Which query correctly finds these customers using EXCEPT?
hard📝 Application Q9 of 15
SQL - Set Operations
You want to find customers who bought products last year but not this year. You have tables customers_2023 and customers_2024 with columns customer_id and product_id. Which query correctly finds these customers using EXCEPT?
ASELECT customer_id FROM customers_2023 EXCEPT SELECT customer_id, product_id FROM customers_2024;
BSELECT customer_id, product_id FROM customers_2023 EXCEPT SELECT customer_id FROM customers_2024;
CSELECT customer_id FROM customers_2023 EXCEPT SELECT customer_id FROM customers_2024;
DSELECT customer_id FROM customers_2023 INTERSECT SELECT customer_id FROM customers_2024;
Step-by-Step Solution
Solution:
Step 1: Identify columns to compare
We want customers who bought last year but not this year, so compare customer_id only.
Step 2: Ensure matching columns in EXCEPT
Both SELECTs must have same columns; selecting only customer_id works.
Step 3: Write correct query
SELECT customer_id FROM customers_2023 EXCEPT SELECT customer_id FROM customers_2024;
Final Answer:
SELECT customer_id FROM customers_2023 EXCEPT SELECT customer_id FROM customers_2024; -> Option C
Quick Check:
Matching columns needed for EXCEPT [OK]
Quick Trick:EXCEPT requires same columns in both SELECTs [OK]
Common Mistakes:
MISTAKES
Mismatching column counts in SELECTs
Using INTERSECT instead of EXCEPT
Selecting extra columns causing errors
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