🧾 SQL Introduction
Last Updated: 7th January 2026
SQL (Structured Query Language) is a standard language used to communicate with relational databases.Using SQL, we can store, retrieve, update, and delete data from a database.
🤔 Why SQL Is Important?
- Almost every application uses a database
- SQL works with most relational databases
- Easy to learn and very powerful
- Required for backend, data analysis, and data engineering
Used in:
- Web applications
- Banking systems
- Data analysis (with Python, Excel, BI tools)
- Enterprise software
SQL Working Principle?
SQL is used with Relational Database Management Systems (RDBMS) such as:
- MySQL
- PostgreSQL
- Oracle
- SQL Server
- SQLite
It execute in following steps:
- SQL Query → Query Language Processor → DBMS Engine → Database
SQL Query?
An SQL Query is a command written in SQL to perform an operation on the database.
- SQL queries are case-insensitive.
- We Can terminate multiple commands with a semicolon (;).
Examples of operations:
- Fetch data
- Insert new data
- Update existing data
- Delete data
Example of a Simple SQL Query
SELECT * FROM students;
This query means:
- SELECT → choose data
- * → all columns
- FROM students → from the students table
SQL Subquery?
A SQL Subquery is a query inside another query.
Example
SELECT *
FROM students
WHERE marks > (
SELECT AVG(marks)
FROM students
);
This query means:
- SELECT → choose data
- * → all columns
- FROM students → from the students table
- WHERE marks > → where marks are greater than
- SELECT AVG(marks) → average marks from the students table
SQL Clause?
An SQL Clause is a part of an SQL query that defines how data should be selected or filtered.
A query is usually made of multiple clauses combined together.
Commonly Used SQL Clauses
- SELECT → specifies columns
- FROM → specifies table
- WHERE → filters rows
- ORDER BY → sorts result
- GROUP BY → groups rows
- HAVING → filters grouped data
- LIMIT → restricts number of rows
Example Using Multiple Clauses
SELECT name, marks
FROM students
WHERE marks > 60
ORDER BY marks DESC;
Explanation:
- Select name and marks
- From students table
- Only rows where marks are greater than 60
- Sort results in descending order
SQL Operators And Functions?
An SQL Operator is a symbol or keyword used to perform specific operations in an SQL query.
An SQL Function is a predefined operation that performs a specific task in an SQL query.
- Arithmetic Operators (+, -, \*, /, %)
- Comparison Operators (>, <, >=, <=, =, !=, <>)
- Logical Operators (AND, OR, NOT)
- String Operators (LIKE, NOT LIKE, IN, NOT IN)
- Date and Time Operators (DATE, TIME, TIMESTAMP)
- NULL Handling Operators (IS NULL, IS NOT NULL)
- Union and Intersect Operators (UNION, INTERSECT)
- Aggregate Functions (COUNT, SUM, AVG, MAX, MIN)
- Window Functions (RANK, ROW_NUMBER, DENSE_RANK)
SQL Comments?
SQL Comments are used to explain queries or disable code execution.They are ignored by the database engine. They are two Types.
Single-Line Comment (--)
-- This is a single-line comment
SELECT * FROM students;
Multi-Line Comment(/* */)
- Improve code readability
- Helpful for teams
- Useful for debugging
/*
This is a
multi-line comment
*/
SELECT name FROM students;
⚠️ Important SQL Rules
- SQL keywords are not case-sensitive (SELECT = select)
- Table and column names depend on database settings
- SQL statements usually end with a semicolon ;
