Comparing differences (error) between two tables in SQL

Question

I want to calculate a delta between the rows of two tables with the exact same structure, but not necessarily the same data.

table 1 (predictions)

Id | Name | GP | G | A

table 1 (actual data)

Id | Name | GP | G | A

result (matched by Id):

Id | (GP1-GP2) AS DeltaGP | (G1-G2) AS DeltaG | (A1-A2) AS DeltaA |

Catch my drift? This is to caclulate the error of a prediction model in SSAS

Here is my code:

    SELECT P.[Player_id]
          ,P.[PlayerName]
          ,sum(P.[Games Played])-sum(S.[GamesPlayed]) AS GP
          ,sum(P.[Total Goals])-sum(s.[TotalGoals]) AS Goals
          ,sum(P.[Total Assists])-sum(s.[TotalAssists]) AS Assists   
FROM [PredictionsPlayersHistory] as P   
JOIN [V_StatsPlayers] AS S on p.pLAYER_id = s.Player_id    
where S.LastPlayedDate >= '2010-02-01' and P.Player_id
    = 8471675   group by P.[Player_id],P.[PlayerName]   
order by Goals desc, Assists desc, GP asc

problem is the results arent right:

Predictions (SUM)

PlayerName GP Goals Assists

Sidney Crosby 56 28 34

Actual data (SUM)

PlayerName GP Goals Assists

Sidney Crosby 26 17 24

Results

Sidney Crosby 28 -42 -98

Answer 1

You need to join the two tables on the id column:

SELECT a.Id,a.Name, a.GP - p.GP AS DeltaGP, a.G - p.G AS DeltaG,  a.A - p.A AS DeltaA
FROM dbo.table1 AS a --actual values
JOIN dbo.table2 AS p --predictions
ON a.Id = p.Id;

For an introduction to SQL Server Joins check out my post series here: http://sqlity.net/en/1146/a-join-a-day-introduction/

---

EDIT: If you have multiple rows per player in each table you need to aggregate them before you join:

SELECT  a.Id,
        a.Name,
        a.GP - p.GP AS DeltaGP,
        a.G - p.G AS DeltaG,
        a.A - p.A AS DeltaA
FROM    (
          SELECT  Id,
                  Name,
                  SUM(GP) AS GP,
                  SUM(G) AS G,
                  SUM(A) AS A
          FROM    dbo.table1
          GROUP BY Id,
                  Name
        ) AS a --actual values
JOIN    (
          SELECT  Id,
                  SUM(GP) AS GP,
                  SUM(G) AS G,
                  SUM(A) AS A
          FROM    dbo.table2
          GROUP BY Id
        ) AS p
        --predictions
  ON a.Id = p.Id;

Depending on you requirements you might want to use AVG instead of SUM.