Databases · Junior · Question 15 of 75

What is the difference between `COUNT(*)` and `COUNT(phone)`?

1COUNT(phone) counts distinct phone numbers
2They always return the same number
✓Correct answer: COUNT(*) counts all rows; COUNT(phone) counts only rows where phone is not NULL

Explanation

Aggregate functions ignore NULLs, except COUNT(*). AVG(score) also averages only non-NULL values.

Further reading

PostgreSQL: Aggregate functions reference ↗

Test yourself

Take the Databases · Junior quiz: questions and options are shuffled, and you get a review of your mistakes at the end.