1 WHERE condition.

14. What are indexes in SQL?

Indexes speed up data retrieval activities. They create a sorted data structure based on one or more columns, making it easy to find certain rows in a database. Query performance can be improved by using indexes.

15. Explain GROUP BY in SQL.

The GROUP BY clause arranges rows from a table according to the values in one or more columns. It is typically used with aggregate functions such as SUM, COUNT, AVG, MIN, and MAX to conduct computations on grouped data.

16.  Describe ORDER BY in SQL.

The ORDER BY clause sorts the result set of a query according to one or more columns. You can choose whether each column should be sorted ascending or descending. For example:

SELECT * FROM tab_1, ORDER BY abc DESC

17.  Explain the differences between WHERE and HAVING in SQL.

The WHERE clause is used to limit individual rows before they are grouped, for as when filtering rows before performing a GROUP BY operation. In contrast, the HAVING clause is used to filter groups of rows that have already been grouped, such as filtering groups based on aggregate values. 

18. What is normalization?

It is the process of examining provided relation schemas based on their functional dependencies and primary keys in order to attain the following desirable properties:

• Minimizing Redundancy
• Minimizing insertion, deletion, and update anomalies 

19. What is a SQL comment?

An easily understood explanation of what a particular piece of code provides. SQL code comments can be single-line (preceded by a double dash –) or multi-line (e.g., /*comment_text*/). When the SQL engine runs, it neglects code comments. The addition of SQL code comments is intended to make the code more comprehensive for future readers.

20. Explain SQL operator?

A reserved character, a string of characters, or a keyword used in SQL queries to carry out a specific operation. SQL operators are used with the WHERE clause whenever needed.

21. What are the aggregate functions?

SQL aggregate functions calculate a set of values and produce a single result.

SUM: This calculates the sum of the values in a column.

COUNT: Counts the number of rows given in a column.

AVG: Calculate the average of the values in a column.

MIN: Determine the column’s minimal value.

MAX: Tells the maximum value.

22. What is a TRIGGER in SQL?

Triggers are the database’s quiet guardians, such as data entry, updating, or deletion. They are gatekeepers who enforce the regulations and will not accept any unauthorized alterations that may result in illicit data alteration. The triggers are monitors with an undetectable break to safeguard the prize.

The syntax to generate the trigger function is as follows:

CREATE TRIGGER trigger_name. 

23. How do you change a value in SQL?

The UPDATE statement alters pre-existing records in a table. It entails specifying the target table for the update, the exact columns to be changed, and the new values to be used.

24. What is a clause?

A condition applied to a SQL query to filter the data and acquire the desired result. Examples include WHERE, LIMIT, HAVING, LIKE, AND, OR, ORDER BY, and so on.

25. Write the statements used with SELECT query?

The most frequent ones include FROM, GROUP BY, JOIN, WHERE, ORDER BY, LIMIT, and HAVING.

26. How to make a table?

Using the CREATE TABLE statement. For example, to build a table with two columns of predefined data types, we use the following syntax:

CREATE TABLE table_name (col_1, 

                                                 col_2). 

27. Explain different kinds of joins with examples.

INNER JOIN: Tells us the rows that have matching values from both the tables.

RIGHT JOIN: Combines all rows that have matching values from both right table with the left table.

LEFT JOIN: Combines all rows that have matching values from both left table with the right table.

FULL JOIN: Combines all rows from either table that match, even mismatched rows from both tables.

Example:

SELECT column_name

FROM table_1

LEFT JOIN table_2

ON table1.column_name=table2.column_name

28. Explain ACID in SQL.

The word ACID stands for Atomicity, Consistency, Isolation, and Durability. They are critical qualities that assure the dependability and integrity of database transactions.

Atomicity: a single, indivisible unit of transactions

Consistency: transactions change the database from one consistent state to another

Isolation: transactions are segregated from one another

Durability: committed transactions are permanent and survive system failures

29. What is the definition of a SQL transaction?

A SQL transaction refers to set of SQL operations considered as a single unit of work. Transactions ensure that database activities are executed successfully or rolled back completely in the event of failure.

30. Which scalar functions are you familiar with?

LEN () (in other SQL dialects – LENGTH ()) returns the length of a string, including blank spaces.

UCASE () (in other SQL dialects – UPPER ()) returns a string transformed to uppercase.

