Which aggregate function counts the number of non-null values in a column per group?

• A. COUNT(*)
• B. COUNT(column_name)
• C. SUM(column_name)
• D. AVG(column_name)

Answer: (B)

Counting non-null values per group relies on applying the count to a specific column. When you group rows with GROUP BY and use a count on a column, SQL tallies only the rows where that column is not null, giving you the number of non-null entries for each group. This differs from counting all rows with COUNT(), which includes rows where the column is null. While SUM and AVG also work with numeric data and skip nulls, they return a total and an average, not a count of non-null values. For example, if a group has values 5, 7, and NULL in that column, COUNT(column) yields 2, SUM(column) yields 12, AVG(column) yields 6, and COUNT() yields 3.