Top 50 SQL Query Interview Questions and Answers

SQL, or Structured Query Language, is a domain-specific programming language used in database management systems to manage data. Because Database Management Systems (DBMS) are used in nearly every software application, SQL programming abilities are extremely sought and necessary in the industry. Candidates must pass an interview in which they will be given various SQL query interview questions to be considered for a job.

The following is a selected set of SQL interview questions and answers that will most likely be asked during the SQL interview. Depending on their expertise and other criteria, candidates for 3 years experienced professionals are likely to be asked SQL basic interview questions to advanced level SQL query interview questions; there can also be SQL developer interview questions.

Mid-level SQL Query interview questions

21. The distinctions between clustered and non-clustered indexes should be stated.

Clustered index: This type of index is used to sort data rows by their key values. A clustered index is similar to a phone book’s contents. We may start with the letter ‘David’ (for ‘David, Thompson’) and discover information for all Davids right next to each other in the book. Because the data is stored adjacent to each other, it is much easier to retrieve data using range-based searches. Furthermore, the clustered index is linked to the data storage method. Per table, only one clustered index is possible.
Non-clustered indexes hold data in one location while indexing in another. The index contains pointers that go to the data’s location. There can be multiple non-clustered indexes for a table since the index in the non-clustered index is stored in separate places.

Index that is clustered

Methods for storing records in memory used for sorting and storing records physically Data is stored in the index’s leaf nodes.
The size is large. Fast access to a vast amount of data
Extra disc space isn’t necessary.
The kind of key A clustered index is the primary key of a table by default.
The most important feature is that it enhances data retrieval performance.

Index that is not clustered

Used to put data rows in a logical order. For physical data files, pointers are used.
Methods of data storage Never keep data in the index’s leaf nodes.
Size Small and Data Access is Slow in comparison.
Additional disc space is available. Indexes must be stored individually.
The kind of key may be used with the table’s unique constraint, which works as a composite key.
The primary feature should be generated on the columns that are utilised in joins.

22. What exactly is the distinction between SQL and MySQL?

SQL

It’s a database query language that’s structured.
It’s a query and database management system.
SQL is the same every time.
SQL only supports a single storage engine.
In SQL, the server is self-contained.

MySQL

It’s a database administration system.
It provides for structured data management, storage, and modification.
MySQL is always being updated.
Multiple storage engines are supported by MySQL.
The database is blocked by the server during backup sessions.

23. Explain the distinctions between SQL and PL/SQL.

Since this is a more detailed SQL query interview question you can state how many differences you want to

SQL

SQL is a structured query language for databases.
SQL is a type of query that may be used to run DML and DDL instructions.
SQL is a data-oriented and declarative language.
It is mostly used to manipulate data.
It allows you to communicate with the database server.
There can’t be any PL/SQL code in it.

PL/SQL

It’s a database programming language that leverages SQL.
A block of code called PL/SQL is used to write a full procedure or function.
PL/SQL is a procedural and object-oriented programming language.
It is employed in the development of an application.
It doesn’t allow you to communicate with the database server.
Because it is an extension of SQL, it can contain SQL.
Next, we have some SQL query interview questions in the line-up.

24. In a database, what is the ACID property?

In this SQL query interview question, ACID stands for Atomicity, Consistency, Isolation, and Durability in its complete form. ACID characteristics are used to assess the transaction’s reliability.

Atomicity relates to whether a transaction succeeded or failed, where a transaction is a single logical operation on data.
This means that if one part of a transaction fails, the entire transaction fails, leaving the database state unaltered.
The term “consistency” refers to the data meeting all of the validity criteria. The transaction is never exited from the database until it has completed its state.
The fundamental goal of isolation is to manage concurrency.
Durability assures that a transaction will take place regardless of what occurs in the meantime, such as a power outage, a fire, or another type of disruption.

25. What is the purpose of SQL group functions?

For each group, group functions act on a sequence of rows and produce a single result. Some of the most often used as group functions are COUNT (), MAX (), MIN (), SUM (), AVG (), and VARIANCE ().

