For this question, I am referring to the specific case where you have table T, it has primary key K, but K is a foreign key. Is this valid? And how would you write it in SQL99?
All the other questions I've seen on here are just asking whether or not a primary key can be a foreign key for another table. That's not what I'm asking about. I'm asking about a table which has a foreign key where that is the primary key of that table.
If I understand you correctly, you want to create a hierarchical table, for instance:
create table hierarchical
(
id number primary key,
parent_id number
);
alter table hierarchical add constraint
fk_parent_id foreign key(parent_id) references hierarchical(id);
This kind of table can contain employees/managers for instance.
A column can be a primary key as well foreign key. For example, refer to the following:
A column can be both a primary key and a foreign key. For example:
create table A
(
id int not null
, constraint PK_A primary key (id)
);
create table B
(
id int not null
,constraint PK_B primary key (id)
,constraint FK_B_ID foreign key (id) references A(id)
);
Though, this requires data to be present in Table B first.
Related
I have two tables:
Article
Subscription
In the Article table I have two columns that make up the primary key: id, sl. In the Subscription table I have a foreign key 'idsl`.
I use this constraint :
constraint FK_idsl
foreign key (idsl) references CSS_SubscriptionGroup(id, sl)
But when I run the query, I getting this error:
Number of referencing columns in foreign key differs from number of referenced columns, table X
In Article Table I have two fields that are the primary key: id,sl. In the Subscription Table I have a foreign key 'idsl`
This design is broken - it is apparent that the composite primary key in Article(id, sl) has been mangled into a single compound foreign key in table Subscription. This isn't a good idea.
Instead, you will need to change the design of table Subscription to include separate columns for both id and sl, of the same type as the Article Table, and then create a composite foreign key consisting of both columns, referencing Article in the same order as the primary key, e.g:
CREATE TABLE Article
(
id INT NOT NULL,
sl VARCHAR(50) NOT NULL,
-- Other Columns
CONSTRAINT PK_Article PRIMARY KEY(id, sl) -- composite primary key
);
CREATE TABLE Subscription
(
-- Other columns
id INT NOT NULL, -- Same type as Article.id
sl VARCHAR(50) NOT NULL, -- Same type as Article.sl
CONSTRAINT FK_Subscription_Article FOREIGN KEY(id, sl)
REFERENCES Article(id, sl) -- Same order as Article PK
);
Edit
One thing to consider here is that by convention a column named table.id or table.tableid should be unique, and is the primary key for the table. However, since table Article requires an additional column sl in the primary key, it implies that id isn't unique.
correct syntax for relation:
CONSTRAINT FK_OtherTable_ParentTable
FOREIGN KEY(OrderId, CompanyId) REFERENCES dbo.ParentTable(OrderId, CompanyId)
You must try like this:
constraint FK_idsl foreign key (id,sl) references CSS_SubscriptionGroup(id,sl)
Is it possible to have a foreign key that references another foreign key in a different table, or can it only reference primary and unique keys?
A foreign key can reference any field defined as unique. If that unique field is itself defined as a foreign key, it makes no difference. A foreign key is just to enforce referential integrity. Making a field a foreign key doesn't change the field itself in any way. If it is a unique field, it can also be the target of another FK.
For example:
create table Table1(
PK int identity primary key,
...
);
create table Table2( -- 1-1 relationship with Table1
PKFK int primary key,
...,
constraint FK_Table2_1 foreign key( PKFK ) references Table1( PK )
);
create table Table3( -- relates to Table2
PKFKFK int primary key,
...,
constraint FK_Table3_2 foreign key( PKFKFK ) references Table2( PKFK )
);
I know of no DBMS where this is not the case. And I agree with Horse, there is nothing wrong with the practice.
Is it possible to have a foreign key that references another foreign key in a different table
Yes. In fact contrary to accepted answer, the referenced FK column doesn't even have to be unique! - at least in MySQL. see https://www.db-fiddle.com/f/6RUEP43vYVkyK2sxQQpBfj/0 for a demo of the same.
which brings up the question that if the FK is not unique in the parent table, then who is the parent row? The purpose of FKs is to establish parent-child relationship.
