I have a problem with this table in Postgres, it give me this error:
ERROR: cannot use subquery in check constraint
LINE 66: check(Artista in(Select ID_Artista
create table DirigeF(
Artista int references Artista(ID_Artista) on delete cascade,
Film int references Film(ID_Contenuto) on delete cascade,
check(Artista in(Select ID_Artista
from Artista
where tipologia='REGISTA'or'AR')),
constraint DirigeF_PK primary key(Artista, Film)
);
I want to check that Artista in table DirigeF has tipologia='REGISTA' from another table.
As the error suggests, you cannot do this with a check constraint. One option is a trigger. Another is a foreign key constraint -- but that needs to be carefully arranged.
First you need a column that indicates whether the type in Artista is "REGISTA" or "AR". That would be:
alter table artista add is_regista_ar bool generated always as
(tipologia in ('REGISTA', 'AR'));
Then create a unique constraint or index:
alter table artista add unq_artista_tipologia_id
unique (is_regista_ar, id_artista)
Note: This requires Postgres 12+. But something similar can be done in earlier versions.
Then, add a boolean column to your table that is always true:
create table DirigeF (
Artista int references Artista(ID_Artista) on delete cascade,
Film int references Film(ID_Contenuto) on delete cascade,
is_regista_ar bool generated always as true,
constraint fk_artista_tipo_artista foreign key (is_regista_ar, Artista) references Artista(is_regista_ar, ID_Artista),
constraint DirigeF_PK primary key (Artista, Film)
);
Related
I have a simple table like below.
create table chemlab.rule_header (
id serial PRIMARY KEY,
name varchar(50),
grade varchar(20),
class_tag varchar(20), --tag added to sammple if match
parent_id int REFERENCES chemlab.rule_header(id) DEFAULT NULL,
unique( grade, class_tag )
)
But afterwards, I found that I need to add ON DELETE action, the default is NO ACTION. I couldn't figure out how to change the action.
Now I have to DROP & ADD
ALTER table chemlab.rule_header
DROP CONSTRAINT rule_header_parent_id_fkey ;
ALTER TABLE rule_header
ADD CONSTRAINT rule_header_parent_id_fkey
FOREIGN KEY (parent_id) REFERENCES chemlab.rule_header(id) ON DELETE RESTRICT;
So what is the correct syntax to alter an action on foreign key constraint ?
Well, this not directly altering FOREIGN KEY constraint, and there are DROP and ADD still, though this is only one statement:
ALTER table chemlab.rule_header
DROP CONSTRAINT rule_header_parent_id_fkey,
ADD CONSTRAINT rule_header_parent_id_fkey
FOREIGN KEY (parent_id) REFERENCES chemlab.rule_header(id) ON DELETE RESTRICT;
Take a look at the documentation at https://www.postgresql.org/docs/current/sql-altertable.html. There are options to alter a few things about a constraint (like DEFERRABLE) but not for changing the action, as I understand you need.
I am trying to add foreign key to my existing column using below query
ALTER TABLE Sub_Category_Master
ADD FOREIGN KEY (Category_ID) REFERENCES Category_Master(Category_ID)
but I'm getting an error
The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK__Sub_Categ__Categ__5812160E". The conflict occurred in database "shaadikarbefikar_new", table "shaadikarbefikar.Category_Master", column 'Category_ID'.
Well, the error clearly tells you that Category_ID in your Sub_Category_Master table contains some values that are not present in Category_Master (column Category_ID). But that's exactly the point of having a foreign key constraint - making sure your child table (Sub_Category_Master) only uses defined values from its parent table.
Therefore, you must fix those "voodoo" values first, before you're able to establish this foreign key relationship. I would also strongly recommend to explicitly name that constraint yourself, to avoid those system-generated, but not really very useful constraint names like FK__Sub_Categ__Categ__5812160E:
ALTER TABLE Sub_Category_Master
ADD CONSTRAINT FK_SubCategoryMaster_CategoryMaster
FOREIGN KEY (Category_ID) REFERENCES Category_Master(Category_ID)
ALTER TABLE Sub_Category_Master
ADD CONSTRAINT FKSub_Category_Master_Category_ID FOREIGN KEY (Category_ID)
REFERENCES Category_Master(Category_ID);
CREATE TABLE Orders
(
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
I have not seen any clear, concise examples of this anywhere online.
