Counts the number of rows in a group depending on the specified parameters.
COUNT ( * | expression | DISTINCT column-name )
* Returns the number of rows in each group.
expression Returns the number of rows in each group where expression is not the NULL value.
DISTINCT column-name Returns the number of different values in the column with name column-name. Rows where the value is the NULL value are not included in the count.
The following statement returns each unique city, and the number of rows with that city value:
SELECT city , Count(*) FROM employee GROUP BY city