DA0-002 Data Acquisition and Preparation Practice Question
A data analyst is profiling a dataset and finds that the 'email' column contains some NULL values. Which SQL query can be used to count how many rows have a NULL email?
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
✓
SELECT COUNT(*) FROM table WHERE email IS NULL
COUNT(*) counts all rows; WHERE email IS NULL filters only null rows.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
SELECT COUNT(email) FROM table WHERE email = NULL
Why it's wrong here
NULL cannot be compared with =; also COUNT(email) excludes nulls.
- ✗
SELECT SUM(CASE WHEN email IS NULL THEN 1 END) FROM table
Why it's wrong here
This also works but is less straightforward.
- ✗
SELECT COUNT(ISNULL(email)) FROM table
Why it's wrong here
ISNULL returns 1 if null else 0; COUNT counts all rows.
- ✓
SELECT COUNT(*) FROM table WHERE email IS NULL
Why this is correct
Correct: counts all rows with null email.
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Written by Johnson Ajibi, MSc IT Security
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