SQL减去使用case函数从现有列生成的值

我已经根据现有的文本等级列创建了数字的附加列

例如，如果结果为C，则有结果列14

Tar  TarVal    TA   TAVal
A    20        A-   19
B    17        B+   18
C    14        B+   18

我想做的是减去两个结果列，然后用结果创建一个新列，例如

TAVal-TarVal

：

Tar  TarVal    TA   TAVal    Difference
A    20        A-   19       -1
B    17        B+   18       1
C    14        B+   18       4

这是迄今为止我所掌握的代码，但到目前为止，我一直无法对每一列进行减法：

    select subject.target AS Tar, 
    (case subject.target
    when 'A*' then 23 when 'A*-' then 22 when 'A+' then 21 when 'A' then 20
    when 'A-' then 19 when 'B+' then 18 when 'B' then 17 when 'B-' then 16
    when 'C+' then 15 when 'C' then 14 when 'C-' then 13 when 'D+' then 12
    when 'D' then 11 when 'D-' then 10 when 'E+' then 9 when 'E' then 8
    when 'E-' then 7 when 'F+' then 6 when 'F' then 5 when 'F-' then 4
    when 'G+' then 3 when 'G' then 2 when 'G-' then 1 when 'U' then 0 end) as TarVal,

    subject.result AS TA, 
    (case subject.result 
    when 'A*' then 23 when 'A*-' then 22 when 'A+' then 21 when 'A' then 20
    when 'A-' then 19 when 'B+' then 18 when 'B' then 17 when 'B-' then 16
    when 'C+' then 15 when 'C' then 14 when 'C-' then 13 when 'D+' then 12
    when 'D' then 11 when 'D-' then 10 when 'E+' then 9 when 'E' then 8
    when 'E-' then 7 when 'F+' then 6 when 'F' then 5 when 'F-' then 4 
    when 'G+' then 3 when 'G' then 2 when 'G-' then 1 when 'U' then 0 end) as TAVal

    from subject 
    join student on subject.upn=student.upn 
    where subject.datacollection='March 2013' and student.stuyear=11 and 
    subject.name='English'
    order by student.surname, student.forename

---

出于兴趣，我尝试在结果集中创建不在原始表中的列的术语是什么？但不确定这个词有多普遍，或者是否还有其他词。

;WITH SQ AS (
select
    student.surname, student.forename,
    subject.target AS Tar, 
    (case subject.target
    when 'A*' then 23 when 'A*-' then 22 when 'A+' then 21 when 'A' then 20
    when 'A-' then 19 when 'B+' then 18 when 'B' then 17 when 'B-' then 16
    when 'C+' then 15 when 'C' then 14 when 'C-' then 13 when 'D+' then 12
    when 'D' then 11 when 'D-' then 10 when 'E+' then 9 when 'E' then 8
    when 'E-' then 7 when 'F+' then 6 when 'F' then 5 when 'F-' then 4
    when 'G+' then 3 when 'G' then 2 when 'G-' then 1 when 'U' then 0 end) as TarVal,

    subject.result AS TA, 
    (case subject.result 
    when 'A*' then 23 when 'A*-' then 22 when 'A+' then 21 when 'A' then 20
    when 'A-' then 19 when 'B+' then 18 when 'B' then 17 when 'B-' then 16
    when 'C+' then 15 when 'C' then 14 when 'C-' then 13 when 'D+' then 12
    when 'D' then 11 when 'D-' then 10 when 'E+' then 9 when 'E' then 8
    when 'E-' then 7 when 'F+' then 6 when 'F' then 5 when 'F-' then 4 
    when 'G+' then 3 when 'G' then 2 when 'G-' then 1 when 'U' then 0 end) as TAVal

    from subject 
    join student on subject.upn=student.upn 
    where subject.datacollection='March 2013' and student.stuyear=11 and 
    subject.name='English'
)
SELECT Tar, TarVal, TA, TAVal, TAVal - TarVal Difference
FROM SQ
order by surname, forename;