With an existing table, how do I add a foreign key which references this table? For example:
CREATE TABLE dbo.Projects(
ProjectsID INT IDENTITY(1,1) PRIMARY KEY,
Name varchar(50)
);
How would I write a command to add a foreign key which references the same table? Can I do this in a single SQL command?
I'll show you several equivalent ways of declaring such a foreign key constraint. (This answer is intentionally repetitive to help you recognise the simple patterns for declaring constraints.)
Example: This is what we would like to end up with:
Case 1: The column holding the foreign keys already exists, but the foreign key relationship has not been declared / is not enforced yet:
In that case, run this statement:
ALTER TABLE Employee
ADD FOREIGN KEY (ManagerId) REFERENCES Employee (Id);
Case 2: The table exists, but it does not yet have the foreign key column:
ALTER TABLE Employee
ADD ManagerId INT, -- add the column; everything else is the same as with case 1
FOREIGN KEY (ManagerId) REFERENCES Employee (Id);
or more succinctly:
ALTER TABLE Employee
ADD ManagerId INT REFERENCES Employee (Id);
Case 3: The table does not exist yet.
CREATE TABLE Employee -- create the table; everything else is the same as with case 1
(
Id INT NOT NULL PRIMARY KEY,
ManagerId INT
);
ALTER TABLE Employee
ADD FOREIGN KEY (ManagerId) REFERENCES Employee (Id);
or, declare the constraint inline, as part of the table creation:
CREATE TABLE Employee
(
Id INT NOT NULL PRIMARY KEY,
ManagerId INT,
FOREIGN KEY (ManagerId) REFERENCES Employee (Id)
);
or even more succinctly:
CREATE TABLE Employee
(
Id INT NOT NULL PRIMARY KEY,
ManagerId INT REFERENCES Employee (Id)
);
P.S. regarding constraint naming: Up until the previous revision of this answer, the more verbose SQL examples contained CONSTRAINT <ConstraintName> clauses for giving unique names to the foreign key constraints. After a comment by #ypercube I've decided to drop these clauses from the examples, for two reasons: Naming a constraint is an orthogonal issue to (i.e. independent from) putting the constraint in place. And having the naming out of the way allows us to focus on the the actual adding of the constraints.
In short, in order to name a constraint, precede any mention of e.g. PRIMARY KEY, REFERENCES, or FOREIGN KEY with CONSTRAINT <ConstraintName>. The way I name foreign key constraints is <TableName>_FK_<ColumnName>. I name primary key constraints in the same way, only with PK instead of FK. (Natural and other alternate keys would get the name prefix AK.)
You can add the column and constraint in one operation
ALTER TABLE dbo.Projects ADD
parentId INT NULL,
CONSTRAINT FK FOREIGN KEY(parentid) REFERENCES dbo.Projects
Optionally you could specify the PK column in brackets after the referenced table name but it is not needed here.
If the table already exists: Assuming you don't already have a column to store this data. If you do then skip this step.
ALTER TABLE [dbo].[project]
ADD [fkProjectsId] INT;
GO
ALTER TABLE [dbo].[projects]
ADD CONSTRAINT [FK_Projects_ProjectsId] FOREIGN KEY ([fkProjectsId]) REFERENCES [dbo].[Projects] ([ProjectsID])
GO
I came upon this code, marked "error," in an application I'm to update. Running it on a test database gives a cyclical reference error:
The referential relationship will result in a cyclical reference that is not allowed (Constraint name = descriptions_fk_2)
I named the constraints to see which one caused the problem.
