SQLite：如何更新具有與另一列兩行之差的值的列？

Question

如何更新具有與另一列兩行之間的差異（百分比）的值的列？

從：

 y | coeff | decreased %
---|-------|------------
15 | 1.35  |
16 | 1.22  |
17 | 1.14  |
18 | 1     |

至：

 y | coeff | decreased %
---|-------|------------
15 | 1.35  |
16 | 1.22  |  9.629
17 | 1.14  |  6.557
18 | 1     | 12.280

Answer 1

使用此UPDATE可以獲取coeff的先前值，即使id不是連續的：

update tablename
set decreased = 100.0 * (1.0 - coeff / (
    select t.coeff from tablename t
    where t.y = (select max(p.y) from tablename p where p.y < tablename.y)
  )
)

觀看演示

Answer 2

我會這樣：

update tbl a
set dec_p  = select (a.coeff-b.coeff)/a.coeff as dec
             from a left join tbl b on (a.y=b.y+1)

Answer 3

如果您的y確實是連續無間隙的，則使用left join ：

select t.*, 100 - (t.coeff * 100.0 / tprev.coeff) as decreased
from t left join
     t tprev
     on tprev.y = t.y - 1;