使用上一行中的值来计算下一行中的值

Question

I am trying to create a report that pulls the date from a previous row, does some calculation and then displays the answer on the row below that row. The column in question is "Time Spent".

Eg I have 3 rows.

+=====+===============+============+====+
|name |  DatCompleted | Time Spent | idx|
+=====+===============+============+====+
|  A  |    1/1/17     | NULL       | 0  |
+-----+---------------+------------+----+
|  B  |    11/1/17    | 10 days    | 1  |
+-----+---------------+------------+----+
|  C  |    20/1/17    | 9 days     | 2  |
+=====+===============+============+====+

Time Spent C = DatCompleted of C - DateCompleted of B

Apart from using a crazy loop and using row x row instead of set I can't see how I would complete this. Has anyone ever used this logic before in SQL? If how did you go about this?

Thanks in advance!

Answer 1

Most databases support the ANSI standard LAG() function. Date functions differ depending on the database, but something like this:

select t.*,
       (DateCompleted - lag(DateCompleted) over (order by DateCompleted)) as TimeSpent
from t;

In SQL Server, you would use datediff() :

select t.*,
       datediff(day,
                lag(DateCompleted) over (order by DateCompleted), 
                DateCompleted
               ) as TimeSpent
from t;

Answer 2

You can do this by using ROW number syntax is

ROW_NUMBER ( ) OVER ( [ PARTITION BY value_expression , ... [ n ] ] order_by_clause)

For reference you can use ROW_NUMBER

Answer 3

You have an index already (similar to rownumber above). Join to itself.

Select table1.*
   ,TimeSpent=DateDiff("d",table1.DateCompleted,copy.DateCompleted)
from table1
   join table1 copy on table.idx=copy.idx-1