I have a bridge relation resolving M:N relationship.
How should I write the code for SQL Server using constraint to define a composite FK derived from these two tables?
I tried doing something like this:
constraint cat_treat_id_fk
foreign key(cat_id,treat_id)
references cat(cat_id), treat(treat_id)
but it does not work.
The canonical table design for this in SQL Server is:
create table Cat_Treat
(
treat_id int not null references Treat,
cat_id int not null references Cat,
constraint pk_Cat_Treat primary key (treat_it,cat_id),
index ix_Cat_Treat_cat_id (cat_id)
)
I think you draw the edges on that diagram pointing in the wrong directions.
The M:N relationship can be defined as:
create table cat_treat (
treat_id int not null references treat (treat_id),
cat_id int not null references cat (cat_id),
primary key (treat_id, cat_id)
);
Related
Is it possible to create multiple relations from one table, to another table?
I have a table containing purchases, each of these purchases have a origin_country and a destination_country.
I would like to have relations (as foreign keys) to a single PK on a table from these two columns from the same table.
i have tried the following queries:
alter table Purchases
add constraint FK_Purchases_OriginCountries
foreign key (FK_OriginCountryCode) references dbo.countries
go
alter table Purchases
add constraint FK_Purchases_DestinationCountries
foreign key (FK_DestinationCountryCode) references dbo.countries
go
But end up getting a conflict, I can't however find documentation that this is not possible...
[23000][547] The ALTER TABLE statement conflicted with the FOREIGN KEY
constraint "FK_Purchases_DestinationCountries". The conflict occurred
in database "Market", table "dbo.countries", column 'ID'.
Is this relationship intentionally not possible, or did i just make a mistake?
Thank you
Yes you can.
The error is not a result of trying to create two foreign keys back to a single table, that's perfectly fine. Try running this to see it work:
create table t(i int primary key);
create table u
(
j int foreign key references t(i),
k int foreign key references t(i)
);
The problem you have is that you have some data in your Purchases table where the value in the column on which you are trying to create the foreign key does not exist in the countries table's ID column.
To find them run a query like this:
select p.*
from dbo.purchases p
where not exists
(
select *
from dbo.countries
where ID = p.FK_DestinationCountryCode
)
Note that I think your column names are a little weird here, You shouldn't call a column FK_DestinationCountryCode just because it has a foreign key on it, and a "code" is not the same kind of thing as an "ID". Your purchases table's columns should probably be called DestinationCountryID and OriginCountryID.
So, I've got a table created like so:
create table CharacterSavingThrow
(
CharacterCode int not null,
constraint FK_CharacterSavingThrowCharacterID foreign key (CharacterCode) references Character(CharacterCode),
FortitudeSaveCode int not null,
constraint FK_CharacterSavingThrowFortitudeSaveCode foreign key (FortitudeSaveCode) references SavingThrow(SavingThrowCode),
ReflexSaveCode int not null,
constraint FK_CharacterSavingThrowReflexSaveCode foreign key (ReflexSaveCode) references SavingThrow(SavingThrowCode),
WillSaveCode int not null,
constraint FK_CharacterSavingThrowWillSaveCode foreign key (WillSaveCode) references SavingThrow(SavingThrowCode),
constraint PK_CharacterSavingThrow primary key clustered (CharacterCode, FortitudeSaveCode, ReflexSaveCode, WilSaveCode)
)
I need to know how I would reference the primary key of this table from another table's constraint? Seems like a pretty simple question, either it's possible or not, right? Thanks for your guys's help!
Yes - totally easy - you just have to specify the complete compound index, e.g. your other table also needs to have those four columns that make up the PK here, and then the FK constraint would be:
ALTER TABLE dbo.YourOtherTable
ADD CONSTRAINT FK_YourOtherTable_CharacterSavingThrow
FOREIGN KEY(CharacterCode, FortitudeSaveCode, ReflexSaveCode, WilSaveCode)
REFERENCES dbo.CharacterSavingThrow(CharacterCode, FortitudeSaveCode, ReflexSaveCode, WilSaveCode)
The point is: if you have a compound primary key (made up of more than one column), any other table wanting to reference that table also must have all those columns and use all those columns for the FK relationship.
Also, if you're writing queries that would join those two tables - you would have to use all columns contained in the compound PK for your joins.
That's one of the main drawbacks of using four columns as a PK - it makes FK relationships and JOIN queries awfully cumbersome and really annoying to write and use. For that reason, in such a case, I would probably opt to use a separate surrogate key in the table - e.g. introduce a new INT IDENTITY on your dbo.CharacterSavingThrow table to act as primary key, that would make it a lot easier to reference that table and write JOIN queries that use that table.
I had a lot of trouble implementing the technique described in an Alexander Kuznetsov article. Basically, the article describes a way to create a FK between one table and alternate tables, and still maintain full constraints on those relationship.
Here's part of Alexander's code:
CREATE TABLE dbo.Vehicles(
ID INT NOT NULL,
[Type] VARCHAR(5) NOT NULL,
CONSTRAINT Vehicles_PK PRIMARY KEY(ID),
CONSTRAINT Vehicles_UNQ_ID_Type UNIQUE(ID, [Type]),
CONSTRAINT Vehicles_CHK_ValidTypes CHECK([Type] IN ('Car', 'Truck'))
)
CREATE TABLE dbo.Cars(ID INT NOT NULL,
[Type] AS CAST('Car' AS VARCHAR(5)) PERSISTED,
OtherData VARCHAR(10) NULL,
CONSTRAINT Cars_PK PRIMARY KEY(ID),
CONSTRAINT Cars_FK_Vehicles FOREIGN KEY(ID, [Type])
REFERENCES dbo.Vehicles(ID, [Type])
)
I finally got it working after errors and confirmed bugs. But when I generate my EF models from the new schema, it is missing a relationship between two of my tables.