26. What exactly do you mean when you say “character manipulation function”?

Character data types are manipulated using character manipulation routines.

The following are some of the character manipulation functions:

UPPER: The string is returned in uppercase.
INITCAP: It turns the string’s initial letter to uppercase while keeping the rest in lowercase.
It is used to determine the span of a string.

27. What is the meaning of AUTO INCREMENT?

For this SQL query interview question, you can say that when a new record is added to a table, SQL’s AUTO INCREMENT function is used to produce a unique number.

Because each record’s main key is unique, we make it an AUTO INCREMENT field so that it is incremented when a brand-new record is entered.

The AUTO-INCREMENT value is set to 1 by default and is increased by 1 whenever a new record is added.

28. What is the difference between the commands TRUNCATE and DELETE?

DELETE: This query is used to remove or delete one or more tables from the database.
TRUNCATE: The TRUNCATE statement deletes all data from a table.

The following are the differences between DELETE and TRUNCATE commands:

DELETE is a DML command, while TRUNCATE is a DDL command.
TRUNCATE does not allow us to truly execute and trigger, but DELETE allows us to do so.
TRUNCATE will not work if foreign key restrictions reference a table. If we have a foreign key, we must delete it using the DELETE command.

Moving to the next SQL developer interview questions.

29. In SQL, what is a “TRIGGER”?

The trigger is an automated procedure that occurs when an event in the database server occurs. It aids in the preservation of the table’s integrity. When instructions like insert, update, and delete are issued, the trigger is triggered.

30. What is the purpose of the COALESCE function?

In this SQL developer interview question, the COALESCE function accepts a collection of inputs and returns the first value that is not null.

31. What do normalisation and denormalization mean to you?

In databases, normalisation and denormalization are the two most used approaches.

By arranging fields and tables in databases, normalisation helps to reduce data redundancy and dependence. It entails creating tables and establishing links between them according to predetermined principles.

These principles may be used to reduce duplication and inconsistent dependencies, making it more adaptable.

Denormalization is the polar opposite of normalisation. To speed up complicated searches involving several tables to join, we simply inject redundant data. By adding redundant data or clustering the data, we may improve the read speed of a database.

32. What are some typical SQL clauses for SELECT queries?

For this SQL developer interview question you could just name the SELECT statement clauses, if the interviewer asks for an in depth explanation, you may go on to explain it.

In SQL, there are several SELECT statement clauses. The following are some of the most regularly used clauses:

FROM

The FROM clause specifies which tables and views may be used to analyse data. The tables and views specified in the question must be available at the time the question is asked.

WHERE

The WHERE clause specifies the parameters that will be utilised to limit the results table’s content. Subselects may be used to test for fundamental connections or relationships between a column and a group of columns.

BY GROUPING

For each group of unique values in a set of columns or expressions, the GROUP BY clause is often used in aggregate functions to generate a single result row.

ORDER BY

The ORDER BY clause allows you to specify which columns should be used to sort the table’s results.

HAVING

The HAVING clause filters the GROUP BY clause’s results by utilising an aggregate function.

33. What is your understanding of the stuff () function?

The stuff function removes a portion of a string and then replaces it with another portion that begins at a given point.

34. What is a point of view? Give a specific example.

Views are virtual tables used to limit the tables that we wish to see, and they are nothing more than the result of a SQL query with a name. Views take up less storage space because they aren’t physically present.

Consider the following scenario. Let’s assume we want to conduct various operations on the data in the employee table below that have the gender ‘Female.’ From the whole employee database, we may build a view-only table for the female employees.

35. What are the different types of SQL views?

In SQL, there are four different types of views. They are as follows:

Simple Observation

A view with no GROUP BY clause or other features that is based on a single table.

Complicated view

A complicated view is one that is made up of multiple tables and includes functions as well as a GROUP BY clause.

Inline View is a view based on a FROM Clause subquery that creates a temporary table and streamlines a complex query.
A view that stores both the definition and the details is known as a materialised view. It creates data copies by maintaining them physically.

Interview Questions for Experienced SQL Professionals

36. What is the definition of a stored procedure? Give a specific example.

For this SQL query interview question you could give the exact definition or you can frame it in your own words.

