Which aggregate function in SQL displays the number of values in the specified…

Concept

SQL aggregate functions reduce a set of rows into one summary value, and how many rows they cover depends on the argument passed to them: COUNT(column_name) walks only the rows where that column holds a value, skipping any row where it is NULL, while COUNT(*) walks every row of the result set regardless of any column's NULL status.

Application

The question asks for the aggregate function that reports how many values a specified column holds while ignoring NULLs — that is precisely COUNT(column_name), written here as count(). Passing a column name to COUNT() applies exactly the NULL-skipping behaviour described above.

Cross-check

• len() (LENGTH() in some dialects) measures the number of characters in one string value; it has no notion of rows or NULLs at all.

• NUMBER names a numeric data type in dialects such as Oracle; it cannot be invoked as a function over a column's rows.

• num() carries no defined meaning in SQL; it is not a built-in operation.

Clarifying the reported confusion

A frequently reported point of confusion is that COUNT(*) — a different call form — always includes rows with NULLs in its total, since it counts rows rather than a specific column's values. That fact is correct, but it describes COUNT(*), not COUNT(column_name). The two forms behave differently by design: COUNT(column_name) is the NULL-ignoring form asked about here, and count() remains the correct answer for a column-specific, NULL-ignoring count.