SQL CHEAT SHEET — Save This Post!
Most Asked Queries in Tech Interviews!

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SQL is asked in 90% of tech interviews —
TCS, Infosys, Amazon, even AI roles need it!
Master these queries TODAY!

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BASIC QUERIES

SELECT * FROM employees;
-> Get all data from table

SELECT name, salary FROM employees
WHERE salary > 50000;
-> Filter rows with condition

SELECT * FROM employees
ORDER BY salary DESC LIMIT 5;
-> Top 5 highest paid employees

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AGGREGATE FUNCTIONS

SELECT COUNT(*) FROM employees;
-> Total number of rows

SELECT AVG(salary) FROM employees;
-> Average salary

SELECT department, COUNT(*) FROM employees
GROUP BY department;
-> Count employees per department

SELECT department, AVG(salary) FROM employees
GROUP BY department
HAVING AVG(salary) > 60000;
-> Departments with avg salary > 60k

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JOINS — Most Asked in Interviews!

INNER JOIN -> Only matching rows in both tables
SELECT e.name, d.dept_name
FROM employees e
INNER JOIN departments d
ON e.dept_id = d.id;

LEFT JOIN -> All rows from left + matching right
SELECT e.name, d.dept_name
FROM employees e
LEFT JOIN departments d
ON e.dept_id = d.id;

SELF JOIN -> Table joined with itself
SELECT e1.name, e2.name AS manager
FROM employees e1
JOIN employees e2
ON e1.manager_id = e2.id;

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SUBQUERIES — Asked in Advanced Rounds!

Find employees earning more than average:
SELECT name FROM employees
WHERE salary > (
SELECT AVG(salary) FROM employees
);

Find 2nd highest salary (Classic Question!):
SELECT MAX(salary) FROM employees
WHERE salary < (
SELECT MAX(salary) FROM employees
);

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WINDOW FUNCTIONS — Modern SQL!

Rank employees by salary:
SELECT name, salary,
RANK() OVER (ORDER BY salary DESC) as rnk
FROM employees;

Running total of salaries:
SELECT name, salary,
SUM(salary) OVER (ORDER BY id) as running_total
FROM employees;

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MUST-KNOW CONCEPTS:

Primary Key -> Unique identifier for each row
Foreign Key -> Links 2 tables together
Index -> Speeds up search queries
Normalization -> Reduce data redundancy
Transaction -> ACID properties (all or nothing)

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TOP 5 SQL INTERVIEW QUESTIONS:

1. Find duplicate records in a table
2. Find employees with no manager
3. Find departments with zero employees
4. Find Nth highest salary
5. Find employees who joined this month

Practice these on a free SQL site!

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PRACTICE FREE ON:
SQLZoo.net
HackerRank SQL section
LeetCode Database problems

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