SQL

sql count number of rows after group by

Select Count(*)
From    (
        Select ...
        From TempTable
        Group By column_1, column_2, column_3, column_4
        ) As Z

SQL COUNT() with GROUP BY

SELECT country, COUNT(*) AS customers
FROM Customers
GROUP BY country;

sql count matching group by

# To count the number of teachers in each department, including 
# rows where COUNT(t.name) == 0
SELECT d.name,COUNT(t.name) FROM teacher t
RIGHT JOIN dept d ON (t.dept=d.id) GROUP BY d.name;