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✅ UNION & UNION ALL in SQL

 ✅ UNION & UNION ALL in SQL  

🧠 1. What is UNION?  

UNION is used to combine results from multiple SELECT queries  

"Merge data from two tables into one result”  


⚡ 2. Rules for UNION  

  •  Same number of columns  
  •  Same datatype/order of columns  


📊 Example Tables  

👨‍💼 employees_2025

Name  

- Ajit

- Neha  


👨‍💼 employees_2026  

name  

- Sujit 

- Neha  


🔥 3. UNION Example

SELECT name FROM employees_2025

UNION

SELECT name FROM employees_2025;


✔️ Removes duplicates automatically  


✅ Result  

name  

- Ajit

- Neha  

- Sujit  


⚡ 4. UNION ALL

SELECT name FROM employees_2025

UNION ALL

SELECT name FROM employees_2026;


  • Keeps duplicates  
  • Faster than UNION  


✅ Result  

name  

- Ajit

- Neha  

- Sujit  

- Neha  



🔥 5. UNION vs UNION ALL  

UNION  

  •  Removes duplicates
  •  Slower  
  • Doesn't keep all rows  


UNION ALL  

  • Doesn't remove duplicates  
  • Faster  
  • Keeps all rows  


⚡ 6. ORDER BY with UNION

SELECT name FROM employees_2025

UNION

SELECT name FROM employees_2026

ORDER BY name;


🎯 7. Practice Tasks  

1. Combine employee names using UNION  

2. Combine employee names using UNION ALL  

3. Identify duplicate removal  

4. Sort UNION result using ORDER BY  

5. Compare UNION vs UNION ALL output  


🧠 Why UNION ALL?  

👉 UNION → removes duplicates  

👉 UNION ALL → keeps duplicates + faster


#union #union_alll

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