SQL Group By

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。

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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.

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

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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

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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.

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