SQL Interview Questions

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SQL Interview Questions

RDBMS (Relational Database Management System) or SQL (Structured Query Language) is one of the most common database systems which is being used in software and app development to this date. As a result, if you are thinking about making a career in software development, you must know all the ins and outs of the SQL database. In this article, we are going to help you prepare for your very first interview, which is based on your knowledge of SQL and databases. If you don’t know the answer to the interviewer’s questions, don’t feel bad about it. You are in the learning stage, and it’s easy to make mistakes or not know about things in general.

Once the interview is over, make a list of all the questions asked and search the answers for the ones you were not able to provide during the interview. This is one of the best ways to learn a new concept and get yourself to know about the things which are required in the company. Below, we have provided you with a list of some of the most commonly asked questions in an SQL interview.

1. What Do You Understand by the Term DBMS?

DBMS stands for Database Management System, and it is an application that is developed for the user to interact with. Also, it works as an interface between the software and its database. A database is a set of information that is collected by the software. With the help of DBMS, a database can be modified, and its data can be deleted, as well as added. There are two types of DBMS.

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RDBMS: The Relational DBMS stores the data in relations or tables, a typical example of RDBMS is MySQL.

Non-RDBMS: In this form of DBMS, there is no concept of relations, tuples, and even attributes. The working example of this type of RDBMS is MongoDB.

2. How is SQL Different from Other Components of Software?

SQL stands for Structured Query Language, and its most common use is in software development as it becomes the communication between the back end and the database. This is a standard language that is used to perform the tasks like retrieval, updating, insertion, etc.

3. What is a Primary Key?

A Primary Key is a column that is present in SQL, and it could be a single column or a group of columns depending on the database. The Primary Key aims to uniquely identify each row in the table. But in the Primary Key null values are not allowed at all.

4. What are Constraints?

Constraints in SQL are used to provide the information of the data type for a given table. The constraints can be specified when you are creating or making changes in the content of the table. Some of the examples of constraints are NOT NULL, CHECK, DEFAULT, UNIQUE, etc.

5. What is a Table and a Field?

A Table is a way to represent the data in the form of columns and rows. The columns in the table are present in the vertical axis, and the rows are used to define the content horizontally. A Table comes with a specified number of columns which are known as fields. On the other hand, the number of rows is called the records. In a database, the table field can be the Employee Name, Date of Birth, and Employee ID.

6. What Do You Understand by Unique Constraint?

With the unique constraint, your database will ensure that no two values present in the column are the same. This leads a user to find the columns and rows uniquely, unlike the primary key, which can’t be defined multiple times. A user can easily define multiple unique constraints as per requirement.

7. How Do We Create a Table in SQL?

The command to create a table using the SQL goes like this:

CREATE TABLE table_name (

column1 datatype,

column2 datatype,

column3 datatype,

….

);

The above-written command will create the table by table_name; after that, in column 1, there will be the datatype value which can be anything.

8. How to Change a Table Name in SQL?

Changing names in SQL is pretty simple. We can use two commands to do it. The first one is ALTER TABLE. The second command is RENAME TO.

9. What Do You Understand by Foreign Key in SQL?

With the use of the Foreign key, we can check the referential integrity of the two tables. In addition to this, a foreign which is located in the child table will reference the primary key in the parent table. Moreover, when a foreign key is used, it will also help a programmer prevent actions that could mistakenly destroy the link between the parent and the child table.

10. What is Data Integrity?

Data integrity stands for the accuracy as well as the consistency of the data which is being stored by a developer or software in the database. In addition to this, data integrity also defines the integrity constraints which provide the business rules on the data when the database fetches it.

11. What is a Join? What are its Different Types?

Join in the database is a keyword that is used for query data from more tables and whose relationship is defined in between the fields of the different tables. When we are using Join for the two tables, keys of both the tables will also play an essential role in forming a relationship.

Inner Join: We use Inner Join when we want to retrieve the same data present in the two different tables. This is the most used join query in database management.

Left (OUTER) Join: This query will provide a user with all the records/rows which are available on the left and their matching records from the right table.

Right (OUTER) Join: When we use this query, it will retrieve all the records which are present in the rows of the right table and match the records of the left table.

Full (OUTER) Join: This query will help us retrieve all the records where there is a match in between the tables.

12. What Do You Understand by Self Join?

A self-join is quite like a standard join query, but the table is joined by itself in the self join. This query works on the relation of a table to itself. Moreover, the self join uses the Inner join or Left join clause. Also, a table alias will be used to provide different names to the table within the query.

13. What is an SQL Server?

SQL Server is the relational database management system developed by Microsoft. The main reason behind the development of SQL servers is to give developers an alternative to the MySQL and Oracle databases. The SQL server works fine with ANSI SQL, which is a gold standard for SQL language. The SQL server comes with two ways of implementing, the first one being SQL language, and the second is Transact-SQL (T-SQL). The T-SQL is managed and run by the Microsoft company, and it allows a user to declare variables, exception handling, stored procedures, and more. All these added features make it a great language to implement, but you will find other ways to do the same thing in simple SQL. As a result, a lot of companies prefer SQL over T-SQL.

