Aggregate functions (like SUM) often need an added GROUP BY functionality.
集合函数（类似SUM）经常需要用GROUP BY来进行功能性的补充。
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.
GROUP BY...之所以加到SQL中去是因为集合函数(像SUM)每当他们被访问时就会返回集合所有栏目的值，而且没有GROUP BY的话就不能够找出单独一种栏目所累计的值了。
The syntax for the GROUP BY function is:
使用GROUP BY函数的语法为:
SELECT column,SUM(column) FROM table GROUP BY column
GROUP BY Example
举例
This "Sales" Table:
这是张名为"Sales"的表: Company Amount
W3Schools 5500
IBM 4500
W3Schools 7100
And This SQL:
这是条SQL:
SELECT Company, SUM(Amount) FROM Sales
Returns this result:
返回的结果为: 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:
上面这些代码几乎是无效的，因为栏目所返回的数值并不属于我们想要的那种合计。使用 GROUP BY子句可以解决这个问题：
SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company
Returns this result:
返回的结果为： Company SUM(Amount)
W3Schools 12600
IBM 4500
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.
WHERE关键字在使用集合函数时不能使用，所以在集合函数中加上了HAVING来起到测试查询结果是否符合条件的作用。
The syntax for the HAVING function is:
HAVING的使用语法为：
SELECT column,SUM(column) FROM table
GROUP BY column
HAVING SUM(column) condition value
This "Sales" Table:
这是名为"Sales"的表： Company Amount
W3Schools 5500
IBM 4500
W3Schools 7100
This SQL:
SQL语句：
SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company
HAVING SUM(Amount)>10000
Returns this result
返回的结果为 Company SUM(Amount)
W3Schools 12600
原文链接：http://shawnccx.iteye.com/blog/739364