How to calculate week over week changes in SQL?-CodePudding

I have a data table that has 3 columns: Week, Type and units. I want to calculate the week over week changes in units for each Type, and the result table will be like this:

Week         Type   Units   Change
2020-12-01    A      10      null
2020-12-01    B      15      null
2020-12-01    C      20      null
2020-12-01    D      18      null
2020-12-08    A      20       10
2020-12-08    B      15       0
2020-12-08    C      25       5
2020-12-08    D      15      -3
...

I tried to use the lag function but failed maybe because I'm confused what to use in partition by and order by. Can someone please help me?

CodePudding user response：

Since you want the previous "Units" for the same "Type", the partition is on the "Type".

SELECT *
, Units - LAG(Units) OVER (PARTITION BY [Type] ORDER BY [Week]) AS Change
FROM data_table