How To Use Group Functions in the SELECT Clause in SQL Server?

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 fyi_links
GO
COUNT(*) MAX(counts)  MIN(created)
7        972          2003-01-01

SELECT COUNT(*), MAX(counts), MIN(created) 
   FROM fyi_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 fyi_links, because is no WHERE clause. In the second case, the group contains only 3 rows because of the WHERE clause tag = 'DBA'.

⇒Using SELECT Statements and GROUP BY Clauses in SQL Server

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