GROUP BY Components
The GROUP BY clause contains the following components:
- One or more aggregate-free expressions. These are usually references to the grouping columns.
- Optionally, the ALL keyword, which specifies that all groups produced by the GROUP BY clause are returned, even if some of the groups do not have any rows that meet the search conditions.
- CUBE or ROLLUP.
- Typically, the HAVING clause is used with the GROUP BY clause, although HAVING can be specified separately.
You can group by an expression as long as it does not include aggregate functions, for example:
SELECT DATEPART(yy, HireDate) AS Year,
COUNT(*) AS NumberOfHires
FROM Northwind.dbo.Employees
GROUP BY DATEPART(yy, HireDate)
This is the result set.
Year NumberOfHires
1992 3
1993 3
1994 3
(3 row(s) affected)
In a GROUP BY, you must specify the name of a table or view column, not the name of a result set column assigned with an AS clause. For example, replacing the GROUP BY DATEPART(yy, HireDate) clause with GROUP BY Year is not legal.
You can list more than one column in the GROUP BY clause to nest groups; that is, you can group a table by any combination of columns. For example, this query finds the average price and the sum of year-to-date sales, grouped by type and publisher ID:
USE pubs
SELECT type, pub_id, 'avg' = AVG(price), 'sum' = sum(ytd_sales)
FROM titles
GROUP BY type, pub_id
Here is the result set:
type pub_id avg sum
------------ ------ ---------------------- -----------
business 0736 2.99 18722
psychology 0736 11.48 9564
mod_cook 0877 11.49 24278
psychology 0877 21.59 375
trad_cook 0877 15.96 19566
UNDECIDED 0877 NULL NULL
business 1389 17.31 12066
popular_comp 1389 21.48 12875
(8 row(s) affected)