[Solved] You have two SELECT queries:SELECT id FROM orders_2021;andSELECT id FROM orders_2022;You want to get all unique IDs from both years but... Which PostgreSQL set operation combination achieves this? | PostgreSQL

PostgreSQL - Set Operations and Advanced Queries

You have two SELECT queries:
SELECT id FROM orders_2021;
and
SELECT id FROM orders_2022;
You want to get all unique IDs from both years but keep duplicates only from orders_2021. Which PostgreSQL set operation combination achieves this?

AUse <code>UNION ALL</code> between both queries and then <code>DISTINCT</code> on the result.

BUse <code>UNION ALL</code> and then remove duplicates only from the second query before union.

CUse <code>UNION ALL</code> and then subtract duplicates from the second query using <code>EXCEPT</code>.

DUse <code>UNION ALL</code> and then join with the first query to filter duplicates.

Step-by-Step Solution

Solution:

Step 1: Understand the requirement
Keep duplicates from orders_2021 but only unique rows from orders_2022 that are not in orders_2021.
Step 2: Use EXCEPT to remove duplicates from second query
SELECT id FROM orders_2022 EXCEPT SELECT id FROM orders_2021 returns unique IDs in 2022 not in 2021.
Step 3: Combine with UNION ALL
Then SELECT id FROM orders_2021 UNION ALL (previous EXCEPT result) keeps duplicates from 2021 and adds unique 2022 IDs.
Final Answer:
Use UNION ALL and then subtract duplicates from the second query using EXCEPT. -> Option C
Quick Check:
Check if duplicates only from first query remain [OK]

Quick Trick: Subtract duplicates from second query before UNION ALL [OK]

Common Mistakes:

Applying DISTINCT after UNION ALL removes all duplicates
Not removing duplicates from second query
Using JOIN which complicates the logic

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You have two SELECT queries:

Step 1: Understand the requirement

Step 2: Use EXCEPT to remove duplicates from second query

Step 3: Combine with UNION ALL

Final Answer:

Quick Check:

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