Top 50 SQL Interview Questions and Answers for 2024-25

Last updated on July 26th, 2024 at 11:15 am

This article will cover 50 SQL interview questions and answers that are asked in SQL developer interviews. This article is for freshers, intermediates and experienced professionals who want to ace their next SQL interview.

In this age of digitisation and data dependence, knowing SQL will give an edge in various career prospects. SQL or Structured Query Language, is a database language for accessing and decoding complicated data in databases.

Top SQL Interview Questions

SQL interviews are tough. Interviewers look for individuals who not only know the basics of SQL but also have practical knowledge about it. Here are the Top 10 SQL interview questions and answers.

1. Define SQL and its main categories.
   
   This is one of the basic SQL interview questions asked by any employer. SQL stands for Structured Query Language, and it is used to manage and change databases. Its categories include Data Query Language (DQL), Data Manipulation Language (DML), Data Definition Language (DDL), Data Control Language (DCL), and Transaction Control Language (TCL).
2. Define a JOIN and state its types.
   
   This is one of the most important SQL interview questions for developers. A JOIN function combines data from more than one table by using a common column to connect them. There are several forms of JOINs, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN. These JOIN variants determine how data from the relevant tables are combined and obtained.
3. Differentiate between DELETE and TRUNCATE commands.
   
   This is one of the most important SQL interview questions for experienced professionals. Specialists employ the DELETE command to eliminate certain rows from a database based on a criterion, allowing them to carefully eliminate records. TRUNCATE, on the other hand, deletes all rows from a database table without any constraints. TRUNCATE is quicker and requires less computing power than DELETE, but it does not record specific row removals.
4. Define a primary key and a foreign key in SQL.
   
   A primary key serves as a distinctive identification for each entry in a database, ensuring the accuracy of data and entity originality. In contrast, a foreign key is an assortment of columns that creates a link between tables by mentioning another table's primary key.
5. Define normalization in SQL.
   
   Normalization is a strategy for streamlining storing information in the form of a database, decreasing duplication, and improving data quality. This method comprises breaking down the tables into simpler, interconnected tables and connecting them.
6. Define SQL dialects.
   
   SQL dialects refer to the numerous free and commercial versions of SQL. SQL syntax is extremely identical across all variants, with just minor differences in extra capability.
7. State the main applications of SQL.
   
   The basic applications of SQL are:

• Manage database tables by creating, deleting, and updating them, as well as accessing and modifying their data.
• Extract and consolidate the pertinent data from a single or many tables.
• Add or Delete certain columns or rows from a data set.

8. State the different types of SQL subqueries.
   
   These are the various types of SQL subqueries:

• Single-row- returns a maximum of one row
• Multi-row- returns a minimum of two rows
• Multi-column- returns a minimum of two columns
• Correlated- a subquery connected to the data contained in the outer query.
• Nested- a subquery within a subquery.

9. Define constraint and state its use.
   
   A collection of criteria that specify the kind of data that may be entered into all the columns of a table. Constraints maintain the confidentiality of information in a database and prevent undesirable activities.
10. Define a schema.
    
    A set of database structural pieces, including tables of information, procedures that are stored, indexes, operations, and triggers. It depicts the general database structure, describes the interconnections between various information items, and assigns various access privileges to them.

Basic Interview Questions and Answers

The smartest way to crack a SQL interview is to have clarity and knowledge of the basics. Additionally, it also increases the chances of cracking the interview. The following set of questions will cover some of the basic SQL interview questions and answers.

1. Define a database.
   
   A database is a planned set of data that is saved and accessed securely from a distant or localized computer system. Databases may be large and complicated, and these databases are created utilizing predetermined architectural and modeling methodologies.
2. Define DBMS.
   
   The acronym DBMS refers to a Database Management System. It is a type of program that is in charge of creating, retrieving, updating, and managing databases. It guarantees that the information that is stored is consistent, structured, and easily available by acting as a liaison between the relational database and its intended users or applications.
3. What is a Cross-Join?
   
   A Cross-join may be described as a cartesian good of both tables involved in the join. The table following the join has the same amount of rows as the combined product of the two tables. If a WHERE clause is implemented in a cross-join, the search query will behave as an INNER JOIN.
4. Define a Subquery and state its types.
   
   A subquery is a query contained within one more query, referred to as a stacked query or internal query. It is employed to limit or improve the data requested by the primary query, hence limiting or improving the original query's result.

