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Top 10+ Frequently Asked SQL Interview Questions

March 15, 2022

What are the most frequently asked SQL interview questions?

SQL, Structured Query Language, stores, retrieves and manipulates data stored in a relational database.

All Relational Database Management Systems (RDMS) such as MySQL, MS Access, Oracle, Sybase, Informix, Postgres and SQL Server use SQL as their standard database language.

In this blog post, we will share advanced SQL interview questions and answers that you may encounter in an interview.

So you can evaluate your current knowledge and learn new things easily.

Let’s take a look at Advanced SQL Interview Questions!

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10+ Advanced SQL Interview Questions and Answers

1. What are the two main index types? Explain the meaning of ‘index’.

Indexes help retrieve information from the database faster and with greater efficiency. They are small and optimized for fast searches.

It is a method that improves performance.

When you query an indexed table, the database first navigates to the index and retrieves the relevant records directly.

The two main types of directories are:
Clustered: A clustered index defines the exact order of data stored in the table.
Unclustered: A nonclustered index just points to data.

sql interview questions

2. Are NULL values same as zero?

No, because NULL doesn’t really equal anything, but it shouldn’t be confused with empty space either.
NULL represents the absence of a character while “zero” is in a numeric format.

A NULL value cannot be equal to or unequal to any value. Therefore, you cannot make any comparisons on this value using operators such as ‘=’ or ‘<>’.

3. What is a Synonym?

Synonyms allow you to create another name for the database object, called the original object, which can be found locally or on another server.

When you rename the objects or change the object’s schema, synonyms allows them to continue using their old names.

Synonyms provide an abstraction layer on the referenced object
They also allow you to change the position of objects without changing the code
You can create the synonyms in the same database to ensure backward compatibility.

You can loosely link the synonyms to referenced objects so they can be deleted from the database without warning.
A synonym is not created for a synonym, nor is a table created with the same name as a synonym.
And, it is not possible to reference a synonym in a DDL statement.

4. What are scalar subqueries and correlated subqueries?

A scalar subquery is a subquery expression that can return exactly one column value from a row, that is, a maximum value.

The value of the scalar subquery expression is the value of the selected list item of the subquery. If the subquery returns 0 rows, the value of the scalar subquery expression is NULL.

There are two types of scalar subqueries: Correlated, Uncorrelated

A subquery is a query within a query and only returns an output value.
Subqueries are performed in cases such as report development, application development.

A Correlated Query is a subquery whose output depends on the inner query used in that query.
The approach of the Correlated query is slightly different from the others, in the Related Subquery the outer query always depends on the inner query.

5. How to find the second highest salary of an employee?

To find the second highest salary, the concept of subquery is used, i.e. first find the highest salary in the table and then put that query in a subquery and find the second highest salary.

To find the highest salary, we insert the above query into another query as written below.

SELECT MAX(Salary) WHERE SALARY < (SELECT MAX(Salary) from Employee);

This query will give you the desired output i.e. the second highest salary.

6. What is the difference between NVL and NVL2 functions?

The nvl function has two parameters, while the nvl parameter has three arguments.
Since you can convert a value, nvl2 likes to combine an nvl with decode:

NVL ( expr1 , expr2 ): If expr1 is empty, NVL returns expr2. If expr1 is not null, NVL returns expr1.

NVL2 ( expr1, expr2, expr3 ): If expr1 is empty, NVL2 returns expr3. If expr1 is not null, NVL2 returns expr2.

sql interview questions

7. What is a data warehouse and what is it used for?

A data warehouse features a centralized repository of information that can be analyzed to make informed decisions.
Data flows from sources to the data warehouse, often on a regular basis. Business analysts, data engineers, data scientists, and decision makers access data through business intelligence (BI) tools, SQL clients, and other analytics applications.

Data warehouses can contain multiple databases.

In each database, data is organized into tables and columns.
In columns: you can define data such as integer, string.
Tables can be arranged within schemas.

When data is retrieved, it is stored in tables by the schema. Query tools use schemas to access and analyze data tables.

8. What is recursive procedure in SQL server?

Recursive stored procedures are procedures that call themselves until they reach the boundary condition.
This procedure helps to use the same set of codes n times.

9. What are variables of SQL?

Variables in SQL always start with @, allowing you to program more efficiently.

There are two different variables in SQL: Local and Global

Local variables can only exist in a single function, while global variables can be located throughout the entire program.

10. What are the main features of MySQL?

MySQL helps to store and manage data.

The main features associated with MySQL are as follows:

Open source
MySQL is open source, free and easy to use.
It uses the GNU General Public license to define rules and regulations regarding the application.

Fast and reliable
MySQL stores data in memory, so data access and manipulation is fast with MySQL.

Scalability is the ability of systems to work easily.
MySQL works with large databases.

Data types
Unsigned/ Signed integers contain multiple data types such as floating point (FLOAT), double (EVEN), character (CHAR), variable character (VARCHAR), text, date, time.

Character Sets
It supports different character sets.

It has a flexible password system and provides a secure interface.

Support for large databases
Provides support for large databases.

Client and utilities
MySQL server comes with many clients and utilities.


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