SQL Basic Interview Questions

RDBMS is one of the most commonly used databases to date, and therefore SQL Skills are indispensable in most of the job roles. In this SQL Interview Questions article, I will introduce you to the most frequently asked questions on SQL (Structured Query Language). This article is the perfect guide for you to learn all the concepts related to SQL, Oracle, MS SQL Server, and MySQL database. Our Top 65 SQL Interview Questions article is the one-stop resource from where you can boost your interview preparation.

Let’s get started!

Q1. What is the difference between SQL and MySQL?

SQL vs MySQL

SQL	MySQL
SQL is a standard language which stands for Structured Query Language based on the English language	MySQL is a database management system.
SQL is the core of the relational database which is used for accessing and managing database	MySQL is an RDMS (Relational Database Management System) such as SQL Server, Informix etc.

Q2. What are the different subsets of SQL?

• Data Definition Language (DDL) – It allows you to perform various operations on the database such as CREATE, ALTER, and DELETE objects.
• Data Manipulation Language(DML) – It allows you to access and manipulate data. It helps you to insert, update, delete and retrieve data from the database.
• Data Control Language(DCL) – It allows you to control access to the database. Example – Grant, Revoke access permissions.

Q3. What do you mean by DBMS? What are its different types?

A Database Management System (DBMS) is a  software application that interacts with the user, applications, and the database itself to capture and analyze data. A database is a structured collection of data.

A DBMS allows a user to interact with the database. The data stored in the database can be modified, retrieved and deleted and can be of any type like strings, numbers, images, etc.

There are two types of DBMS:

• Relational Database Management System: The data is stored in relations (tables). Example – MySQL.
• Non-Relational Database Management System: There is no concept of relations, tuples and attributes.  Example – MongoDB

Q4. What do you mean by table and field in SQL?

A table refers to a collection of data in an organised manner in form of rows and columns. A field refers to the number of columns in a table. For example:

Table: StudentInformation
Field: Stu Id, Stu Name, Stu Marks

Q5. What are joins in SQL?

A JOIN clause is used to combine rows from two or more tables, based on a related column between them. It is used to merge two tables or retrieve data from there. There are 4 types of joins, as you can refer to below:

• Inner join: Inner Join SQL is the most common type of join. It is used to return all the rows from multiple tables where the join condition is satisfied.

• Left Join:  Left Join in SQL is used to return all the rows from the left table but only the matching rows from the right table where the join condition is fulfilled.
• Right Join: Right Join in SQL is used to return all the rows from the right table but only the matching rows from the left table where the join condition is fulfilled.
• Full Join: Full join returns all the records when there is a match in any of the tables. Therefore, it returns all the rows from the left-hand side table and all the rows from the right-hand side table.

Q6. What is the difference between CHAR and VARCHAR2 datatype in SQL?

Both Char and Varchar2 are used for characters datatype but varchar2 is used for character strings of variable length whereas Char is used for strings of fixed length. For example, char(10) can only store 10 characters and will not be able to store a string of any other length whereas varchar2(10) can store any length i.e 6,8,2 in this variable.

Q7. What is a Primary key?

• A Primary Key in SQL is a column (or collection of columns) or a set of columns that uniquely identifies each row in the table.
• Uniquely identifies a single row in the table
• Null values not allowed

Example- In the Student table, Stu_ID is the primary key.

Q8. What are Constraints?

Constraints in SQL are used to specify the limit on the data type of the table. It can be specified while creating or altering the table statement. The sample of constraints are:

• NOT NULL
• CHECK
• DEFAULT
• UNIQUE
• PRIMARY KEY
• FOREIGN KEY

Q9. What is the difference between DELETE and TRUNCATE statements?

DELETE vs TRUNCATE

DELETE	TRUNCATE
Delete command is used to delete a row in a table.	Truncate is used to delete all the rows from a table.
You can rollback data after using delete statement.	You cannot rollback data.
It is a DML command.	It is a DDL command.
It is slower than truncate statement.	It is faster.

Q10. What is a Unique key?

• Uniquely identifies a single row in the table.
• Multiple values allowed per table.
• Null values allowed.

Q11. What is a Foreign key in SQL?

• Foreign key maintains referential integrity by enforcing a link between the data in two tables.
• The foreign key in the child table references the primary key in the parent table.
• The foreign key Constraint prevents actions that would destroy links between the child and parent tables.

Q12. What do you mean by data integrity? 

Data Integrity defines the accuracy as well as the consistency of the data stored in a database. It also defines integrity constraints to enforce business rules on the data when it is entered into an application or a database.

Q13. What is the difference between clustered and non-clustered index in SQL?

The differences between the clustered and non clustered index in SQL are :

1. Clustered index is used for easy retrieval of data from the database and its faster whereas reading from non clustered index is relatively slower.
2. Clustered index alters the way records are stored in a database as it sorts out rows by the column which is set to be clustered index whereas in a non clustered index, it does not alter the way it was stored but it creates a separate object within a table which points back to the original table rows after searching.
3. One table can only have one clustered index whereas it can have many non clustered index.