There are two types of Subquey- Correlated and Non-Correlated

• A correlated subquery can never be regarded as a separate query, however, it may allude to a field in a table specified in the initial query's FROM clause.
• A non-correlated subquery might be regarded as a separate query, with its output inserted into the primary query.

5. Define Data Integrity
   
   Data integrity is the guarantee of data correctness and uniformity throughout its life. It is an essential component of the creation, execution, and upkeep of any device that gathers, organizes, or extracts data. It also establishes integrity limitations, which impose company standards on information as it is input into a program or system.
6. Explain Data Definition Language.
   
   DDL or Data Definition Language permits the execution of queries like CREATE, DROP, and ALTER.
7. Define data manipulation language.
   
   The Data Manipulation Language, or DML, is a method for obtaining and altering database information.
8. Does SQL support programming language features?
   
   SQL, albeit a language, does not facilitate coding since it is not a language for coding, but rather a command-line language.
9. Define a Default constraint.
   
   The DEFAULT constraint fills an area with preset and set values. When no alternative value is specified, the chosen one is applied to all newly created records.
10. Define an ALIAS command.
    
    Aliases are transitory names assigned to tables or columns for the intent of a specific SQL query. It is employed when the title of an area or database is changed from its initial designation, but only temporarily.

SQL Interview Questions and Answers for Freshers

It is preferable for aspiring data analysts, to opt for a data science course with placement. Choosing a data science course that has placement can help to set foot in the industry sooner. Here are some SQL interview questions and answers for those who are just starting to work.

1. What are the different types of SQL statements?
   
   There are four types of SQL statements. They are- Data Manipulation Language (DML), Data Definition Language (DDL), Data Control Language (DCL) and Transaction Control Language (TCL).
2. State the difference between CHAR and VARCHAR data types.
   
   This is one of the most common SQL interview questions for freshers. CHAR is a fixed-length character string type, whereas VARCHAR is a variable-length character data structure. CHAR always utilizes an identical quantity of storage capacity, but VARCHAR utilizes just the space required for the data itself.
3. Differentiate between INNER JOIN AND OUTER JOIN.
   
   The difference between INNER JOIN and OUTER JOIN are as follows:

INNER JOIN	OUTER JOIN
INNER JOIN retrieves the identical and appropriate data between both tables.	OUTER JOIN retrieves every entry from the database fields.
An INNER JOIN returns entries that share similar fields or attributes.	Since every record is sent back, the OUTER JOIN doesn't need an identical column ID.
There exists no inner join variation.	An outer join can be either left or right-side complete (cross join).
Inner joins are useful when several entry points for data are necessary.	Outer joins are ideal if you do not require connected data entries as necessary.
The sluggish velocity that occurs in the inner join causes low efficiency.	Outer joins perform superior to inner joins in SQL.

4. Explain the ACID properties in SQL.
   
   ACID in SQL denotes Atomicity, Consistency, Isolation, and Durability.

• Atomicity: Guarantees that an operation is handled as a single piece of activity that either finishes or does not.
• Consistency: Guarantees that the relational database is in an identical state before and following the operation.
• Isolation: Allows numerous actions to occur simultaneously without influencing one another.
• Durability: Ensures that when an operation has been committed, the modifications are enduring, even if the system fails.

5. Define a SQL index.
   
   A SQL index is an arrangement of data that speeds up retrieval of information activities on the database tables by allowing rapid utilization of rows depending on the contents of specific fields.
6. What is an SQL trigger?
   
   A SQL trigger is a collection of SQL commands that run periodically as a reaction to specific events, including INSERT, UPDATE, or DELETE actions on a database table.
7. Define a SQL transaction.
   
   A SQL transaction is a set of a few SQL statements performed as one piece of work. Transactions protect the integrity of data by enabling activities to be either entirely performed or completely reversed.
8. What is a self-join in SQL?
   
   This is one of the most asked SQL interview questions. A self-join is a type of join procedure that joins a table to itself. It serves to merge rows from an identical table using an associated column.
9. Differentiate between a view and a table in SQL.
   
   A table is an actual storage component that contains data, while a window is an electronic table created from several tables. Views cannot save data, but they do give a means to show data from tables.
10. What is an SQL injection?
    
    This is one of the popular SQL interview questions for freshers. SQL injection is an approach for exploiting user data via website submissions by inserting SQL instructions as statements. Simply said, unscrupulous individuals can use these assertions to influence the program's internet server.

