Description: Returns the number of rows retrieved in a SELECT statement where values of expression are not NULL. Generally used with GROUP BY clause.
Note that COUNT(*) returns the number of rows retrieved by a query, even if their values are NULL.
Examples:
SELECT city_name, COUNT(temperature) FROM weather GROUP BY city_name
returns the number of temperatures recorded, grouped by city name.
SELECT COUNT(*) FROM weather
returns the total number of rows in table weather.
See also: COUNT(DISTINCT)
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