You should know certain SQL concepts if you plan to attend an SQL interview. No matter which RDBMS you use, be it MySQL, Oracle, Microsoft SQL Server, PostgreSQL, or any other, these SQL concepts are common to all popular RDBMS.
In this article, you will see an edited list of SQL interview questions with answers you are likely to hear during a SQL interview. Candidates will likely be asked basic SQL interview questions to advance to the level of SQL interview questions for 3 years experienced professionals, depending on their experience and various other factors. The list below includes all SQL Technical Interview Questions for Beginners, SQL Server Interview Questions for Experienced Candidates, and some tricky SQL Query Interview Questions.

1. What is a DBMS?


A Database Management System (DBMS) is a program that manages the creation, maintenance, and use of a database. A DBMS can be called a file manager that collects data in a database rather than storing it in file systems.

2. What is an RDBMS?


RDBMS stands for Relational Database Management System. An RDBMS stores data in a collection of tables that are linked by standard fields between table columns. It also provides relational operators for manipulating data stored in tables.
Example: SQL Server.
What is SQL?
SQL is a Structured Query Language used to communicate with the Database. This standard language performs tasks such as retrieving, updating, inserting, and deleting data from a database.
Standard SQL statements are Select.

3. What is a database?


A database is nothing but an ordered form of data for easy access, storage, retrieval, and management of data. It is also a structured form of data that can be accessed in many ways.
Example: Database of school management, Database of bank management.

4. What are tables and arrays?


A table is a data set organized into a model with columns and rows. Columns can be compartmentalized as vertical and rows as horizontal. A table has only a specified number of columns called fields but can have any number of rows called records.
Example:.
Table: Employee.
Data: 201456, David, 11/15/1960.

5. What is a primary key?


A primary key is a mixture of fields that uniquely identify a row. This unique kind of special key has an implicit NOT NULL constraint. This means that primary base values ??cannot be NULL.

6. What is a unique key?


A unique key restraint uniquely recognizes each record in the Database. This ensures the uniqueness of a column or set of columns.
A primary key constraint has a unique automatic restriction defined.
Many unique constraints can be defined for each table, but only one primary base control is limited.

7. What is a foreign key?


A foreign key is one table that can be connected to the primary key of another table. A foreign critical reference relationship must be created between two tables with the primary key of the other table.

8. What is a connection?


This keyword is used to query data from multiple tables based on the relationship between table fields. Keys play an essential role when using JOINs.

9. What are the types of joins, and explain each one?


Depending on the table's relationship, different types of joins can be used to retrieve data.

Inner join
An inner join returns rows when there is at least one-row match between the tables.

Right Join
Join the standard return lines between the tables and all the rows of the table on the right. It simply returns all rows from the right-hand table, even if there are no matches in the left-hand table.

Left Join 
Left join return rows are shared between tables and all rows of the table on the left side. It simply returns all rows from the left-hand table, even if there are no matches in the right-hand table.

Full Join
A full join returns rows if matching rows exist in either table. That is, it returns all rows from the table on the left and all from the right.

10. What is normalization?


Normalization is the process of reducing redundancy and dependency by arranging database fields and tables. The main goal of normalization is to add, remove, or modify areas that can be created in a single table.

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11. What is denormalization?


Denormalization is a method for accessing data from higher to lower standard database forms. It also introduces redundancy into a table by incorporating data from related tables.

12. What are all the different normalizations?


Standard forms can be divided into 5 stages which are explained below -.

First Normal Form (1NF):

This should remove all duplicate columns from the table. Creating tables for related data and identifying unique cues.

Second Normal Form (2NF):

Fulfillment of all requirements of the first standard form. Placing subsets of data into separate tables and creating relationships between tables using primary keys.

Third standard form (3NF):

This should satisfy all 2NF requirements. Deleting columns that are not subject to primary key constraints.

Fourth Normal Form (4NF):

It meets all the requirements of the third standard form and should not have multi-valued dependencies.

13. What is a view?


A view is a virtual table containing a subset of the data. Ideas are practically not present, and their storage takes up less space. A picture can have combined data from one or more tables, depending on the relationship.

14. What is an index?


The index is a performance tuning method that allows faster retrieval of records from a table. The index will create an entry for each value, and data retrieval will be faster.

15. What are local and global variables and their dissimilarities?


Local variables are variables that can be used or exist inside a function. Other parts are unknown and cannot be referenced or used. Variables can be created each time this function is called.
Global variables are variables that can be used or exist throughout the program. The same variable announced in global cannot be used in functions. Global variables cannot be created each time this function is called.

16. What is a limitation?


Constraints can be used to specify the data type limit of a table. A condition can be set when creating or modifying a table statement. Sample restrictions are.
NON-ZERO.
CHECK.
THE DEFAULT.
UNIQUE.
PRIMARY KEY.
FOREIGN KEY.

17. What is the distinction between clustered and non-clustered indexes?


A clustered index quickly retrieves data from a database by changing the record store. A database sorts rows by columns, which is called a clustered index.
A non-clustered index does not change how it was stored but creates a separate object in the table. After searching, it points back to the original rows of the table.

18. What are all types of user-defined functions?


There are three types of user-defined functions.
Scalar function.
Inline Table valuable features.
Multiple commands of the particular function.
A scalar return unit is a variant defined by the return clause. The other two types return an array as a return.

19. What is sorting?


Collation is a set of rules specifying how character data can be sorted and compared. This can be used to compare the characters of A and other languages ??and also depends on the width of the characters.
ASCII values ??can be used to compare this character data.

20. What are the different types of shift sensitivity?


Following are the different types of shift sensitivity -.
  • Case Sensitivity - A and a and B and b.
  • Accent sensitivity.
  • Kana Sensitivity - Japanese Kana characters.
  • Width sensitivity - Single-byte character and double-byte character.

Final Thoughts 


Interview questions are important in preparing for a dream role you always aspire for. We hope this blog has helped you realize the top 20 SQL interview questions for freshers and experts and their relevant answers. Best of luck with your future works; keep learning, as that's the way to succeed.