DA0-002 Data Acquisition and Preparation Practice Question

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

  DISTINCT COUNT(customer_id) is not valid SQL syntax; the DISTINCT keyword belongs inside the function as COUNT(DISTINCT customer_id). It is tempting because it expresses the intended logic in plain English, but no major database engine parses it.

• ✗

  COUNT(customer_id)

  Why it's wrong here

  COUNT(customer_id) counts every non-null customer_id row, so repeat orders inflate the total; DISTINCT is required inside the count. It is tempting because it counts customers rather than rows, and would be correct if each customer appeared only once.

• ✓

  COUNT(DISTINCT customer_id)

  Why this is correct

  COUNT(DISTINCT customer_id) eliminates duplicate customer identifiers before tallying, returning the number of unique customers rather than total order rows. This directly satisfies the requirement to count distinct customers, whereas plain COUNT(customer_id) would include repeat purchasers multiple times and overstate the customer base.

• ✗

  COUNT(*)

  Why it's wrong here

  COUNT(*) counts all order rows, including duplicates and nulls, so it returns orders rather than distinct customers. It is tempting as the fastest way to count rows, and would be correct when the question asks for total orders.