MySQL Cheat Sheet

त्वरित संदर्भ

यह एक MySQL cheat sheet है — जो commands आप रोज़ इस्तेमाल करते हैं, एक जगह। अभ्यास करते समय इसे पास रखिए। एक बार ये मुख्य patterns परिचित लगें, ज़्यादातर असली queries बस इनके variations हैं।

Databases और tables

SHOW DATABASES;
CREATE DATABASE shop;
USE shop;
SHOW TABLES;
CREATE TABLE items (id INT PRIMARY KEY AUTO_INCREMENT, name VARCHAR(50), price INT);
DESCRIBE items;
ALTER TABLE items ADD stock INT;
DROP TABLE items;

Insert, update, delete

INSERT INTO items (name, price) VALUES ('Pen', 10);
INSERT INTO items (name, price) VALUES ('Pen', 10), ('Book', 50);  -- कई rows
UPDATE items SET price = 12 WHERE id = 1;
DELETE FROM items WHERE id = 1;

UPDATE और DELETE को हमेशा WHERE जोड़िए, वरना हर row प्रभावित होती है।

Selecting और filtering

SELECT * FROM items;
SELECT name, price FROM items;
SELECT * FROM items WHERE price > 20;
SELECT * FROM items WHERE name LIKE 'P%';
SELECT * FROM items WHERE price BETWEEN 10 AND 50;
SELECT DISTINCT name FROM items;
SELECT * FROM items ORDER BY price DESC LIMIT 5;

Joins

-- inner join (सिर्फ़ matches)
SELECT o.id, c.name
FROM orders o JOIN customers c ON o.customer_id = c.id;

-- left join (सारी left rows, matched या नहीं)
SELECT c.name, o.id
FROM customers c LEFT JOIN orders o ON o.customer_id = c.id;

Aggregates और grouping

SELECT COUNT(*) FROM items;
SELECT AVG(price), MAX(price), MIN(price), SUM(price) FROM items;
SELECT name, COUNT(*) FROM items GROUP BY name;
SELECT name, COUNT(*) FROM items GROUP BY name HAVING COUNT(*) > 1;

Rows filter करने को WHERE, GROUP BY के बाद groups filter करने को HAVING इस्तेमाल कीजिए।

उपयोगी functions

SELECT NOW(), CURDATE();
SELECT UPPER(name), LOWER(name), LENGTH(name) FROM items;
SELECT ROUND(price, 2), CEIL(price), FLOOR(price) FROM items;
SELECT IFNULL(stock, 0) FROM items;
SELECT CONCAT(name, ' - ', price) FROM items;

सारांश

• Databases/tables: CREATE, USE, SHOW, DESCRIBE, ALTER, DROP।
• CRUD: INSERT, SELECT, UPDATE, DELETE (बदलाव को हमेशा WHERE)।
• WHERE, LIKE, BETWEEN से filter; ORDER BY से sort; LIMIT से cap।
• JOIN ... ON से join; unmatched rows रखने को LEFT JOIN।
• COUNT/SUM/AVG और GROUP BY / HAVING से summarise।

आगे, इन्हें अभ्यास कीजिए SQL queries practice में, या गहराई से जाइए 50 MySQL queries से।