You have a table employees with 100 rows. The phone_number column has 80 non-NULL values and 20 NULLs. You want to find how many employees have a phone number and how many total employees there are. Which query gives both counts correctly?
ASELECT COUNT(phone_number) + COUNT(*) AS total FROM employees;
BSELECT COUNT(*) AS with_phone, COUNT(phone_number) AS total_employees FROM employees;
CSELECT COUNT(phone_number) AS with_phone, COUNT(*) AS total_employees FROM employees;
DSELECT COUNT(phone_number) AS total_employees, COUNT(*) AS with_phone FROM employees;
Step-by-Step Solution
Solution:
Step 1: Count employees with phone numbers
COUNT(phone_number) counts only non-NULL phone numbers, so it returns 80.
Step 2: Count total employees
COUNT(*) counts all rows, so it returns 100.
Final Answer:
SELECT COUNT(phone_number) AS with_phone, COUNT(*) AS total_employees FROM employees; -> Option C
Quick Check:
COUNT(phone_number) = 80, COUNT(*) = 100 [OK]
Quick Trick:Use COUNT(column) for non-NULL, COUNT(*) for total rows [OK]
Common Mistakes:
MISTAKES
Swapping counts in SELECT clause
Adding counts instead of separate columns
Confusing which count counts NULLs
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