SQL Interview Preparation Q&A | Part -1

• What is SQL, and why is it used
  
  

SQL (Structured Query Language) is a standard programming language used to manage and manipulate relational databases. It helps to query, insert, update, and delete data efficiently.

SELECT * FROM Customers WHERE Country = India; (code-box)

• What are the different types of SQL statements
  
  

SQL Statements Are Categorized Into Five Types

• DDL (Data Definition Language)
• DML (Data Manipulation Language)
• DQL (Data Query Language)
• TCL (Transaction Control Language)
• DCL (Data Control Language)
  
  

CREATE TABLE Employees ( ID INT PRIMARY KEY, Name VARCHAR(50), Salary DECIMAL(10, 2) );(code-box)

• What is the difference between DELETE and TRUNCATE
  
  

DELETE: Removes specific rows based on a condition. It can be rolled back.

TRUNCATE: Removes all rows from a table. It is faster but cannot be rolled back.

DELETE FROM Employees WHERE ID = 1; TRUNCATE TABLE Employees;(code-box)

• What is a JOIN in SQL
  
  

JOINs are used to combine rows from two or more tables based on a related column. Common types are INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.

SELECT Orders.OrderID, Customers.CustomerName FROM Orders INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID;(code-box)

• What are indexes in SQL
  
  

An index is used to speed up the retrieval of rows by creating a pointer to data in the database. It improves query performance.

CREATE INDEX idx_customer_name ON Customers (CustomerName);(code-box)

• What is normalization in SQL
  
  

Normalization is the process of organizing data to reduce redundancy and improve data integrity. Common forms include 1NF, 2NF, 3NF, and BCNF.

• What is a primary key
  
  

A primary key is a unique identifier for a table row. It ensures that no two rows have the same value for the primary key column.

CREATE TABLE Students ( StudentID INT PRIMARY KEY, Name VARCHAR(50) );(code-box)

• What is a foreign key
  
  

A foreign key is a field that establishes a relationship between two tables. It references the primary key in another table.

CREATE TABLE Orders ( OrderID INT PRIMARY KEY, CustomerID INT, FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID) );(code-box)

• What is a stored procedure
  
  

A stored procedure is a precompiled SQL code that can be executed repeatedly. It helps to encapsulate complex logic and improve performance.

CREATE PROCEDURE GetCustomerOrders AS SELECT * FROM Orders WHERE CustomerID = 1;(code-box)

• What is a transaction in SQL
  
  

A transaction is a sequence of operations performed as a single logical unit of work. Transactions follow ACID properties (Atomicity, Consistency, Isolation, Durability).

BEGIN TRANSACTION; UPDATE Accounts SET Balance = Balance - 500 WHERE AccountID = 1; UPDATE Accounts SET Balance = Balance + 500 WHERE AccountID = 2; COMMIT;(code-box)