SQL Server：两列的单个值上的唯一索引（!!!不组合）

Question

I have a table for teams where each team has two codes. A code for teammembers and a code for the teamleader.

TeamId    Name     MemberCode    LeaderCode
--------------------------------------------
1         Team1    CodeXY        CodeXYZ
2         Team2    CodeAB        CodeBC
...

There are two unique indexes, one on MemberCode and one on LeaderCode securing that MemberCodes and LeaderCodes are unique.

But how can I define the not only MemberCodes itself are unqiue, but MemberCodes and LeaderCodes?

No MemberCode should be a LeaderCode.

Someone got an idea?

PS: A unique index on the two columns like Create Unique index UIDX_12 On tbl (MemberCode, LeaderCode) is no option!

Answer 1

With this data structure, I think you would have to have a trigger.

You can reformat the data, so you have one table and (at least) three columns:

• TeamId
• Code
• CodeType

Then you can add constraints:

• codetype is only 'member' or 'leader'
• code is unique
• teamid is in the teamid table
• teamid / codetype is unique

This will allow you to store exactly one of each of these values for each team (assuming that the values are not NULL ).

In a create table statement, this might look something like:

create table . . .
    check codetype in ('member', 'leader'),
    unique(code),
    teamid references teams(teamid),
    unique (teamid, codetype)
    . . .

Answer 2

You can enforce this constraint with an indexed view . Something like:

create table dbo.MColumnUnique (
    MemberName int not null,
    LeaderName int not null
)
go
create table dbo.Two (ID int not null primary key,constraint CK_Two_ID CHECK (ID in (1,2)))
go
insert into dbo.Two(ID) values (1),(2)
go
create view dbo.MColumnUnique_Enforcer (Name)
with schemabinding
as
    select
        CASE WHEN ID = 1 THEN MemberName ELSE LeaderName END
    from
        dbo.MColumnUnique
            cross join
        dbo.Two
go
create unique clustered index IX_MColumnUnique_Enforcer on dbo.MColumnUnique_Enforcer (Name)
go
insert into dbo.MColumnUnique (MemberName,LeaderName) values (1,2),(3,4) --Works
go
insert into dbo.MColumnUnique (MemberName,LeaderName) values (4,5) --Fails
go
insert into dbo.MColumnUnique (MemberName,LeaderName) values (6,6) --Fails

Where hopefully you can see the parallels between my above structure and your tables.

dbo.Two is just a generally helpful helper table that contains exactly two rows, and is used to perform a limited unpivot on the data into a single column.

Answer 3

You could do it with a trigger, but I would use a CHECK CONSTRAINT.

Create a function that takes a varchar parameter (or whatever the datatype you use for MemberCode and LeaderCode), and returns a bit: 0 if there is no LeaderCode or MemberCode that matches the parameter value, or 1 if there is a match.

Then put a check constraint on the table that specifies:

• MemberCode <> LeaderCode AND
• YourFunction(MemberCode) = 0 AND
• YourFunction(LeaderCode) = 0

EDIT based on Damien's comment:

To prevent the function from including the row you just added, you need to also pass the [code] column (which you say is UNIQUE), and not count the row with that value for [code].