SQL Queries (Structured Query Language)

SQL (Structured Query Language) ek standard language hai jo relational databases se communicate karne ke liye use hoti hai. Isse hum data ko create, retrieve, update, aur delete kar sakte hain.

A. Basic Structure: SELECT-FROM-WHERE

Har SQL query ka dil yehi 3 keywords hain:

• SELECT: Tumhe kya-kya columns chahiye. * ka matlab saare columns.
• FROM: Kis table se data chahiye.
• WHERE: Data filter karne ke liye condition.

Example: SELECT ename FROM Works WHERE cname = 'XYZ';

B. Joins: The Real Power

Asli maza tab hai jab humein data ek se zyada table se nikalna ho. JOIN do tables ko ek common column ke basis par jodta hai.

• INNER JOIN: Sirf wahi rows dikhata hai jahan dono tables me matching value ho.
• LEFT/RIGHT OUTER JOIN: Ek table ki saari rows dikhata hai, bhale hi doosre me match na ho (wahan NULL aa jayega).
• FULL OUTER JOIN: Dono tables ki saari rows dikhata hai.

C. Aggregate Functions with GROUP BY and HAVING

Jab humein data ko summarize karna ho (e.g., total salary, average marks), tab hum Aggregate Functions (COUNT, SUM, AVG, MAX, MIN) use karte hain.

• GROUP BY: Aggregate functions ko groups me apply karta hai. (e.g., har company ki average salary).
• HAVING: GROUP BY se bane hue groups ko filter karta hai. (e.g., sirf unhi companies ko dikhao jinki average salary 50000 se zyada ho).

D. Subqueries (Nested Queries)

Ek query ke andar doosri query. Andar waali query pehle chalti hai aur apna result bahar waali query ko deti hai. Yeh complex problems solve karne me bahut kaam aati hai.