What is the difference between COUNT(*) and COUNT(column_name) when counting rows with NULLs?

• A. COUNT(*) counts rows regardless of NULLs; COUNT(column_name) ignores rows where column_name is NULL.
• B. COUNT(*) ignores NULLs; COUNT(column_name) counts all rows including NULLs.
• C. COUNT(*) and COUNT(column_name) both ignore NULLs.
• D. COUNT(*) counts only non-null values in column_name.

Answer: (A)

Counting with COUNT(*) looks at every row, regardless of NULLs in any column. It counts the total number of rows returned by the query (or after a WHERE/GROUP BY, the rows in that group). In contrast, COUNT(column_name) counts only the non-null values in that specific column; rows where column_name is NULL are not included in the count.

For example, if a table has three rows and the values in column_name are 5, NULL, and 7, COUNT() would be 3, while COUNT(column_name) would be 2. Use COUNT() when you want the total row count; use COUNT(column_name) to count how many rows have a non-null value in that column.