从两列中减去数据，然后得出一列。

Question

I have data like below

id   year   marks    id    year    marks 
1    2017   80       1     2018    100  
2    2017   60       2     2018    70 
3    2017   500      3     2018    600 

My result should be values as 20, 10, 100 in Difference column . All this data should be in a single row.

Answer 1

Somehow, I think you want:

select id, sum(case when year = 2018 then marks else - marks end) as diff
from t
where year in (2017, 2018)
group by id;

Answer 2

Unless I'm missing something, you just want to take a difference of the two marks columns:

SELECT
    t1.id, t1.year, t1.marks AS marks_2017, t2.id, t2.year, t2.marks AS marks_2018,
    t2.marks - t1.marks AS diff
FROM yourTable t1
INNER JOIN yourTable t2
    ON t1.id = t2.id AND t1.year = 2017 AND t2.year = 2018;

Answer 3

SELECT id, year, marks, id, year, new_marks, new_marks - marks AS diff
FROM yourTable where year = '2018'

Answer 4

select 
t1.*, t2.year, t2.marks, t2.marks - t1.marks as diff
from
test t1
inner join
test t2
on t1.id = t2.id and t1.year = 2017 and t2.year = 2018

Demo