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 rel

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?