Joins in DBMS 

Jab data alag-alag tables mein store kiya jaata hai, tab un tables ko ek saath laane ke liye Join use kiya jaata hai.

Join ek SQL operation hai jo 2 ya usse zyada tables ke rows ko combine karta hai based on koi common column.

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 Join kyun use karte hain?

Maan lo aapke paas 2 tables hain:

1. Students – jisme student ke naam aur department ID hai.

2. Departments – jisme department ka naam hai.

Ab aap chahte ho ki har student ke saath uska department ka naam bhi dikhe — ye possible hoga Join ki madad se.

 Example Tables

 Table: Students

 Table: Departments

 Types of Joins

1. INNER JOIN

Sirf wahi rows dikhata hai jahan dono tables mein matching value hoti hai.

SQL Example:

Result:

Diagram:

2. LEFT JOIN (LEFT OUTER JOIN)

Left table (Students) ke sabhi rows show karega, aur right table (Departments) ke sirf matching rows. Match na ho toh NULL show karega.

SQL Example:

Result:

3. RIGHT JOIN (RIGHT OUTER JOIN)

Right table (Departments) ke sabhi rows show karega, aur left table (Students) ke sirf matching rows. Match na ho toh NULL show karega.

SQL Example:

Result:

4. FULL OUTER JOIN

Dono tables ke sabhi rows show karega — match ho toh dono ka data, warna NULL jahan match nahi ho.

Note: MySQL mein direct support nahi hota, par PostgreSQL, SQL Server mein hota hai.

Result:

5. CROSS JOIN

Dono tables ke sabhi combinations deta hai — har row of table 1 ke saath table 2 ki har row.

SQL Example:

Agar 3 students × 3 departments = 9 rows banengi

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 Join Types Summary

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 Conclusion

Joins help karte hain:

• -> Alag-alag tables ka data combine karne mein.

• -> Structured aur normalized data ko ek saath dikhane mein.

• -> Query ko zyada meaningful aur powerful banane mein.