DA0-002 Data Acquisition and Preparation Practice Question
In a dataset of customer orders, you need to count the number of distinct customers who have placed orders. Which SQL aggregate function should you use?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
COUNT(DISTINCT customer_id)
COUNT(DISTINCT column) counts the number of unique non-null values in a column. COUNT(*) counts all rows including duplicates, COUNT(column) counts non-null values including duplicates. DISTINCT alone is not an aggregate function.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
DISTINCT COUNT(customer_id)
Why it's wrong here
Invalid SQL syntax; DISTINCT must go inside the COUNT function.
- ✗
COUNT(customer_id)
Why it's wrong here
Counts all non-null customer IDs including duplicates.
- ✓
COUNT(DISTINCT customer_id)
Why this is correct
Correctly counts unique customer IDs.
- ✗
COUNT(*)
Why it's wrong here
COUNT(*) counts all rows including duplicates, not distinct customers.
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