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Mohit Decodes
Mohit Decodes

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Learning SQL with Handwritten Notes ✍️

#sql #react #frontend #backend

SQL (Structured Query Language) is the standard language for dealing with relational databases. Whether you're a backend developer, data analyst, or a tech enthusiast, SQL is an essential tool in your toolkit.

Today, I learned and created handwritten notes to help solidify the concepts. Here's what I covered:

🔹 What is SQL?

SQL is used to read, manipulate, and store data in relational databases.

Why SQL?

  • Easy to learn & use
  • Powerful for querying large datasets
  • Preferred for data audits over spreadsheets

🧾 Basic SQL Syntax

SELECT Statement 

SELECT * FROM Sales;
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Selects all columns from the Sales table.

SELECT year, month FROM Sales;
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Selects specific columns.

Aliases 

SELECT west AS "West Region" FROM Sales;
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Renames a column temporarily for readability.

🔍 Filtering with WHERE Clause

Basic Comparison 

SELECT * FROM Sales WHERE Country = 'India';
SELECT * FROM Sales WHERE City != 'Delhi';
SELECT * FROM Sales WHERE SalesAmount > 50000;
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🔢 LIMIT Clause 

SELECT * FROM Sales LIMIT 100;
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Limits output to 100 records.

➕ Arithmetic Operations

SQL supports row-wise arithmetic:

SELECT west + south AS total_sales FROM Sales;
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📋 Data Modification

CREATE TABLE 

CREATE TABLE Person (
  PersonID int,
  LastName varchar(255),
  FirstName varchar(255),
  Address varchar(255),
  City varchar(255)
);
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INSERT INTO 

INSERT INTO Person (FirstName, LastName) VALUES ('John', 'Doe');
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UPDATE 

UPDATE Sales SET City = 'Goa' WHERE CustomerID = 1;
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DELETE 

DELETE FROM Sales WHERE CustomerName = 'Bob';
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🕳️ NULL Handling 

SELECT * FROM Sales WHERE Address IS NULL;
SELECT * FROM Sales WHERE Address IS NOT NULL;
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📊 Aggregate Functions

  • COUNT()
  • SUM()
  • AVG()
  • MIN()
  • MAX() 
SELECT COUNT(*) FROM Sales;
SELECT SUM(SalesAmount) FROM Sales;
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📦 GROUP BY & HAVING 

SELECT year, COUNT(*) FROM Sales GROUP BY year;
SELECT month, MAX(high) FROM Sales GROUP BY month HAVING MAX(high) > 400;
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🧠 Logical Operators

  • AND, OR, NOT
  • LIKE, IN, BETWEEN
  • IS NULL, IS NOT NULL

Got stuck? Want to showcase your version? Drop a link or comment below.
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