define a computed column reference another table

Question

I have two database tables, Team (ID, NAME, CITY, BOSS, TOTALPLAYER) and Player (ID, NAME, TEAMID, AGE), the relationship between the two tables is one to many, one team can have many players.

I want to know is there a way to define a TOTALPLAYER column in the Team table as computed?

For example, if there are 10 players' TEAMID is 1, then the row in Team table which ID is 1 has the TOTALPLAYER column with a value of 10. If I add a player, the TOTALPLAYER column's value goes up to 11, I needn't to explicitly assign value to it, let it generated by the database. Anyone know how to realize it?

Thx in advance.

BTW, the database is SQL Server 2008 R2

Answer 1

Yes, you can do that - you need a function to count the players for the team, and use that in the computed column:

CREATE FUNCTION dbo.CountPlayers (@TeamID INT)
RETURNS INT 
AS BEGIN
    DECLARE @PlayerCount INT

    SELECT @PlayerCount = COUNT(*) FROM dbo.Player WHERE TeamID = @TeamID

    RETURN @PlayerCount
END

and then define your computed column:

ALTER TABLE dbo.Team
ADD TotalPlayers AS dbo.CountPlayers(ID) 

Now if you select, that function is being called every time, for each team being selected. The value is not persisted in the Team table - it's calculated on the fly each time you select from the Team table.

Since it's value isn't persisted, the question really is: does it need to be a computed column on the table, or could you just use the stored function to compute the number of players, if needed?

Answer 2

You don't have to store the total in the table -- it can be computed when you do a query, something like:

SELECT teams.*, COUNT(players.id) AS num_players
FROM teams LEFT JOIN  players ON teams.id = players.team_id
GROUP BY teams.id;

This will create an additional column "num_players" in the query, which will be a count of the number of players on each team, if any.