Syntax
COUNT ( [ DISTINCT | ALL ] Expression )
The
DISTINCT qualifier eliminates duplicates. The ALL qualifier retains duplicates.
ALL is assumed if neither ALL nor DISTINCT is specified. For example, if a
column contains the values 1, 1, 1, 1, and 2, COUNT(col) returns a greater
value than COUNT(DISTINCT col).
Only one DISTINCT aggregate expression
per SelectExpression is
allowed. For example, the following query is not allowed:
-- query not allowed
SELECT COUNT (DISTINCT flying_time), SUM (DISTINCT miles)
FROM Flights
An Expression can contain multiple column
references or expressions, but it cannot contain another aggregate or subquery.
If an Expression evaluates to NULL, the aggregate is not processed
for that value.
The resulting data type of COUNT is INTEGER.
-- Count the number of countries in each region,
-- show only regions that have at least 2
SELECT COUNT (country), region
FROM Countries
GROUP BY region
HAVING COUNT (country) > 1
