The COUNT function is one of the most commonly used aggregate functions in SQL for determining the number of items in a group. Here's why the correct answers are B and C:
* A: Incorrect. COUNT can be used with any data type, not just NUMBER. It counts rows without regard to data type.
* B: Correct. COUNT(DISTINCT inv_amt) counts the number of unique non-null values in the column inv_amt. It excludes duplicates and ignores NULL values.
* C: Correct. COUNT(*) counts all rows in a table, including those with duplicates and those with NULLs in any column. It is a total row count regardless of content.
* D: Incorrect. You can use a WHERE clause with COUNT. The WHERE clause filters rows before counting, which is standard in SQL.
* E: Incorrect. COUNT(inv_amt) counts the rows where inv_amt is not NULL. Rows with NULL in the inv_amt column are not counted.