SQL can be defined as a special-purpose language that helps interact with databases. Most applications that use data have strong relational database systems. SQL lets you interact freely with these relational databases. 
 
Let’s take a look at some SQL interview questions you should always be prepared for.
 

Important SQL Questions and Answers:

Q1. What is SQL?
SQL is short for Structured Query Language, with the primary purpose of interacting with databases. SQL is the default query language that is used in communicating with RDBMS (Relational Database Management Systems) and also for carrying out various operations on different data types that are related to data manipulation. To sum up, SQL is a database language required to make or delete databases, while it can also fetch or modify table rows among other things.
 
Q2. What are the different types of commands for SQL?
The different SQL commands to be learned are
1. DDL or Data Definition Language: DDL helps to define the database’s data structure during the starting stage before the database is created. Its main purpose is creating and restructuring database objects. The commands within DDL are
  • Create table
  • Drop table
  • Alter table
2. DML or Data Manipulation Language: DML manipulates existing data within a database. In other words, it allows users to retrieve and manipulate this data. DML is used while performing operations like adding data to the database using the ‘Insert’ command, keeping data up-to-date with the ‘Update’ command and removing data from databases with the ‘Delete’ command. 
 
3. DCL or Data Control Language: DCL essentially controls or restricts access to data within a database. The DCL subsets are most often used for creating objects that are related to user access as well as controlling privilege distribution in authorised users. The ‘Grant’ and ‘Revoke’ commands are used in DCL.
 
4. TCL or Transaction Control Language: TCL commands help in controlling the changes that DML commands make. They also authorise statements regarding assembling in conjunction into transactions. ‘Commit’, ‘Rollback’, ‘Savepoint’, ‘Begin’ and ‘Transaction’ are the commands used in TCL.
 
 
Q3. Describe the different database management system types.
There are four main types of database management systems. 
  1. Hierarchical Database: This database is structured like a tree and data is stored in a hierarchy. To understand this better, a parent structure can have many things, but a child structure can have just one parent.
  2. Network Database: This database is structured like a graph where different points can all be related. It allows a single child structure to have multiple relationships.
  3. Relational Database: This database is structured like a table. The values given through the rows and columns are closely related to each other. Relational databases are easy to understand and use, making them the most widely used databases.
  4. Object-oriented Database: Data values and operations are structured as objects in this database. Each of the objects has multiple relationships amongst themselves.
Q4. What are the applications of SQL?
SQL databases can be used to perform several operations. This includes
  • Creation of new databases
  • Insertion of new data
  • Deleting existing data
  • Record updates
  • Data retrieval
  • Dropping and creating tables
  • Creation of functions and views
  • Conversion of data types
Q5. What are default constraints?
A constraint is used for specifying a set of rules that help to process data and limit the kind of data that will go in a table. A default constraint’s main purpose is to establish a default value in a column so that it can be added to any new record where no value has been specified.
 
Q6. What is meant by field and table in SQL?
Organised data and information stored in the form of columns and rows is called a table. Rows are referred to as tuples and columns as attributes.
The total number of columns made within a table is known as a field. In a database record, a field represents attributes and characteristics. 
 
Q7. What is a primary key?
The main objective of a primary key is uniquely identifying all the table records present. There can be no null values and there must be unique values. One table can have a maximum of one primary key, consisting of either single or multiple fields.
 
Q8. What is a unique key?
A key that accepts only null values and cannot process or accept a duplicate value is known as a unique key. The purpose of having a unique key is to ensure that every row and column is unique.
 
Q9. What is the difference between a Unique key and a Primary key?
Both the primary and unique keys have unique values, but there are two main differences between them.
  • A primary key cannot have null values, while a unique key can.
  • In a table, you can have multiple unique keys, but no more than one primary key.
Q10. What is a foreign key?
A foreign key can be defined as a set of attributes or an attribute that provides a reference to a primary key within another table. In other words, a foreign key links two tables together. 
 
Q11. What is an index?
An index helps to speed up searches in a database. Without an index on columns within the WHERE clause, the SQL server will have to go over entire tables and check every row to ascertain a match. This will slow down operations where large volumes of data are involved.
An index is used to find all the rows that match with specific columns and then skim through these specific data subsets to find a match.
 
Q12. What is the difference between a clustered and non clustered index?
Clustered Index: A clustered index sorts through rows of data using their key values. A clustered index can be thought of as a phone book and its context. You can choose one particular first name and you will find information for all these first names in the same place. As the data is all placed close to each other, you can find data easily using a clustered index for range-based searches. Additionally, clustered indexes are related to the way data gets stored. Only one clustered index can be present in a single table. 
Non-clustered Index: A non-clustered index stores indexes in one location in the database and data at a different location. The index contains pointers that direct users to the data location. Since indexes in a non-clustered index are stored in various places, you can have several non-clustered indexes in a single table.
 
Q13. What is the difference between MySQL and SQL?
Nature: MySQL is a database management system, while SQL is a query language that is used within a database.
Use: MySQL allows the handling, storing and modifying of data in a structured way. SQL is used for querying and operating database systems.
Updates: MySQL updates itself from time to time, but SQL remains the same. 
Storage engine: MySQL can support multiple engines, while SQL supports single storage engines. 
Server: During backup sessions in MySQL, the database is blocked by the server. SQL is not server-dependent and remains unaffected. 
 
Q14. What is the difference between PL/ SQL and SQL?
  • PL/SQL is a programming language for databases that use SQL. SQL is a structured query language.
  • PL/SQL is made up of code that is used to create procedures and functions. SQL is a query that executes DDL and DML commands.
  • PL/SQL is application-oriented and procedural. SQL is data-oriented and declarative.
  • PL/SQL is mainly used to create applications, while SQL is used to manipulate data. 
  • PL/SQL provides no interaction with database servers, while SQL does provide interaction.
  • PL/SQL can contain SQL code as it is an extension of SQL itself. But SQL cannot have PL/SQL code.
Q15. What is ACID in databases?
ACID is short for Atomicity, Consistency, Isolation, Durability. ACID is used for checking the reliability of transactions. 
  • Atomicity means transactions that have failed or been completed. A transaction is a defined logical data operation. This means that if a transaction fails even in part, the entire transaction is considered failed. The consequence is that the database remains unchanged.
  • Consistency refers to the validity guidelines that data should meet. A transaction doesn’t leave a database without completing its state. 
  • Isolation has the primary purpose of managing concurrency
  • The role of durability is ensuring that once transactions take place, they will complete regardless of hazards or disturbances like fire outbreaks, power outages etc. 

Also Read: Complete Guide To Prepare For Microsoft SQL Certification

Q16. What is the purpose of group functions in SQL?
A group function operates on multiple columns and rows and gives back a single result for every group. The most popularly used group functions are COUNT(), AVG(), SUM(), VARIANCE(), MAX() and MIN().
 
Q17. What does Self Join mean?
Self Join is used in SQL to join a table with the same table. Upon meeting set criteria, every row in a table is joined to itself and other rows in the same table.
 
If you are looking to take your career to the next level with SQL, enrolling in a training course and find answers to all the database interview questions there are. Give your career the boost it deserves and enrol today.
 
Armin Vans
Aarav Goel has top education industry knowledge with 4 years of experience. Being a passionate blogger also does blogging on the technology niche.

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