SQL Interview Questions and Answers for Intermediates

These are some of the most asked SQL interview questions and answers to intermediates and developers. 

1. Define a function in SQL. State its use.
   
   A database object that represents a collection of SQL commands frequently employed for a particular task. A function accepts certain parameters as inputs, calculates or manipulates them, and then delivers the output. Functions increase the understanding of code and prevent unnecessary repetition of programming snippets.
2. State the different types of SQL functions.
   
   There are two types of SQL functions- Aggregate and Scalar Functions. Aggregate functions act on many, generally aggregated records from a table's given columns and produce a single result. Scalar functions deal with specific values and ultimately produce a single output.
3. State the different types of aggregate functions.
   
   AVG()- returns the average value
   SUM ()- returns the sums of values
   MIN ()- returns the minimum value
   MAX ()- returns the maximum value
   COUNT ()- returns the number of rows, including NULL values.
   FIRST ()- returns the first value from a column
   LAST ()- returns the last value from a column
4. Define case manipulation functions.
   
   Case manipulation functions are a particular category of the character operations used to manipulate the formatting of textual information.
5. State the difference between local and global variables.
   
   Local variables are only accessible within what function they were defined in. Instead, global variables are identified outside of any function and maintained in persistent memory frameworks, allowing them to be utilized throughout the whole program.
6. State the difference between a primary key and a unique key.
   
   Although both forms of keys assure distinctive values in a table's section, the first distinguishes each record, whilst the second prohibits redundancies in that category.
7. State the difference between renaming a column and giving an alias to do it.
   
   Renaming a column completely alters its real identity in the initially created database. Assigning an alias to a column is to give it an interim title when running a query using SQL, to make the source code more understandable and concise.
8. Can a view be used if the original table is deleted?
   
   No. Any views that utilize the table in question will turn obsolete once the main table is deleted. If we attempt to utilize a view like this, we will get an error notice.
9. Can a view be created based on another view?
   
   Yes. This is additionally referred to as "nested views." Nevertheless, we should steer clear of layering many views since it makes the code harder to comprehend and troubleshoot.
10.  State the different types of SQL relationships.
    
    There are primarily three types of relationships:

• One-to-one- Each record in a particular table matches exactly one record in a different table.
• One-to-many- Each record in a particular table refers to multiple records in a different table.
• Many-to-many- Each entry in the two tables relates to numerous records in the additional table.

SQL Interview Questions and Answers for Experienced Professionals.

These are some important SQL interview questions for experienced professionals.

1. State the different types of case manipulation functions available in SQL.
   
   There are three types of case manipulation functions available in SQL. They are:

• LOWER- This method returns a phrase in lowercase. It accepts a string of characters as a parameter and produces it in lowercase.
• UPPER- This method returns a string in uppercase. It accepts an address as input and comes back it in uppercase. 
• INTCAP- This method returns a string with the initial letter in uppercase as well as the remainder of the characters in lowercase.

2. Which function is used to remove spaces at the end of a string?
   
   The spaces are eliminated using a trim function.
3. Which operator is used in queries for pattern matching?
   
   A Like operator is used to get specific data by looking for a specific pattern in the where clause.
4. What is SQL Order by the statement?
   
   This statement is used to classify the recorded data in either ascending or descending order based on multiple columns.
5. Are NULL values the same as zero or a blank space?
   
   In SQL, zero or blank spaces are distinguishable and contrasted with one another. One null could not be considered equivalent to a different null. Null indicates that data may not be given or that no data exists.
6. Why is group functions required in SQL?
   
   Group functions or aggregate functions are used to group values of multiple rows as input on certain criteria to form one single value.
7. Define Nested Triggers.
   
   A nested trigger incorporates its data alteration mechanism.
8. State the operator that is incorporated for appending two strings.
   
   The “Concentration operator” is used to append two strings in SQL.
9. Define a cursor.
   
   The pointer represents a Temporary Storage or Work Station. The database engine allocates it when the user performs DML actions on the table. Cursors are employed to save tables of data.
10. Explain the ON DELETE CASCADE constraint.
    
    In MySQL, an 'ON DELETE CASCADE' constraint is employed to purge entries from the youngest table whenever the main table's entries are eliminated.

Conclusion

Understanding SQL is critical for data administration. This guide explored essential SQL principles with simple questions and straightforward answers. Keep in mind that practice makes perfect. Continuously improve the skills by carrying out actual tasks and staying informed on the newest innovations in SQL technology. 

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