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)
2. 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)
3. 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)
4. 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)
5. 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)
6. 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.
7. 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)
8. 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)
9. 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)
10. 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)
