日期:2013-10-31  浏览次数:20493 次


SQLServer2000中UNION与UNION ALL的区别

UNION 运算符:

将两个或更多查询的结果组合为单个结果集,该结果集包含联合查询中的所有查询的全部行。这与使用联接组合两个表中的列不同。

使用 UNION 组合两个查询的结果集的两个基本规则是:

所有查询中的列数和列的顺序必须相反。

数据类型必须兼容。

       这种多结果的查询组合为单一结果集在实际中使用的非常方便。但在使用中也有有着问题,如下实例可以进一步说明问题。

问题描述:

       为了进一步分析与统计企业中关键部件的生产进度情况,采用了一个表Key_Item_Cal,其结构如图一:



图一

通过联合查询,将查询分为三个方面:

1、提出数据的基础明细,其代码如下:

SELECT dbo.Key_item.Key_item_name AS Item_Name,

      dbo.H_MORVE.QTY_RECVD1 AS Quantity, dbo.Key_item.Style AS Style

FROM dbo.Key_item LEFT OUTER JOIN

      dbo.H_MORVE ON dbo.Key_item.Key_item = dbo.H_MORVE.ITEM

WHERE (dbo.Key_item.Key_item <> 1)

Order by Style asc

其结果为:

2、提出数据的一级汇总明细,其代码如下:

SELECT dbo.Key_item.Key_item_name AS Item_Name,

      SUM(dbo.H_MORVE.QTY_RECVD1) AS Quantity,MAX(dbo.Key_item.Style) AS Style

FROM dbo.Key_item LEFT OUTER JOIN

      dbo.H_MORVE ON dbo.Key_item.Key_item = dbo.H_MORVE.ITEM

WHERE (dbo.Key_item.Key_item <> 1)

GROUP BY dbo.Key_item.Style,dbo.Key_item.Key_item_name

order by Style

其结果如图如示:



3、提出数据的二级汇总明细,其代码如下:

SELECT MAX(dbo.Key_item.Key_item_name) as Item_Name,

SUM(dbo.H_MORVE.QTY_RECVD1) as Quantity,

      dbo.Key_item.Style as Style

FROM dbo.Key_item LEFT OUTER JOIN

      dbo.H_MORVE ON dbo.Key_item.Key_item = dbo.H_MORVE.ITEM

GROUP BY dbo.Key_item.Style

ORDER BY dbo.Key_item.Style asc

其结果如图所示:



       总体的设计就如以上所示,但通过UNION联接的时候出现了新的问题。通过以下例子,就可以看出UNION与UNION ALL的区别

方案一,其代码如下:

SELECT dbo.Key_item.Key_item_name AS Item_Name,

      dbo.H_MORVE.QTY_RECVD1 AS Quantity, dbo.Key_item.Style AS Style

FROM dbo.Key_item LEFT OUTER JOIN

      dbo.H_MORVE ON dbo.Key_item.Key_item = dbo.H_MORVE.ITEM

WHERE (dbo.Key_item.Key_item <> 1)

UNION (

SELECT dbo.Key_item.Key_item_name AS Item_Name,

      SUM(dbo.H_MORVE.QTY_RECVD1) AS Quantity,MAX(dbo.Key_item.Style) AS Style

FROM dbo.Key_item LEFT OUTER JOIN

      dbo.H_MORVE ON dbo.Key_item.Key_item = dbo.H_MORVE.ITEM

WHERE (dbo.Key_item.Key_item <> 1)

GROUP BY dbo.Key_item.Style,dbo.Key_item.Key_item_name

UNION

SELECT MAX(dbo.Key_item.Key_item_name) as Item_Name, SUM(dbo.H_MORVE.QTY_RECVD1) as Quantity,

      dbo.Key_item.Style as Style

FROM dbo.Key_item LEFT OUTER JOIN

      dbo.H_MORVE ON dbo.Key_item.Key_item = dbo.H_MORVE.ITEM

GROUP BY dbo.Key_item.Style)

ORDER BY dbo.Key_item.Style asc

GO

其结果如下:



问题:

       通过以上结果,可以发现GR180.14.2.1后机架、PY165G.14.2.1B后机架和PY165K.14.2.1后机架的明细与其二级汇总值明显不符。而二级汇总的值是正确,为什么明细与汇总值不符?

针对这个问题,我采用了第二种方案。

方案二,其代码如下:

SELECT dbo.Key_item.Key_item_name AS Item_Name,

      dbo.H_MORVE.QTY_RECVD1 AS Quantity, dbo.Key_item.Style AS Style

FROM dbo.Key_item LEFT OUTER JOIN

      dbo.H_MORVE ON dbo.Key_item.Key_item = dbo.H_MORVE.ITEM

WHERE (dbo.Key_item.Key_item <> 1)

UNION ALL(

SELECT dbo.Key_item.Key_item_name AS Item_Name,

      SUM(dbo.H_MORVE.QTY_RECVD1) AS Quantity,MAX(dbo.Key_item.Style) AS Style

F