How to select all data in SQL?

Use the SELECT * FROM table_name; command.

What is the difference between INNER and LEFT JOIN?

INNER JOIN returns records with matching values in both tables. LEFT JOIN returns all records from the left table, and the matched records from the right table.

What is a Bitmap Index?

A Bitmap Index uses a grid of bits (0s and 1s) to represent whether a value is present in a row. It is best for columns with low cardinality.

Quick Reference

Ultimate SQL Cheat Sheet

A comprehensive guide to essential SQL commands, operations, and optimization techniques. Perfect for interviews, exams, and daily data work.

Basic Queries

Select All Columns

SELECT * FROM users;

Select Specific Columns

SELECT name, email FROM users;

Filter Data

SELECT * FROM users WHERE country = 'USA';

Sort Results

SELECT * FROM users ORDER BY created_at DESC;

JOIN Operations

INNER JOIN (Default)

Returns records with matching values in both tables.

SELECT * FROM orders o JOIN users u ON o.user_id = u.id;

LEFT JOIN

Returns all records from the left table, and matched records from the right.

SELECT * FROM users u LEFT JOIN orders o ON u.id = o.user_id;

Aggregations

Group By & Count

SELECT country, COUNT(*) FROM users GROUP BY country;

Filter Aggregated Data

Use HAVING instead of WHERE for aggregated data.

SELECT country, COUNT(*) FROM users GROUP BY country HAVING COUNT(*) > 5;

Advanced Functions

Coalesce (Fallback Defaults)

Returns the first non-null value.

SELECT COALESCE(phone, 'N/A') FROM users;

Window Functions

Compute ranks without collapsing rows.

SELECT name, salary, RANK() OVER (ORDER BY salary DESC) FROM employees;

Indexing Types

B-Tree Index

Default standard. Best for exact matches and range queries.

Hash Index

Lightning-fast for exact matches only. No range support.

Bitmap Index

Best for low-cardinality columns (e.g., Status, Gender).

Optimization Tips

Use Explain

Check execution plans before optimizing.

EXPLAIN QUERY PLAN SELECT * FROM users WHERE id = 5;

Avoid Leading Wildcards

This prevents index usage (Full Table Scan).

-- Slow WHERE name LIKE '%son';