如果要查看结果，必须先选择表，请尝试以下操作

select CASE when ColB = 'March' THEN (Select ColA From yourtable where ColB = 'January') END     
from yourtable

我认为你最好是自己加入：

SELECT a.b Month1 ,b.b Month2
FROM Table1 a
LEFT JOIN Table1 b
 ON a.a = b.a+2

演示：SQL小提琴
这适用于所有月份，但您可以将其限制在您的示例中：

SELECT b.a
FROM Table1 a
LEFT JOIN Table1 b
 ON a.a = b.a+2
WHERE a. = 'March'

您已经接近了，可以这样做（但这将返回9个空行和一个有效行）

SELECT
      CASE WHEN ColB = 'March' 
        THEN (Select ColA FROM tablename WHERE ColB = 'January') 
      END
FROM tablename

或者可以不用CASE就完成

SELECT
  (SELECT colA FROM tablename WHERE colB = 'January')
FROM tablename
WHERE colB = 'March'

参见此处的第二次运行示例

听起来你需要一个通用的解决方案。不幸的是，像这样存储月份名称并不是很有效。但是你可以通过一个self-join来实现：

with t as (
      select t.*,
             (case colB
                   when 'January' then 1
                   when 'February' then 2
                   when 'March' then 3
                   when 'April' then 4
                   when 'May' then 5
                   when 'June' then 6
                   when 'July' then 7
                   when 'August' then 8
                   when 'September' then 9
                   when 'October' then 10
                   when 'November' then 11
                   when 'December' then 12
              end) as monthnum
      from t
     )
select colA,
       (select top 1 colA from t t2 where t2.monthnum = t.monthnum - 2) as ColAMinus2
from t