LCASE () (in other SQL dialects – LOWER ()) returns a string transformed to lowercase.

INITCAP () returns a string changed to the title case.

MID () (in other SQL languages, SUBSTR ()) – extract a substring from a string.

ROUND () delivers a numerical value rounded to the supplied number of decimals.

NOW (): Tells the present time and date.

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31. What is the default data ordering for the ORDER BY statement, and how can you change it?

By default, the order is ascending. To set it to descending, use the DESC keyword as follows:

SELECT * 

FROM table_name

ORDER BY col_1 DESC 

32. How can you avoid SQL injection?

SQL injection is a security issue that occurs when SQL queries mishandle untrusted data, resulting in the danger of unauthorized access or data manipulation. To prevent SQL injection, use techniques such as parameterized queries, prepared statements, input validation, and tight access controls.

33. Create a SQL query to locate the names of employees that begin with ‘A’.

The LIKE operator from SQL can be used for this purpose. It is used to retrieve filtered data by looking for a certain pattern in the where clause.

The syntax for using LIKE is:

SELECT column1, column2 

FROM table_name 

WHERE column_name 

LIKE pattern

LIKE: Operator Name

Pattern: The exact value taken from the pattern to obtain related data in the result set.

The required query is:

SELECT * 

FROM Employees 

WHERE EmpName 

LIKE 'A%'

34. What is the sequence of appearance for the typical statements in the SELECT query?

SELECT, FROM, JOIN, ON, WHERE, GROUP BY, HAVING, ORDER BY, LIMIT

35. In what order does the interpreter execute the common statements in the SELECT query?

FROM, JOIN, ON, WHERE, GROUP BY, HAVING, SELECT, ORDER BY, AND, LIMIT. 

36. What are ctes (Common Table Expressions)?

Common Table Expressions (CTEs) are temporary result sets that you can mention in SQL queries, such as SELECT, INSERT, UPDATE, or DELETE operations. They are defined using the ‘WITH’ keyword and help to streamline complex searches by breaking them down into more digestible components.

37. Explain the MERGE statement.

The SQL MERGE statement is used to insert, update, or delete rows from a destination table based on the results of a source table or query. It combines the functionality of many distinct statements (INSERT, UPDATE, and DELETE) into a single comprehensive statement, making it very useful for ensuring data synchronization between databases. 

38. Name the function that removes spaces from the end of a string.

In SQL, the trim function removes the spaces at the end of the string.

Syntax:

Trim(s), where s is any string. 

39. What are the different possible values for a BOOLEAN data field?

In some SQL dialects, such as PostgreSQL, the BOOLEAN data type is expressly defined and accepts TRUE, FALSE, or NULL. Other variants, like Microsoft SQL Server, use the BIT datatype to store Boolean values as integers 1 (true) or 0 (false).

40. Explain how DELETE and TRUNCATE are different?

DELETE is a reversible Data Manipulation Language (DML) command that deletes one or more rows from a table based on the WHERE clause. Instead, TRUNCATE is an irreversible DDL (Data Definition Language) operation that deletes all entries from a table. DELETE is slower than TRUNCATE. Also, we cannot use the TRUNCATE command on a table with a foreign key.

41. What’s the distinction between DROP and TRUNCATE statements?

DROP removes a table from the database altogether, including its structure, constraints, relationships with other tables, and access privileges. TRUNCATE removes all rows from a table without changing its structure or constraints. DROP performs slower than TRUNCATE. Both are irreversible Data Definition Language (DDL) commands.

42. Explain AND, OR statement with an example.

In SQL, the AND and OR operators are used to filter data and produce accurate results based on requirements. The AND and OR operators are applied to the WHERE clause.

These two operators are known as conjunctive operators.

AND Operator: This operator displays only records where both condition 1 and condition 2 are True.

OR Operator: This operator returns records in which either condition 1 or condition 2 is True. This means that either condition 1 or condition 2 is true.

In conclusion, mastering SQL interview questions is critical for data analysts, data engineers, and business analysts seeking to excel in their respective industries. This article will help all the individuals preparing for SQL interview.

Understanding and practicing these questions improves your ability to effectively address the issues presented in SQL-related interviews, paving the road for a successful career in data management and analysis. Remember, each question is an opportunity to demonstrate your analytical skills and technical experience, both of which are required for any aspiring professional in these crucial professions.