CREATE TABLE items (
id INT NOT NULL UNIQUE IDENTITY,
name NCHAR(100) NOT NULL UNIQUE,
PRIMARY KEY (id)
);
CREATE TABLE sources (
id INT NOT NULL UNIQUE IDENTITY,
item_id INT NOT NULL,
PRIMARY KEY (id),
FOREIGN KEY (item_id)
REFERENCES items(id) ON UPDATE NO ACTION ON DELETE CASCADE
);
CREATE TABLE descriptions (
id INT NOT NULL UNIQUE IDENTITY,
item_id INT NOT NULL,
source_id INT NOT NULL,
PRIMARY KEY (id),
CONSTRAINT descriptions_fk_1 FOREIGN KEY (item_id)
REFERENCES items(id) ON UPDATE NO ACTION ON DELETE CASCADE,
CONSTRAINT descriptions_fk_2 FOREIGN KEY (source_id)
REFERENCES sources(id) ON UPDATE NO ACTION ON DELETE CASCADE
);
Why is this a cyclical reference? The descriptions table is linked to two separate tables, but none of them link back to descriptions.
It's not strictly cyclical - but there are multiple cascade paths. So you could cascade delete a row in items two ways:
1) description -> item
2) description -> source -> item
And, for that reason, it's disallowed.
I believe it's a performance concern, as PostGres will allow cycles like that and will just work it out, but deletes under those circumstances can be quite slow.
For some further reading about why it's disallowed, please see this answer.
I am designing a test database in SQL Server 2008 R2 and/or SQL Azure. (All of my code will run on both, so far.)
I have a table with a foreign key, and I need to add a constraint that references a field in the foreign table.
Normally I would have the foreign table manage it's own validation checks, but there are cases where that is impossible (or illogical). I've provided some sample-code that displays what I am trying to accomplish.
CREATE TABLE CustomerOrder
(
ID INT NOT NULL IDENTITY PRIMARY KEY,
CustomerID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES Customer(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
ProductID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES Product(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
Quantity INT NOT NULL DEFAULT 1,
IsPaid BIT NOT NULL DEFAULT 0
)
GO
CREATE TABLE RMA
(
ID INT NOT NULL IDENTITY PRIMARY KEY,
CustomerOrderID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES CustomerOrder(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
-- Add constraint to prevent RMAs from being
-- created for orders that have not been paid.
-- This could be a column constraint, or a table constraint.
CHECK ( CustomerOrderID.IsPaid = 1 )
-- ERROR: 'The multi-part identifier "CustomerOrderID.IsPaid"
-- could not be bound.'
)
GO
In this example, it doesn't make sense to put the CHECK constraint in the CustomerOrder table, because a row in the CustomerOrder table is perfectly happy being unpaid as long as there are no RMAs for the order. Furthermore, a constraint in the CustomerOrder table would still need to reference the RMA table to confirm whether there is an RMA, so the same issue remains.
I've also tried:
CHECK (EXISTS(SELECT co.ID FROM CustomerOrder co
WHERE co.ID=CustomerOrderID AND
co.IsPaid=1))
-- ERROR: 'Subqueries are not allowed in this context.
-- Only scalar expressions are allowed.'
Since this is a static constraint for basic data validation, and will never be referenced by any other object, I'd like to avoid making this into a scalar function or stored procedure.
However, in order to avoid using a scalar function, I will need to define the constraint within SQL (preferrably at the same time my database is deployed and the table is created).
What SQL syntax could I use here to define this type of constraint?
Another option (since you asked for a solution without adding triggers or enlarging the foreign key), is removing the IsPaid column and adding another table for paid orders:
CREATE TABLE CustomerOrder
(
ID INT NOT NULL IDENTITY PRIMARY KEY,
CustomerID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES Customer(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
ProductID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES Product(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
Quantity INT NOT NULL DEFAULT 1
)
GO
CREATE TABLE CustomerOrderPaid
(
ID INT NOT NULL PRIMARY KEY
FOREIGN KEY REFERENCES CustomerOrder(ID)
ON DELETE NO ACTION ON UPDATE CASCADE
)
GO
CREATE TABLE RMA
(
ID INT NOT NULL IDENTITY PRIMARY KEY,
CustomerOrderID INT NOT NULL UNIQUE
FOREIGN KEY REFERENCES CustomerOrderPaid(ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
)
GO