I have the following T-SQL to create 3 SQL tables:
create table dbo.Posts
(
Id int identity not null
constraint PK_Posts_Id primary key clustered (Id),
Active bit not null
constraint DF_Posts_Active default (0)
);
create table dbo.PostsLocalized
(
Id int not null,
Culture int not null
constraint CK_PostsLocalized_Culture check ([Culture] in ('1', '2', '3')),
[Text] nvarchar (200) not null,
constraint PK_PostsLocalized_Id_Culture primary key clustered (Id, Culture)
);
create table dbo.Tags
(
Id int identity not null
constraint PK_Tags_Id primary key clustered (Id),
Name nvarchar not null
);
create table dbo.PostsLocalized_Tags
(
PostLocalizedId int not null,
TagId int not null,
constraint PK_PostsLocalized_Tags_Post_PostLocalizedId_TagId primary key clustered (PostLocalizedId, TagId)
);
Then I have added the following constraints:
alter table dbo.PostsLocalized
add constraint FK_PostsLocalized_Id foreign key (Id) references dbo.Posts(Id) on delete cascade on update cascade;
alter table dbo.PostsLocalized_Tags
add constraint FK_PostsLocalized_Tags_PostLocalizedId foreign key (PostLocalizedId) references PostsLocalized(Id) on delete cascade on update cascade,
constraint FK_PostsLocalized_Tags_TagId foreign key (TagId) references Tags(Id) on delete cascade on update cascade;
But I get the following error:
There are no primary or candidate keys in the referenced table 'PostsLocalized' that match the referencing column list in the foreign key 'FK_PostsLocalized_Tags_PostLocalizedId'.
How can I solve this?
Thank You,
Miguel
SQL Server mandates that foreign key references be to a primary key or unique key. The foreign key reference has to be to all the columns that constitute the primary/unique key. The documentation says:
In a foreign key reference, a link is created between two tables when
the column or columns that hold the primary key value for one table
are referenced by the column or columns in another table. This column
becomes a foreign key in the second table.
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. A FOREIGN KEY
constraint can contain null values; however, if any column of a
composite FOREIGN KEY constraint contains null values, verification of
all values that make up the FOREIGN KEY constraint is skipped. To make
sure that all values of a composite FOREIGN KEY constraint are
verified, specify NOT NULL on all the participating columns.
The primary key in PostsLocalized contains the culture column, so you need to add it into the foreign key reference.
Your PK on PostsLocalized table is complex consisting of two columns - id and culture and you are trying to create FK on only one of those columns which is not possible.
You'll have to either add Culture column on PostsLocalized_Tags and use them both in foreign key or remove Culture from your PK on PostLocalized
Is it possible to have a table's foreign key be part of another table's composite primary key?
For example, if I have two tables, one contains information on all active projects of different users and another containing information on what equipment is being used by the projects:
Project Table:
Composite Primary Keys: UserId, ProjectId (neither are unique by themselves)
Equipment Table:
Composite Primary Keys: UserId, ProjectId, EquipmentId (neither are unique by themselves)
Now is it possible to set the ProjectId in the equipment table to be a foreign key from the project table? When I try, I get an error saying that the column in Project Table do not match an existing primary key or unique constraint?
No.
When you create a foreign key, the key that you "point to" in the other table must be a UNIQUE or PRIMARY KEY constraint. You cannot establish a foreign key that points to a column that allow duplicate values. It would be very hard to imagine how the data should "act" if you update one of the duplicate values in the other table (for instance).
To do what you want you must establish a Projects table in which ProjectID is UNIQUE or a PRIMARY KEY and then point foreign keys in both the other tables to that table.
Parenthetically, you use the term "Primary Keys" to describe the columns in each table that make up the primary key. In fact, each table can have one and only one primary key. That key can be composed of one or more columns, but the key itself is still referred to in the singular. This is an important difference when using the primary key to optimize searches.
