sql - SQL：我想使用来自不同表的两个同名列计算新列中的值

Question

我想做的是：

一月表

尖点数：1、2、3

电流系数：0.123、0.140、0.045

二月表

尖点数：1、2、3

电流系数：0.184、0.163、0.033

然后是 FEB 表中的一个新列，用于查找每个 CUSIP 号码的二月因子/一月因子的值：

=.123/.184

=.140/.163

ETC..

这是我到目前为止所拥有的，它从上个月提取了因素并将其显示在 2 月表下方

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