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1 Answers

Anubhav Sharma
Anubhav Sharma 12 May 2026
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UNION and JOIN are both used to combine data in SQL, but they work very differently.

UNION

UNION combines rows from multiple SELECT queries into a single result set.

Example

SELECT Name FROM Employees
UNION
SELECT Name FROM Customers

Result

John
Aman
Sara

It stacks results vertically.

UNION Rules

  • Number of columns must match
  • Data types should be compatible
  • Removes duplicate rows automatically

UNION ALL

Keeps duplicates.

SELECT Name FROM Employees
UNION ALL
SELECT Name FROM Customers

JOIN

JOIN combines columns from related tables using a condition.

Example

SELECT 
    Orders.OrderId,
    Customers.Name
FROM Orders
INNER JOIN Customers
ON Orders.CustomerId = Customers.Id

Result

101   John
102   Sara

It combines data horizontally.

JOIN Types

JOIN Type Meaning
INNER JOIN Matching records only
LEFT JOIN All left + matched right
RIGHT JOIN All right + matched left
FULL JOIN All records from both
CROSS JOIN Cartesian product

Visual Difference

UNION

Table A
1
2

Table B
3
4

UNION Result
1
2
3
4

JOIN

Customers          Orders
1 John             101 CustomerId=1

JOIN Result
1 John 101

Main Difference

Feature UNION JOIN
Combines Rows Columns
Works On Multiple SELECT results Related tables
Direction Vertical Horizontal
Requires Relation No Yes
Duplicate Handling Removes duplicates No duplicate removal
Column Count Must Match Yes No

Real-World Example

UNION Use Case

Combine data from:

  • CurrentEmployees
  • FormerEmployees
SELECT Name FROM CurrentEmployees
UNION
SELECT Name FROM FormerEmployees

JOIN Use Case

Show order with customer details.

SELECT 
    Customers.Name,
    Orders.Total
FROM Customers
JOIN Orders
ON Customers.Id = Orders.CustomerId

Performance Difference

UNION

  • Slower because duplicates are checked
  • UNION ALL is faster

JOIN

Performance depends on:

  • Indexes
  • Join condition
  • Table size

Conclusion

Use:

  • UNION → Combines rows vertically
  • JOIN → Combines columns horizontally
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