It do not know if that's a good design practice but for sure it is possible to have a composite foreign key of one table that is the part of the composite primary key of other table.
Say we have a table test1 having a composite primary key (A, B)
Now we can have a table say test2 having primary key (P, Q, R) where in (P,Q) of test2 referencing (A,B) of test2.
I ran the following script in the MySql database and it works just fine.
CREATE TABLE `test1` (
`A` INT NOT NULL,
`B` VARCHAR(2) NOT NULL,
`C` DATETIME NULL,
`D` VARCHAR(45) NULL,
PRIMARY KEY (`A`, `B`));
CREATE TABLE `test2` (
`P` INT NOT NULL,
`Q` VARCHAR(2) NOT NULL,
`R` INT NOT NULL,
`S` DATETIME NULL,
`T` VARCHAR(8) NULL,
PRIMARY KEY (`P`, `Q`, `R`),
INDEX `PQ_idx` (`P`,`Q` ASC),
CONSTRAINT `PQ`
FOREIGN KEY (`P`, `Q`)
REFERENCES `test1` (`A`,`B`)
ON DELETE CASCADE
ON UPDATE CASCADE);
In the above mentioned case, the database is expecting the combination of (A,B) to be unique and it is, being a primary key in test1 table.
But if you try to do something like following, the script would fail. The database would not let you create the test2 table.
CREATE TABLE `test2` (
`P` INT NOT NULL,
`Q` VARCHAR(2) NULL,
`R` DATETIME NULL,
`S` VARCHAR(8) NULL,
`T` VARCHAR(45) NULL,
INDEX `P_idx` (`P` ASC),
INDEX `Q_idx` (`Q` ASC),
CONSTRAINT `P`
FOREIGN KEY (`P`)
REFERENCES `test1` (`A`)
ON DELETE CASCADE
ON UPDATE CASCADE,
CONSTRAINT `Q`
FOREIGN KEY (`Q`)
REFERENCES `test1` (`B`)
ON DELETE CASCADE
ON UPDATE CASCADE);
In the above mentioned case database would expect the column A to be unique individually and the same follows for column B. It does not matter if combination of (A,B) is unique.
#Larry Lustig
The foreign key can be part of primary key in the other table.
source: Dependent relationship
Check relationship between tables: Zdarzenie(Event) and TypZdarzenia (type of event)
Naive question with the answer "No" , I believe, but still would like to ask.
Table_parent
pk_parent_surrogate
parent_natural_unique_key
Table_child
pk_child_surrogate
child_natural_NOT_unique
Is that true that the only possible declarative relationship among main database vendors is
pk_parent_surrogate ----------< pk_child_surrogate
and we can't have declarative constraint or foreign key in other words for pair
parent_natural_unique_key -------< child_natural_NOT_unique
My answer here is based on my MS SQL knowledge - although I believe the same answer is correct for ANSI standards as well, i'm not 100% sure...
YES - you CAN do this as long as you've got a unique constraint on the column in your parent table that you want to use as the anchor column for the key.
You can create a FOREIGN KEY constraint as part
of the table definition when you create a table.
If a table already exists, you can add a
FOREIGN KEY constraint, provided that the
FOREIGN KEY constraint is linked to an existing
PRIMARY KEY constraints or UNIQUE constraint in
another, or the same, table. A table can contain
multiple FOREIGN KEY constraints.
And as an example of this sort of key...
use tempdb
CREATE TABLE parent(
pk int identity primary key,
candidate_key int unique not null)
CREATE TABLE child(
pk int identity primary key,
join_key int references parent(candidate_key))
See here for more information.
Try code like this:
create table testunique (id int identity(1,1) primary key, otherid int)
go
create unique index ixOther on testunique(otherid)
go
create table testFK (id int identity(1,1) primary key, someid int)
go
alter table testFK add constraint fkTest foreign key (someid) references testunique(otherid)
Rob