The GROUP BY Clause specifies the groups into which output rows are to be placed. It also determines if aggregate functions are included in the SELECT clause (select list), and calculates a summary value for each group.

In other words, whenever the SELECT clause of your SELECT statement includes column values and aggregate functions, you must include the GROUP BY clause. The GROUP BY clause must list all of the column values used in the SELECT clause.

How to Use GROUP BY and Null Values

If the grouping column contains a null value, that row becomes a group in the results. If the grouping column contains more than one null value, the null values are put into a single group. This behavior is defined in the SQL-92 standard. The royalty column in the titles table contains some null values, for example: SELECT royalty, AVG(price * 2) AS AveragePrice
FROM pubs.dbo.titles
GROUP BY royalty

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Here is the result set: royalty AveragePrice ----------- -------------------------- (null) (null) 10 32.89 12 30.94 14 23.90 16 45.90 24 5.98 (6 row(s) affected) This SELECT statement can be easily changed to remove the null values by adding a WHERE clause: ///////////////////////////// Delete Below /////////////////////////////

Grouping Rules you must know

1. Every non-aggregate column that appears in the SELECT clause must also appear in the GROUP BY clause.
2. You may not use aliases in the HAVING clause. MySQL allows usage of aliases in the HAVING clause, but you may want to avoid this to keep your code as cross-database compatible as possible.
3. You may use aliases in the ORDER BY clause.
4. To filter rows based on aggregate functions, use the HAVING clause.
5. You should declare column aliases for any calculated fields in the SELECT clause.
6. You may not use aggregate functions in the WHERE clause.

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