Top 20 SQL basic beginner level Interview Questions

1. What is SQL?

SQL (Structured Query Language) is a standard language used to manage and manipulate relational databases. It allows users to create, read, update, or delete data, provides structured queries, and is easy to understand to perform these operations effectively. Almost all data-driven applications depend on SQL to store and retrieve the data.

2.What is a database?

A database is a systematically organized data collection and typically stored in a table format consisting of rows and columns. A Database Management system (DBMS) helps to manage storage, manipulation, and retrieval of data

3. What are the various types of SQL commands?

The various types of SQL commands are :

DDL (Data Definition Language): CREATE, ALTER, DROP, TRUNCATE.

DML (Data Manipulation Language): SELECT, INSERT, UPDATE,   DELETE.

DCL (Data Control Language): GRANT, REVOKE.

TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT.

4.What is the Primary Key?

A Primary Key is used to avoid duplication of data in a table, whose values uniquely identify a row in the table. It ensures that no two rows have the same value in the primary key column(s), and it does not allow NULL values.

5.  What is Foreign Key?

A Foreign key is a set of attributes in a table that refers to the primary key of another table. It establishes and enforces a relationship between the two tables, ensuring data integrity.

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6. What is the difference between Primary Key and Unique Key?

Primary Key CREATE TABLE Employees ( EmpID INT PRIMARY KEY,   Name VARCHAR(100) );

Unique Key CREATE TABLE Users ( UserID INT PRIMARY KEY,   Email VARCHAR(100) UNIQUE );

7. What is the difference between CHAR and VARCHAR data types?

CHAR: It stores data in fixed-length storage. If the input is smaller than the defined size, it is padded with spaces. VARCHAR2: It stores data with variable length storage. It    uses space only for the actual data storage, making it more storage-efficient for shorter values.

8. What is normalization?

Normalization is the process of structuring data in a database to reduce duplication and enhance data integrity. It helps to minimize large tables into smaller, related tables and establish relationships between them to maintain consistency and avoid anomalies.

9.What is the purpose of the DEFAULT constraint?

When no value is specified during an INSERT operation, the DEFAULT constraint automatically assigns a default value to a column. This helps to maintain consistency and enhances data entry efficiency.

10. What are the different operators in SQL?

• Arithmetic Operators: +, -, *, /, %
• Comparison Operators: =, !=, <>, >, <, >=, <=
• Logical Operators: AND, OR, NOT
• Set Operators: UNION,INTERSECT,EXCEPT
• Special Operators: BETWEEN, IN, LIKE, IS NULL,IS NOT NULL, EXISTS

11.What are the different types of  joins in SQL?

INNER JOIN: It returns only the matching rows from both tables.

LEFT JOIN (or LEFT OUTER JOIN):Returns all rows from the left table and the matching rows from the right table.If there is no match, NULLs are shown for right table columns.

RIGHT JOIN (or RIGHT OUTER JOIN):Returns all rows from the right table and matching rows from the left table.If there is no match, NULLs are shown for left table columns.

FULL JOIN (or FULL OUTER JOIN):Returns all rows from both tables.

CROSS JOIN: Returns the Cartesian product of both tables

12. What are aggregate functions in SQL?

Aggregate functions perform calculations and return a single result based on a group or set of values The aggregate functions include:

• COUNT(): Returns the number of rows.
• SUM(): Returns the total sum of values.
• AVG(): Returns the average of values.
• MIN(): Returns the smallest value.
• MAX(): Returns the largest value.

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13. What is the difference between DELETE , DROP and TRUNCATE commands?

DELETE Command: Delete rows individually and records each deletion in the transaction log, making rollback possible. It can have a WHERE clause to delete specific records.

DROP Command: It completely removes the table structure and its data from the database. Once DROP Command is executed, the table is permanently deleted and can't be recovered unless restored from a backup.

TRUNCATE Command: It quickly removes all the rows from the table at once without logging each row deletion. It does not support a WHERE clause and is faster than DELETE for large datasets.

14. What are the types of constraints in SQL?

Constraints are applied to table columns to enforce data integrity and improves accuracy in a database.

NOT NULL: Ensures that a column cannot have NULL values.

UNIQUE: Ensures that all values in a column are different . (no duplicates).

PRIMARY KEY: Uniquely identifies each row in a table.

FOREIGN KEY: Maintains referential integrity by linking to a primary key in another table.

CHECK: Ensures that all values in a column meet a specific condition.

DEFAULT: Assigns a default value for a column, if no value is specified.

15. What are the differences between SQL and NoSQL databases? SQL Databases:

•  Use Structured Organized tables with rows and columns
•  It has Predefined Schema
•  It Supports ACID properties
• Use Cases: It is Ideal for complex queries and structured data
• Examples: MySQL, PostgreSQL, Oracle

NoSQL Databases:

• Use flexible, schema-less formats ( For eg. key-value, document, graph)
• It has dynamic or schema-less structure
• NoSQL is designed for horizontal scaling
• It is best for large-scale, unstructured, or real-time data needs
• Example: MongoDB, Cassandra, Redis

16. What is a subquery?

A subquery is a query nested within the another query, typically used in the WHERE , FROM, or SELECT clause to filter or retrieve data based on the results of another query, making it easier to handle complex filtering 

17. What is Stored Procedure?

Stored Procedures are created to perform one or more DML operators on the database. It is nothing but a group of SQL Statements that accepts some input in the form of parameters, performs some tasks, and may or may not return a value.

18.What is the difference between the WHERE and HAVING clauses?

WHERE clause: It is used to filter records before any grouping takes place and applying conditions to individual rows.

HAVING clause: It is used to filter data after the GROUP BY clause and applying conditions to groups of rows

19. What is a cursor?

A cursor is a database object used to retrieve and manipulate rows from a query result one at a time.

Used in stored procedures and loops to process individual rows.

Example in SQL:

DECLARE cursor_name CURSOR FOR

SELECT name FROM students;

20. What is a trigger in SQL?

Trigger is a special type of stored procedure that automatically runs when an event occurs in the database server. DML triggers run when a user tries to modify data through a data manipulation language (DML) event. DML events are INSERT, UPDATE, or DELETE statements on a table or view. If you found these questions helpful, there’s a lot more where that came from!

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