如何约束这样的条件？ SQL服务器

Question

I need to create a table TEAMS and create a constraint that the coach ( TeamCoachName ) can just be a coach for just 1 team ( TeamName ).

/* Table number 2: Teams */
CREATE TABLE TEAMS
(
    TeamName VARCHAR(255) UNIQUE,
    YearOfFounding INT,
    TeamOwnerName VARCHAR(255),
    StadiumName VARCHAR(255),
    GeographicArea VARCHAR(255) 
         CHECK (GeographicArea  IN ('North','Central','South')),
    TeamCoachName VARCHAR(255),

    CONSTRAINT Names UNIQUE (TeamName, TeamCoachName),
    CONSTRAINT OneTeamCoach
)

Answer 1

这是您的“ Team表，如果一个教练只能与1个团队行关联，则它只是TeamCoachName上的UNIQUE CONSTRAINT 。

Answer 2

You can do this with a UDF that takes a TeamName and a CoachName and returns "true" if the CoachName is associated with any other team.

Then check the value of that function in the CHECK constraint.

(Although actually Jamie raises a good point. If TeamName is also UNIQUE, then all you need is a UNIQUE constraint on CoachName . The only way you would need my approach is if the same TeamName could appear in multiple rows.)