http://www.cnblogs.com/yinzhenzhixin/archive/2009/01/07/1371064.html
在SQL语句中，UNION关键字多用来将并列的多组查询结果(表)合并成一个结果(表)，简单实例如下：
SELECT [Id],[Name],[Comment] from [Product1]
UNION
SELECT [Id],[Name],[Comment] from [Product2]
上面的代码可以实现将从Product1和Product2两张表合并成一个表，如果您只是希望合并两张表中符合特定条件的记录抑或是合并两张表各自的前N条记录，那么您的代码可能会像下面这样写：
SELECT [Id],[Name],[Comment] from [Product1] WHERE LEN([Name]) > 5
UNION
SELECT [Id],[Name],[Comment] from [Product2] WHERE [Id] IN (11,20) AND [Comment] IS NOT NULL
SELECT TOP N [Id],[Name],[Comment] from [Product1]
UNION
SELECT TOP N [Id],[Name],[Comment] from [Product2]
This is so easy！但是假如您希望从包含Type字段的某表中根据Type分别随机筛选N条记录并将结果合并成一张表，您可能会像下面这样写：
SELECT TOP N [Id],[Name],[Comment] from [Product] WHERE [Type]='TYPE1' ORDER BY NEWID()
UNION
SELECT TOP N [Id],[Name],[Comment] from [Product] WHERE [Type]='TYPE2' ORDER BY NEWID()
UNION
SELECT TOP N [Id],[Name],[Comment] from [Product] WHERE [Type]='TYPE3' ORDER BY NEWID()
UNION
SELECT TOP N [Id],[Name],[Comment] from [Product] WHERE [Type]='TYPE4' ORDER BY NEWID()
UNION
SELECT TOP N [Id],[Name],[Comment] from [Product] WHERE [Type]='TYPE5' ORDER BY NEWID()
UNION
SELECT TOP N [Id],[Name],[Comment] from [Product] WHERE [Type]='TYPE6' ORDER BY NEWID()
UNION
SELECT TOP N [Id],[Name],[Comment] from [Product] WHERE [Type]='TYPE7' ORDER BY NEW
