Problems with Null Values and Dangling Tuples in DBMS

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 Null Value kya hoti hai?

Database mein NULL ka matlab hota hai:
 -> koi value nahi hai,
 -> value abhi pata nahi hai, ya
 -> value applicable hi nahi hai.

Note: NULL ka matlab 0 ya empty string (“”) nahi hota.

 Null Value ka Example

Sochiye ek Employee table hai:

Yahan Rohan ke liye ManagerID = NULL hai, matlab uska manager abhi pata nahi hai.

 Null Values ke Problems

1.  Logic Confuse ho jata hai
          Null ke saath = use nahi kar sakte:

      SELECT * FROM Employee WHERE ManagerID = NULL;


         Ye query kuch bhi return nahi karegi. Sahi syntax hoga:

      WHERE ManagerID IS NULL


2. Join karte waqt dikkat
         Agar 2 tables join kar rahe ho aur koi NULL value ho, to matching records nahi milte.

3. Functions ka result galat ho sakta hai
      SUM, AVG jaise functions NULL ko ignore karte hain.

4. Constraints weak ho jaate hain
         NULL values ka use karne se foreign key constraints kaam nahi karte, data inconsistent ho sakta hai.

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 Dangling Tuple kya hoti hai?

Dangling tuple wo record hota hai jo kisi doosre table ki value ko reference karta hai, lekin wo value exist hi nahi karti.

 Dangling Tuple ka Example

 Department Table

 Employee Table

Yahan Karan ka DeptID = 503 hai, lekin Department table mein 503 ka record hi nahi hai. Ye ek dangling tuple hai.

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 Dangling Tuples ke Problems

1. Referential Integrity Break hoti hai
         Foreign key kisi existing record ko refer nahi kar rahi hoti.

2. Join karne mein dikkat
         Join karne par data mismatch hota hai — kuch records chhoot jaate hain.

3. Data inconsistent ho jaata hai
         Fake ya incomplete relationships bante hain, jo galat data dikhate hain.

 In problems ka solution kya ho sakta hai?

 Null Value ke liye:

• Query mein IS NULL ya IS NOT NULL use karo.

• NULL avoid karne ki koshish karo — default values ya constraints use karo.

 Dangling Tuples ke liye:

• Foreign Key constraints lagao, taaki sirf valid values hi insert ho.

• Pehle parent table ka data daalo, phir dependent table ka.

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 Summary Table