Top 25 SQL Interview Questions You Must Prepare

Q1. 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.

Q2. What do you mean by DBMS? What are its different types?

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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

Q3. 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

Q4. 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 in 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.

Q5. 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.

Q6. 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

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 is the difference between DELETE and TRUNCATE statements?

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.

Q9. What is a Unique key?

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

Q10. 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.

Q11. 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.

Q12. 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.

Q13. 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.

Q14. 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.

Q15. What are the different operators available in SQL?

There are three operators available in SQL

Arithmetic Operators
Logical Operators
Comparison Operators

Q16. 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.

Q17. 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 = .

Q18. 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.

Q19. List the ways to get the count of records in a table?

To count the number of records in a table in SQL, you can use the below commands:

SELECT * FROM table1

SELECT COUNT(*) FROM table1

SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table1) AND indid < 2

Q20. Write a SQL query to find the names of employees that begin with ‘A’?

To display name of the employees that begin with ‘A’, type in the below command:

1	`SELECT * FROM Table_name WHERE EmpName like 'A%'`

Q21. Write a SQL query to get the third-highest salary of an employee from employee_table?

SELECT TOP 1 salary FROM (SELECT TOP 3 salaryFROM employee_tableORDER BY salary DESC) AS empORDER BY salary ASC;

Q22. What is the need for group functions in SQL?

Group functions work on the set of rows and return one result per group. Some of the commonly used group functions are: AVG, COUNT, MAX, MIN, SUM, VARIANCE.

Q23. What is the main difference between ‘BETWEEN’ and ‘IN’ condition operators?

BETWEEN operator is used to display rows based on a range of values in a row whereas the IN condition operator is used to check for values contained in a specific set of values.

Example of BETWEEN:

SELECT * FROM Students where ROLL_NO BETWEEN 10 AND 50;

Example of IN:

SELECT * FROM students where ROLL_NO IN (8,15,25);

Q24. What is CLAUSE in SQL?

SQL clause helps to limit the result set by providing a condition to the query. A clause helps to filter the rows from the entire set of records.

For example – WHERE, HAVING clause.

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Q25. What is the difference between ‘HAVING’ CLAUSE and a ‘WHERE’ CLAUSE?

HAVING clause can be used only with SELECT statement. It is usually used in a GROUP BY clause and whenever GROUP BY is not used, HAVING behaves like a WHERE clause.
Having Clause is only used with the GROUP BY function in a query whereas WHERE Clause is applied to each row before they are a part of the GROUP BY function in a query.

Top 25 SQL Interview Questions You Must Prepare