SQL Interview Questions

Top 10 SQL Interview Questions and Answers You Must Prepare for 2021

Relational databases such as SQL are the most extensively used database management systems in the world. The amount of data available to us is rapidly increasing and the need to access and analyze this data is the topmost need. There’s a high market demand for individuals with SQL skills and thus, learning the top SQL interview questions will help you stand out from the crowd and ensure that you get your dream job. In this blog, we will talk about the top SQL interview questions and answers. The blog covers SQL Interview Questions for fresher’s and SQL interview questions for experienced professionals.

What is SQL?

Structured Query Language or SQL is the primary language that is used to interact with databases. Through SQL, we are able to extract data from the database, we can update this data, and we can modify it as in when required. This particular query language has been in existence for a really long time and is widely used across most of the industries in the world. Let’s assume that an organization records all its employees’ details in a database. Using SQL, we are able to find out valuable information and insights with the help of this data within a short period of time.

How do you create a table in SQL? 

Creating a table in SQL is fairly simple. All you have to do is follow this command.

CREATE TABLE table_name (

     column3 datatype,

     column2 datatype,

     column1 datatype,

   ….

);

Let us further understand how this command works with the help of an example. First, we should use the CREATE TABLE command, followed by the name of the table. Within braces, we must list out the columns and the data type corresponding to them. Here, we are creating a table with details of students in a class.

CREATE TABLE student (

     name varchar(30),

     age int,

     gender varchar(15),

   ….

);

How can you delete a table in SQL? 

In SQL, if you wish to delete a table, there are two commands that you can use. One is the DROP command, and another one is the TRUNCATE command. If you want to completely delete the entire table from the database, you should use the DROP TABLE command. This is the command that you must use:

DROP TABLE table_name; 

This command will delete all the data within the table, and will also delete the table itself. Now, if you wish to delete the data within a table but not the entire table itself, you must use the TRUNCATE command.

TRUNCATE TABLE table_name;

This command will help you delete the data within a given table.

Can you change a table name in SQL? How can you do so? 

Yes. It is possible to change a table name in SQL. To change the table name, you should use the following command.

ALTER TABLE table_name

RENAME TO new_table_name;

We start off by selecting the table which we want to alter the name. After this, we use a command to rename that particular table to another name. To select the table whose name you wish to change, use the ALTER TABLE command followed by the name of the table. After this, use the RENAME TO command followed by the new name that you wish to give. Let us assume we want to change the “student” table to “student_information”, we will use the following command to do so.

ALTER TABLE student

RENAME TO student_information;

How can you delete a row in SQL? 

Deleting a row in SQL is a very simple process and requires a simple command. We can only delete rows from an existing table, and we must be careful to mention the table name correctly.

DELETE FROM table_name

WHERE [condition];

Here, we are starting the command with the DELETE FROM query, followed by the name of the table where the row we want to delete is present. We will then give the WHERE clause and state the condition in which the row should be deleted. Going forward with the same student table example we have seen so far if we want to delete the rows in that table where the student is of age 18, use the following command.

DELETE FROM student

WHERE [age=18];

How can you create a database in SQL? 

A repository within SQL, where we can store multiple tables is known as a database. The command used is very simple and is as follows:

CREATE DATABASE database_name;

How can you insert the date in SQL? 

If the RDBMS that you are working with is MYSQL, all you have to do is follow this command.

“INSERT INTO tablename (col_name, col_date) VALUES (‘DATE: Manual Date’, ‘2021-2-4’)”;

What do you understand by primary key in SQL?

To understand what a primary key is, we must first know what we mean by constraints in SQL. A constraint is the rules that are enforced on data columns in a table. These constraints are used to limit the type of data that can be added to a table. They are either column level or table-level constraints. The primary key is one such constraint in SQL.

What do you understand by PL SQL?

Procedural Language Constructs for Structured Query Language or PL SQL was first introduced by Oracle. They introduced this concept to overcome the limitations they faced while using plain SQL. What PL SQL does is, it adds procedural language to the regular SQL. We must keep in mind that PL SQL can be used only in Oracle databases and you cannot work with it if you do not have an Oracle database. In the normal SQL, we can use DDL and DML queries. In the case of PL SQL, we can create triggers, functions, and other procedural constructs as well.

What do you understand by ETL in SQL?

Extract, Transform, and Load. ETL is a 3-step process that starts by extracting data from various sources. After this data has been extracted and collated, we have raw data available with us. Now, the raw data needs to be transformed into a cleaner format. This is where the next step of transforming comes into the picture. After this, the last step is to load this cleaned data into tools that will help us gain insights.

Conclusion 

This brings us to the end of the blog on SQL Interview Questions. These SQL questions and answers are relevant for freshers as well as experienced professionals and will help you brush up your knowledge before you head into the interview. We wish you all the best! And hope that this blog was of help to you. Happy Learning.

Larachris

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