MS SQL Server Concepts and Programming Question:

If group functions are used in the SELECT clause, all rows that meet the criteria defined in the WHERE clause will be treated as a single group. The group functions will be apply all rows in that group as a whole. The final output of the SELECT statement is the resulting values of the group functions, not the rows in the group. Here are two good examples of using group functions :

SELECT COUNT(*), MAX(counts), MIN(created)
FROM ggl_links
GO
COUNT(*) MAX(counts) MIN(created)
7 972 2003-01-01

SELECT COUNT(*), MAX(counts), MIN(created)
FROM ggl_links WHERE tag = 'DBA'
GO
COUNT(*) MAX(counts) MIN(created)
3 972 2005-01-01

In first case, the group contains all the rows in table ggl_links, because is no WHERE clause. In the second case, the group contains only 3 rows because of the WHERE clause tag = 'DBA'.