Solution:Step 1: Understand UNION ALL vs UNIONUNION ALL returns all rows including duplicates; UNION removes duplicates.Step 2: Fix query to remove duplicatesReplacing UNION ALL with UNION removes duplicates automatically.Final Answer:Replace UNION ALL with UNION -> Option AQuick Check:UNION removes

[Solved] Consider this query:SELECT id FROM table1 UNION ALL SELECT id FROM table2;It returns duplicate rows. How can you fix it to return only unique rows? | PostgreSQL

PostgreSQL - Set Operations and Advanced Queries

Consider this query:

SELECT id FROM table1 UNION ALL SELECT id FROM table2;

It returns duplicate rows. How can you fix it to return only unique rows?

AReplace UNION ALL with UNION

BAdd DISTINCT inside each SELECT

CUse INTERSECT instead of UNION ALL

DAdd GROUP BY id at the end

Step-by-Step Solution

Solution:

Step 1: Understand UNION ALL vs UNION
UNION ALL returns all rows including duplicates; UNION removes duplicates.
Step 2: Fix query to remove duplicates
Replacing UNION ALL with UNION removes duplicates automatically.
Final Answer:
Replace UNION ALL with UNION -> Option A
Quick Check:
UNION removes duplicates, UNION ALL does not [OK]

Quick Trick: Use UNION (not UNION ALL) to remove duplicates [OK]

Common Mistakes:

Using UNION ALL when duplicates are unwanted
Adding DISTINCT inside each SELECT unnecessarily
Using INTERSECT which returns only common rows

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Consider this query:

Step 1: Understand UNION ALL vs UNION

Step 2: Fix query to remove duplicates

Final Answer:

Quick Check:

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