What is the Difference Between UNION and UNION ALL in SQL?

UNION and UNION ALL are SQL operators used to combine results from multiple SELECT queries.

In simple words:

• UNION combines results and removes duplicates
• UNION ALL combines results and keeps duplicates

Simple Understanding

Why UNION and UNION ALL are Important

Large applications often store data in:

• Multiple tables
• Multiple databases
• Partitioned systems

UNION operators help combine these results into a single output.

Real-Time Example

Suppose a learning platform stores:

• Java students
• Python students

Java Students Table

Python Students Table

What is UNION?

UNION combines results from multiple SELECT queries and removes duplicate rows.

UNION Syntax

SELECT column_name FROM table1

UNION

SELECT column_name FROM table2;

Example

SELECT name FROM java_students

UNION

SELECT name FROM python_students;

Result

Important Observation

Duplicate value:

Rahul

appears only once.

Why?

Because:

UNION automatically removes duplicates.

UNION Internal Flow

First Query Result
        |
        v
Second Query Result
        |
        v
Combine Results
        |
        v
Remove Duplicates
        |
        v
Final Output

What is UNION ALL?

UNION ALL combines results from multiple SELECT queries and keeps duplicate rows.

UNION ALL Syntax

SELECT column_name FROM table1

UNION ALL

SELECT column_name FROM table2;

Example

SELECT name FROM java_students

UNION ALL

SELECT name FROM python_students;

Result

Important Observation

Duplicate value:

Rahul

appears twice.

Why?

Because:

UNION ALL does not remove duplicates.

UNION ALL Internal Flow

First Query Result
        |
        v
Second Query Result
        |
        v
Combine Results Directly
        |
        v
Final Output

Main Difference Between UNION and UNION ALL

Why UNION is Slower

UNION performs:

• Duplicate checking
• Sorting or hashing operations

This increases processing time.

Why UNION ALL is Faster

UNION ALL:

• Simply combines results
• Does not check duplicates

This improves performance.

Performance Comparison

UNION
   |
   v
Combine + Remove Duplicates
   |
   v
Higher Processing

UNION ALL
   |
   v
Direct Combine
   |
   v
Faster Processing

Rules for UNION and UNION ALL

Both queries must have:

• Same number of columns
• Compatible data types

Correct Example

SELECT id, name FROM students

UNION

SELECT id, name FROM teachers;

Invalid Example

SELECT id, name FROM students

UNION

SELECT id FROM teachers;

Error Reason

Number of columns does not match.

ORDER BY with UNION

ORDER BY should be used:

At the end of final query.

Example

SELECT name FROM java_students

UNION

SELECT name FROM python_students

ORDER BY name;

UNION vs JOIN

Visualization

UNION

Table1 Rows
      +
Table2 Rows

JOIN

Table1 Columns <-> Table2 Columns

Real-Time Banking Example

Banking systems may combine:

• Savings account transactions
• Current account transactions

Example

SELECT transaction_id, amount
FROM savings_transactions

UNION ALL

SELECT transaction_id, amount
FROM current_transactions;

Why UNION ALL?

Because:

• All transaction records are needed

Real-Time E-Commerce Example

E-commerce systems may combine:

• Online orders
• Store orders

Example

SELECT customer_name
FROM online_orders

UNION

SELECT customer_name
FROM store_orders;

Why UNION?

Because:

• Duplicate customers should appear only once

Real-Time Analytics Example

Analytics systems heavily use:

UNION ALL

because:

• Performance is critical
• All records are needed

Advantages of UNION

• Removes duplicates automatically
• Produces clean result sets
• Useful for unique reporting

Advantages of UNION ALL

• Faster performance
• No duplicate removal overhead
• Best for large datasets

Disadvantages of UNION

• Slower performance
• Additional sorting overhead

Disadvantages of UNION ALL

• Duplicate data may appear

Best Practices

• Use UNION when unique results are required
• Use UNION ALL for better performance when duplicates are acceptable
• Prefer UNION ALL in analytics and reporting systems
• Ensure column counts and data types match

Related Learning Topics

• Mastering MySQL Joins
• What is GROUP BY in SQL?
• MySQL Aggregate Functions
• What is a Table in SQL?
• MySQL Performance Optimization

Professional Interview Answer

UNION and UNION ALL are SQL operators used to combine results from multiple SELECT queries. UNION removes duplicate rows and returns only unique results, whereas UNION ALL keeps all rows including duplicates. UNION is slower because it performs duplicate checking and sorting operations, while UNION ALL is faster because it directly combines result sets without duplicate removal. UNION is commonly used when unique data is required, whereas UNION ALL is preferred for better performance in large-scale reporting and analytics systems.

Why Interviewers Like This Answer

• Clearly explains duplicate handling
• Includes performance comparison
• Shows query execution understanding
• Provides real-world examples
• Explains enterprise usage scenarios

Frequently Asked Questions

What is difference between UNION and UNION ALL?

UNION removes duplicates, while UNION ALL keeps duplicates.

Which is faster UNION or UNION ALL?

UNION ALL is faster because it does not remove duplicates.

Why UNION is slower?

UNION performs duplicate checking and sorting operations.

Can UNION combine different column counts?

No, both queries must return same number of columns.

When should UNION ALL be used?

UNION ALL should be used when all records including duplicates are required and performance is important.