Does COUNT(*) Always Return a Value, Even with No Matching Rows?

*COUNT() Always Returns a Value: Even with No Matches**

The COUNT(*) function is essential for determining the number of rows in a table or query result. A key question is: does COUNT(*) always return a result, even if no rows meet the query's conditions?

Consider this query:

SELECT COUNT(*) AS num FROM table WHERE x = 'y'

The answer is a definitive yes. COUNT(*) consistently returns a numerical value, even when zero rows satisfy the WHERE clause. Unlike functions like MAX() or SUM(), which might return NULL with no matching rows, COUNT(*) always provides a precise count (which may be zero).

This inherent behavior stems from COUNT(*)'s function: it counts the presence or absence of rows and accurately reflects the row count regardless of query conditions. This reliability is crucial for data analysis and reporting, ensuring empty result sets are interpreted as valid data points, not errors.

However, this behavior changes when using GROUP BY. With GROUP BY, COUNT(*) counts rows within each group. If no groups exist, the result set is empty.

In short, COUNT(*) always produces a numerical output, irrespective of whether the query finds any matching rows. This makes it a fundamental tool in data retrieval and analysis.

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