A stored procedure is SQL code that has been prepared and may be saved and reused. To put it another way, a stored procedure is a function that consists of a number of SQL statements that are used to access the database system. We may combine several SQL statements into a stored procedure and run it whenever and wherever we need it.

A stored procedure may be used to provide modular programming; that is, we can write it once, save it, then call it as many times as needed. When opposed to doing several queries, this allows for quicker execution.

37. What are your thoughts on Joins? Define the various kinds of joins?

The Join clause is used to join rows from two or more tables together based on a common column. There are several sorts of joins that may be used to obtain data, depending on the tables’ relationships.

There are four different kinds of joins:

Joining from within: In a nutshell, an inner join yields entries that have the same values in both tables.
Left Join: Left Join retrieves all rows from the left-hand-side table, if no matches in the right-hand-side table is available, as well as rows that are common between the tables.
Right Join: Right Join recovers all rows from the right-hand-side table, no matches in the left-hand-side table are there, as well as rows that are common between the tables.
Full Join: This operation returns all rows from the left-hand table as well as all rows from the right-hand table.

Moving to the next SQL query interview questions.

38. Provide an explanation of the Inner Join.

The records that have matching values in two tables are returned by an inner join.

39. What exactly do you mean when you say “temporary table”?

We can store and process intermediate results with the assistance of a temporary table. These temporary tables are generated and can be removed automatically once they’re no longer needed. They are extremely useful in situations where transitory data must be stored.

40. What exactly do you mean when you say “self join”?

In SQL, a self-join is used to link two tables together. Each row of the table is connected to itself and other rows of the table based on certain requirements.

41. What is the difference between the operators Union and Union All?

To combine the results of two or more select statements, use the Union operator.

All records from both tables, including duplicates, are returned by Union All.

42. What exactly is a cursor?

A collection of data The cursor is a control that allows you to move between rows or documents in a table. It may be thought of as a pointer to a specific row inside the collection of rows. Cursors are in handy for doing database operations like extraction, insertion, and deletion.

43. What does the Intersect operator do?

The Intersect operator joins two select statements together, returning only records that are common to both of them. So, if we obtain Table A and Table B over here and then use the Intersect operator on these two tables, we’ll only receive entries that are common to the select statements of these two tables.

44. In SQL, what’s the difference between the BETWEEN and IN operators?

Use the BETWEEN operator to express rows depending on a collection of values. Numbers, text, and dates can all be used as values. Between two given ranges, the BETWEEN operator delivers the total number of values. The IN condition operator is used to find values inside a specific range of values. The IN operator is used when there are many values to pick from.

45. What’s the difference between a HAVING clause and a WHERE clause?

This is one of the most asked SQL Query interview questions. In SQL, the difference between the HAVING and WHERE clauses are that the WHERE clause can’t be utilised with aggregates, but the HAVING can. The WHERE clause only works with data from a single row, not aggregated data.

46. What is the definition of normalisation?

Normalization is the process of arranging a database’s columns and tables to reduce redundancy and dependence. The primary goal of normalisation is to add, remove, or alter fields in a single table.

47. What is Denormalization, and how does it work?

Denormalization is a database access technique that converts data from higher to lower normal forms. It’s also the practice of integrating data from similar tables into a table to add redundancy.

48. What exactly is an index?

An index is a performance optimization strategy for retrieving records from a table more quickly. An index generates an entry for each value, making data retrieval faster.

49. What is the definition of a stored procedure?

A stored procedure is a database access function that consists of several SQL statements. A stored procedure is a collection of SQL statements that may be executed whenever and wherever they are needed.

50. What exactly is a Data warehouse?

A Data warehouse is a central storehouse for data from a variety of sources. These data are then aggregated, converted, and made available for online processing and mining. Data Marts are a subset of data found in the midst of warehouses.

Conclusion:

These were some SQL interview questions and answers. All of the SQL developer interview questions for freshers and SQL server interview questions for experienced applicants and certain SQL query interview questions are included in the list above. They can be an excellent preparation resource for preparation.

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SQL Query Interview Questions and Answers