14. What is Pl SQL?

The Pl SQL stands for the procedural SQL language, which is developed by using the structured query language. Oracle designed PI SQL to overcome the limitations of plain SQL. As a result, we can say that Pl SQL is an upgrade to the SQL language.

A user needs to know that Pl SQL only runs on the database, which is formed using the Oracle database. Thus, if you are not working on the Oracle database, then you cannot use the Pl SQL queries to perform specific tasks.

With the use of simple SQL, a user can efficiently perform DDL and DML queries, but with the implementation of Pl SQL, one can create functions, triggers, and even other procedural constructs.

15. How to See All the Tables Present in SQL?

Each database management system has a different set of queries to display all the tables. To see all the tables present in MySQL, we need to use show tables:

For Oracle, we need to write the following command:

SELECT

table_name

FROM

User_tables;

For SQL server, the command is:

SELECT

*

FROM

Information_schema.tables;

16. What Do You Understand by Clustered Index and Non-Clustered Index in SQL?

The clustered index is used to retrieve the data from the database; if we compare it with the non-clustered index, we can see the clustered index is much faster.

A clustered index works by sorting out rows from the columns. Thus, it alters the record which is stored by the developer in the database. On the other hand, a non-clustered index does not change the information provided in the database in any way possible. It creates a separate object in the given table, which points back to the rows of the original table after searching it out.

Any given table will have one clustered index, whereas the same table can have more than one non-clustered index.

17. What is Denormalization?

Denormalization is used when a user wants to access the data from higher to lower forms of a database. This will help database managers boost the performance of the entire infrastructure of their database, and this redundancy is introduced by the denormalization in the given tables. The redundancy feature comes in because of combining the data from various tables into a single table.

18. What are Different Types of Index?

There are three different types of indexes in the database, and they are:

  • Unique index: With the use of a unique index, all the fields which are present in the table will be unique. This index will be applied automatically when we define the primary key.
  • Clustered index: It will record the physical order in which the table is present, and the search done by this index will be based on the key values.
  • Unclustered index: This index will maintain the logical order of the data present in the table. A single table can have 999 types of unclustered indexes.

19. What are Union, Minus, and Intersect Commands in SQL?

  • The Union helps a user to get the combined result of two or more SELECT statements from the database.
  • The Minus operator is used when too many duplicates are present in the result-set, which was obtained using the SELECT query on the database. The Minus operator will remove any duplicates which are present in the result.
  • The Intersect is a clause that combines the result-set fetched by the two SELECT statements in which the record for the one matches the other, and then it will return the intersection in the result set.

20. What is Schema in SQL?

A database is made of different entities like tables, stored procedures, functions, database owners, and others. To make all of these entities interact with each other, we need to use the schema. So in simple terms, a schema is the logical relationship between all the different entities which are present in your database.

Also, the schema is used to provide information about who has access to specific tables in the database. Moreover, a user can easily modify the relationship between the different entities which are present in the database.

21. How Do You Implement Multiple Conditions by Using WHERE Clauses?

In SQL, we can quickly implement multiple conditions using OR and AND operators. Let’s take an example to display how you can use the WHERE clause in a command:

SELECT * FROM employees WHERE last_name = ‘Sachin’ AND salary <=20000;

From the above command, we are providing two conditions in the command. From the first condition, we make sure that the command will only extract those names from the employee Sachin. The second condition will filter out those Sachin whose salary is less than 20,000.

22. How to Copy Tables in SQL?

This is one of the most important commands which everyone working on SQL must know. A user can take the help of a SELECT INTO statement to copy the data from one table to the other. In addition to this, in SQL, we can choose what we want to copy to the new table. Given below is an example of how we can copy all the columns present in one table to the other.

SELECT *

INTO new table

FROM old table

WHERE condition;

On the other hand, if we are looking to copy a specific column of the first table, we have to write this command:

SELECT column1, column2, column3, …

INTO new table

FROM old table

WHERE condition;

23. Are NULL Values Same as the Zero or a Blank Space?

Many people mistake saying that the NULL value is the same as zero and the blank space. A NULL value represents a value that is not available or unknown to the user. On the other hand, a zero is a number, and the blank space present in the database is a character.

Conclusion

These are some of the most important theoretical questions which you should know before going to the interview room. In addition, you can also find several other questions on the Internet. But in the end, it is up to you to memorize the questions and their answers or to understand their logic by practicing the SQL databases. The second option is more preferable, and we recommend you follow it. We wish you all the best and hope this blog will help you crack your upcoming SQL-based job interview.

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