Q14. Write a SQL query to display the current date?

In SQL, there is a built-in function called GetDate() which helps to return the current timestamp/date.

Q15.What do you understand by query optimization?

The phase that identifies a plan for evaluation query which has the least estimated cost is known as query optimization.

The advantages of query optimization are as follows:

• The output is provided faster
• A larger number of queries can be executed in less time
• Reduces time and space complexity

SQL Interview Questions

Q16. What do you mean by Denormalization?

Denormalization refers to a technique which is used to access data from higher to lower forms of a database. It helps the database managers to increase the performance of the entire infrastructure as it introduces redundancy into a table. It adds the redundant data into a table by incorporating database queries that combine data from various tables into a single table.

Q17. What are Entities and Relationships?

Entities:  A person, place, or thing in the real world about which data can be stored in a database. Tables store data that represents one type of entity. For example – A bank database has a customer table to store customer information. The customer table stores this information as a set of attributes (columns within the table) for each customer.

Relationships: Relation or links between entities that have something to do with each other. For example – The customer name is related to the customer account number and contact information, which might be in the same table. There can also be relationships between separate tables (for example, customer to accounts).

Q18. What is an Index?

An index refers to a performance tuning method of allowing faster retrieval of records from the table. An index creates an entry for each value and hence it will be faster to retrieve data.

Q19. Explain different types of index in SQL.

There are three types of indexes namely:

Unique Index:

This index does not allow the field to have duplicate values if the column is unique indexed. If a primary key is defined, a unique index can be applied automatically.

Clustered Index:

This index reorders the physical order of the table and searches based on the basis of key values. Each table can only have one clustered index.

Non-Clustered Index:

Non-Clustered Index does not alter the physical order of the table and maintains a logical order of the data. Each table can have many nonclustered indexes.

Q20. What is Normalization and what are the advantages of it?

Normalization in SQL is the process of organizing data to avoid duplication and redundancy. Some of the advantages are:

• Better Database organization
• More Tables with smaller rows
• Efficient data access
• Greater Flexibility for Queries
• Quickly find the information
• Easier to implement Security
• Allows easy modification
• Reduction of redundant and duplicate data
• More Compact Database
• Ensure Consistent data after modification

Q21. What is the difference between DROP and TRUNCATE commands?

DROP command removes a table and it cannot be rolled back from the database whereas TRUNCATE command removes all the rows from the table.

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Q22. Explain different types of Normalization.

There are many successive levels of normalization. These are called normal forms. Each consecutive normal form depends on the previous one.The first three normal forms are usually adequate.

• First Normal Form (1NF) – No repeating groups within rows
• Second Normal Form (2NF) – Every non-key (supporting) column value is dependent on the whole primary key.
• Third Normal Form (3NF) – Dependent solely on the primary key and no other non-key (supporting) column value.

Q23. What is the ACID property in a database?

ACID stands for Atomicity, Consistency, Isolation, Durability. It is used to ensure that the data transactions are processed reliably in a database system.

• Atomicity: Atomicity refers to the transactions that are completely done or failed where transaction refers to a single logical operation of a data. It means if one part of any transaction fails, the entire transaction fails and the database state is left unchanged.
• Consistency: Consistency ensures that the data must meet all the validation rules. In simple words,  you can say that your transaction never leaves the database without completing its state.
• Isolation: The main goal of isolation is concurrency control.
• Durability: Durability means that if a transaction has been committed, it will occur whatever may come in between such as power loss, crash or any sort of error.

Q24. What do you mean by “Trigger” in SQL?

Trigger in SQL is are a special type of stored procedures that are defined to execute automatically in place or after data modifications. It allows you to execute a batch of code when an insert, update or any other query is executed against a specific table.

Q25. What are the different operators available in SQL?

There are three operators available in SQL, namely:

1. Arithmetic Operators
2. Logical Operators
3. Comparison Operators

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Q26.  Are NULL values same as that of zero or a blank space? 

A NULL value is not at all same as that of zero or a blank space. NULL value represents a value which is unavailable, unknown, assigned or not applicable whereas a zero is a number and blank space is a character.

Q27. What is the difference between cross join and natural join?

The cross join produces the cross product or Cartesian product of two tables whereas the natural join is based on all the columns having the same name and data types in both the tables.

Q28. What is subquery in SQL?

A subquery is a query inside another query where a query is defined to retrieve data or information back from the database. In a subquery, the outer query is called as the main query whereas the inner query is called subquery. Subqueries are always executed first and the result of the subquery is passed on to the main query. It can be nested inside a SELECT, UPDATE or any other query. A subquery can also use any comparison operators such as >,< or =.

Q29. What are the different types of a subquery?

There are two types of subquery namely, Correlated and Non-Correlated.

Correlated subquery: These are queries which select the data from a table referenced in the outer query. It is not considered as an independent query as it refers to another table and refers the column in a table.

Non-Correlated subquery: This query is an independent query where the output of subquery is substituted in the main query.