[Solved] Why does PostgreSQL remove duplicates by default in UNION but not in UNION ALL? — Ans: To ensure result sets represent true sets without duplicates | PostgreSQL

PostgreSQL - Set Operations and Advanced Queries

Why does PostgreSQL remove duplicates by default in UNION but not in UNION ALL?

ABecause UNION ALL only works with numeric data

BTo ensure result sets represent true sets without duplicates

CTo avoid syntax errors in UNION queries

DBecause UNION ALL is slower and less efficient

Step-by-Step Solution

Solution:

Step 1: Understand set theory in SQL
UNION represents a mathematical set which does not allow duplicates.
Step 2: Explain difference with UNION ALL
UNION ALL returns all rows including duplicates, useful when duplicates matter.
Final Answer:
To ensure result sets represent true sets without duplicates -> Option B
Quick Check:
UNION removes duplicates to model sets [OK]

Quick Trick: UNION models sets, so removes duplicates by default [OK]

Common Mistakes:

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Why does PostgreSQL remove duplicates by default in UNION but not in UNION ALL?