我学习的是MySQL，学习写sql语句过程中遇到Group By 和 Union。
大家乐意看这两个链接，写的很好
Group By： www.cnblogs.com/rainman/archive/2013/05/01/3053703.html
Union : http://www.jb51.net/article/48933.htm
博主只是为了加强记忆才写博客的，基本上是盗用
首先我们来看什么是group By。
Group By 就是分组的意思，根据***分组
这是原始表
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select 类别, sum(数量) as 数量之和
from A
group by 类别
执行后
aaarticlea/png;base64,iVBORw0KGgoAAAANSUhEUgAAAJIAAABGCAIAAACotlrgAAAChUlEQVR4nO2ZUZKEIAxEvf+pvBn7YZWTIQkwQoLJdn9MWQrpwAsq41FKOaBwurCdGZV4XMAWT8AWUsAWUsAWUlbY7oD84GNMJPZdmMZgs+uA/vKWXI+tG937HY1WW3sK6Jnt2E6Sp1Zn2smGRRc8PzNeZ4uxifUo5kq7nPq6nEzmWc6nNL9iy+5VDUZjKoYS3rXa+DTRjgtzGG/Mc6giiAP5KYcu+NFUtzzb+Mm92A6ywsRkGiupmvrBBMTxbsPWHkOjeDdiq6z5r9aybdF25yXyU8K2GwBtkfFS3YhNbKxhm19n5zezV2ATByYO8pBqfDz1wWSe5ax155dEiy4zLcI2bFX0Rirvwaa5V91HarFrKoLXQjWC2L5J8uPu1S3YTjJ9tG+35niEQd93rTatauj5qtC6fWeSGWzG06iCsAVWS4zZaDMzzPXYXqXE4zrwdTukLmwloxKPC9jiCdhCCthCCthCCthCCthCCthWxqd7KmtfYHsenMbXjq2sHWx2yXO1Zcbmcw+hdp7xc94kbxfPKnGLn3+1Adu8L1bbmvhpsZXvL0Q+dqbB+ctkwmebvxKPC9ji6YMNCqaSuip3p2AiYAspYAspYAspYAspYAspYAspP2z3ZsNT1o5VfLqnsvbd8Feym0wdK0LasZW1g81t5uDi6fhfsDnfKoFtjVl14OboE7/6y9DUF9jWxwe2NY4+8d0GiGfbVHD+MplwA+CvxOMCtnj6YIOCqaSuyt0pmAjYQgrYQgrYQgrYQgrYQgrYQgrYpoLfO6jC9r52vgXYlkSu5tBhMoFtvUtCbD73EGpnHd+fWcH3tuUumbG5CdjWmDm4+DhqqBJiK7mebeJY3LD9AW9KNe/E3M1WAAAAAElFTkSuQmCC" alt="" />
相信大家看出来了，这是按照类别分组，结果里只剩下abc三个类别，数量之和就是相同类别间的相加。
Union Union all
union:联合的意思，即把两次或多次查询结果合并起来。
要求：两次查询的列数必须一致
推荐：列的类型可以不一样，但推荐查询的每一列，想对应的类型以一
Employees_China:
E_ID E_Name
01 Zhang, Hua
02 Wang, Wei
03 Carter, Thomas
04 Yang, Ming
Employees_USA:
E_ID E_Name
01 Adams, John
02 Bush, George
03 Carter, Thomas
04 Gates, Bill
使用 UNION 命令实例
列出所有在中国和美国的不同的雇员名：
SELECT E_Name FROM Employees_China
UNION
SELECT E_Name FROM Employees_USA
结果：
E_Name
Zhang, Hua
Wang, Wei
Carter, Thomas
Yang, Ming
Adams, John
Bush, George
Gates, Bill
使用 UNION ALL 命令实例
UNION ALL 命令和 UNION 命令几乎是等效的，不过 UNION ALL 命令会列出所有的值。
实例：
列出在中国和美国的所有的雇员：
SELECT E_Name FROM Employees_China
UNION ALL
SELECT E_Name FROM Employees_USA
结果
E_Name
Zhang, Hua
Wang, Wei
Carter, Thomas
Yang, Ming
Adams, John
Bush, George
Carter, Thomas
Gates, Bill
888
