Group by ,Having, Aggregate functions and top 10

1. 今天实验的收获很大，最重要的就是Mr dennis帮我指出了我的一个严重错误！the serious error about querying the ten items having the least total quantities.
I didn’t understand the use of “group by” and “having” until now! They are just and mainly used for aggregate functions.
2.  http://www.w3schools.com/sql/sql_groupby.asp
SQL GROUP BY and HAVING
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Aggregate functions (like SUM) often need an added GROUP BY functionality.
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GROUP BY...
GROUP BY... was added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called, and without the GROUP BY function it was impossible to find the sum for each individual group of column values.
The syntax for the GROUP BY function is:
SELECT column,SUM(column) FROM table GROUP BY column

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GROUP BY Example
This "Sales" Table:
Company Amount
W3Schools 5500
IBM 4500
W3Schools 7100
And This SQL:
SELECT Company, SUM(Amount) FROM Sales
Returns this result:
Company SUM(Amount)
W3Schools 17100
IBM 17100
W3Schools 17100
The above code is invalid because the column returned is not part of an aggregate. A GROUP BY clause will solve this problem:
SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company
Returns this result:
Company SUM(Amount)
W3Schools 12600
IBM 4500

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HAVING...
HAVING... was added to SQL because the WHERE keyword could not be used against aggregate functions (like SUM), and without HAVING... it would be impossible to test for result conditions.
The syntax for the HAVING function is:
SELECT column,SUM(column) FROM table
GROUP BY column
HAVING SUM(column) condition value
This "Sales" Table:
Company Amount
W3Schools 5500
IBM 4500
W3Schools 7100
This SQL:
SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company
HAVING SUM(Amount)>10000
Returns this result
Company SUM(Amount)
W3Schools 12600

2. http://mycodeblog.blogspot.com/2006/11/how-to-select-top-results-in-group-by.html
How to Select Top Results in a Group By to use Microsoft SQL Server:
SET ROWCOUNT 10
SELECT field1, field2, COUNT(*)
FROM table
GROUP BY field1, field2
ORDER BY COUNT(*) DESC

3. http://www.kekecn.com/blog/article.asp?id=146
Microsoft SQL Server
select top 3 e_name,e_wage from employee order by e_wage desc