SQL:我想在一个新列中计算一个值，使用两个来自不同表的同名列

Question

我想做的是:

表1月

Cusip Number: 1,2,3

电流因数:0.123,0.140,0.045

表2月

Cusip Number: 1,2,3

电流因数:0.184,0.163,0.033

然后在FEB表中创建一个新列，用于查找每个CUSIP号的FEB因子/jan因子的值:

= .123/.184

= .140/.163

等。

这是我目前所做的，它从上个月提取因子并将其显示在Feb表

SELECT [CURRENT FACTOR]
FROM DBO.mbs012013, DBO.mbs022013
WHERE DBO.mbs012013.[CUSIP NUMBER] = DBO.mbs022013.[CUSIP NUMBER]

Answer 1

Update February 
  Set factor= (1.0 * January.factor/February.factor )
  from February 
  Join Janaury on .... whatever links Febuary to January which had better be a 1-1 join

Answer 2

  SELECT February.CUSIP, February.factor, (1.0 * January.factor/February.factor ) as Ratio
  from February 
  INNER JOIN Janaury on January.CUSIP = February.CUSIP