The problem is that, in order to have a FK on two columns, there must be an index or unique constraint on both those columns. However, in my case, I also have another table with a FK to a single column in the base table (Vehicles, in Alexander's code).
Since you cannot have more than one PK in a table, this means I cannot have a FK to a PK on both sides. The PK can be for one or two columns, and the other FK will need to reference the non-PK unique constraint.
Unfortunately, Entity Framework will only create relationships for you when there is a FK to a PK. That's the problem. Can someone who understand DB design better than I spot any other alternatives here?
Note: I realize some will see the obvious fix as simply modifying the model to manually add the additional relationship. Unfortunately, we are using a database project and are constantly using automated systems to regenerate the project and model from an updated database. So manual steps are really not practical.
You can't have more than one PK, but you can have more than one unique constraint, and in SQL Server you can create a foreign key constraint that references a unique constraint (one or multiple columns). Here is an example of two tables that roughly look like your model.
CREATE TABLE dbo.Vehicles
(
VehicleID INT PRIMARY KEY,
[Type] VARCHAR(5) NOT NULL UNIQUE,
CONSTRAINT u1 UNIQUE(VehicleID, [Type])
);
CREATE TABLE dbo.Cars
(
CarID INT PRIMARY KEY,
VehicleID INT NOT NULL
FOREIGN KEY REFERENCES dbo.Vehicles(VehicleID),
[Type] VARCHAR(5) NOT NULL
FOREIGN KEY REFERENCES dbo.Vehicles([Type]),
CONSTRAINT fk1 FOREIGN KEY (VehicleID, [Type])
REFERENCES dbo.Vehicles(VehicleID, [Type])
);
Note that Cars has three foreign keys: one points to the PK of vehicles (VehicleID), one points to the unique constraint on Vehicles([Type]), and one points to the multi-column unique constraint on Vehicles(VehicleID, [Type]). I realize this is not equivalent to what you are trying to do but should demonstrate that SQL Server, at least, is capable of doing everything you seem to want to do (I'm having a hard time concluding what you're actually because you keep swapping concepts between what Alex did, what you're trying to do but failing, and what you've done successfully).
Are you saying that EF will not recognize a foreign key that references a unique constraint? If so, does that affect constraints that have more than one column, or all unique constraints? If this is the case, that's a shame, because it is certainly supported in SQL Server. Seems like this would either be a bug or an intentional omission (given that the standard doesn't strictly allow FKs against unique constraints). I wonder if there are any bugs reported on Connect?
I have no idea how to force EF to recognize it, but I do know that just about all the people I know who use database projects end up performing pre- or post-deployment modifications and these can be relatively automated.
Would it be possible in SQL Server 2008 to have a table created with 2 columns that are at the same time primary and foreign keys? If yes, how would such a code look like? I've searched and came up with nothing.
Sure, no problem:
CREATE TABLE dbo.[User]
(
Id int NOT NULL IDENTITY PRIMARY KEY,
Name nvarchar(1024) NOT NULL
);
CREATE TABLE [Group]
(
Id int NOT NULL IDENTITY PRIMARY KEY,
Name nvarchar(1024) NOT NULL
);
CREATE TABLE [UserToGroup]
(
UserId int NOT NULL,
GroupId int NOT NULL,
PRIMARY KEY CLUSTERED ( UserId, GroupId ),
FOREIGN KEY ( UserId ) REFERENCES [User] ( Id ) ON UPDATE NO ACTION ON DELETE CASCADE,
FOREIGN KEY ( GroupId ) REFERENCES [Group] ( Id ) ON UPDATE NO ACTION ON DELETE CASCADE
);
This is quite commonly used to model many-to-many relations.
These are totally different constructs.
A Primary Key is used to enforce uniqueness within a table, and be a unique identifier for a certain record.
A Foreign Key is used for referential integrity, to make sure that a value exists in another table.
The Foreign key needs to reference the primary key in another table.
If you want to have a foreign key that is also unique, you could make a FK constraint and add a unique index/constraint to that same field.
For reference purposes, SQL Server allows a FK to refer to a UNIQUE CONSTRAINT as well as to a PRIMARY KEY field.
It is probably not a good idea since often you want to allow duplicate foreign keys in the table. Even if you don't now, in the future, you might, so best not to do this. See Is it fine to have foreign key as primary key?
Just a quick note - from Microsoft pages (http://msdn.microsoft.com/en-us/library/ms189049.aspx)...
"A foreign key constraint does not have to be linked only to a primary key constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table."
Not used often, but useful in some circumstances.
I have a table called objectives, each objective has zero to many cause-effect relationships with other objectives, these relationships I have to be stored in the database, let me know if there's a way to relate this table records.
There is not a way to relate the records without creating an additional table (you would need N-1 additional columns on your current table to model the N possible effects of a cause).
Creating an additional table like the one below should serve your purpose.
CREATE TABLE cause_effect (
cause integer NOT NULL,
effect integer NOT NULL,
CONSTRAINT cause_effect_pkey PRIMARY KEY (cause, effect),
CONSTRAINT cause_effect_cause_fkey FOREIGN KEY (cause)
REFERENCES yourtable (id),
CONSTRAINT cause_effect_effect_fkey FOREIGN KEY (effect)
REFERENCES yourtable (id)
)
Apply FKey behaviour as applies.