I have a table let's say table3 and it contains two foreign keys, each one referencing a different table. I want to learn that, could I define two different ON DELETE action for them.
Let me explain it via an example.
create table table3 (
ID varchar(255),
Name varchar(255),
primary key(ID,Name),
foreign key(ID) References user(id),
foreign key(Name) References shops(StoreName)
on update cascade
on delete cascade // I want to cascade table if id is deleted
on delete no actions); // and do not allowed if StoreName is deleted.
Is there anyone to help me ? Thanks in advance.
Not sure if I totally understand what you're trying to do - but if you want to have ON DELETE CASCADE on the fk reference to the User table, and ON DELETE NO ACTIONS on the fk reference to the Shops table, you need to use this T-SQL:
create table table3 (
ID varchar(255),
Name varchar(255),
primary key(ID,Name),
foreign key(ID) References user(id)
on delete cascade, // I want to cascade table if id is deleted
foreign key(Name) References shops(StoreName)
on update cascade
on delete no actions); // and do not allowed if StoreName is deleted.
You need to specify the ON DELETE .... action right with your foreign key definition
Related
Here how I created tables:
CREATE TABLE TABLE_A(
id uuid NOT NULL, UNIQUE
name text
);
CREATE TABLE TABLE_B(
id uuid NOT NULL, UNIQUE
name text
);
-- custom realization of many-to-many association
CREATE TABLE TABLE_A_B(
id uuid NOT NULL, UNIQUE
a_id uuid REFERENCES TABLE_A(id) ON UPDATE CASCADE,
B_id uuid REFERENCES TABLE_B(id) ON UPDATE CASCADE
);
I've already created tables and now can't update it by adding ON DELETE CASCADE.
And I need now to add ON DELETE CASCADE to staging table TABLE_A_B. How to do it ?(
You use ON DELETE CASCADE:
CREATE TABLE TABLE_A_B(
id uuid NOT NULL UNIQUE,
a_id uuid REFERENCES TABLE_A(id) ON UPDATE CASCADE ON DELETE CASCADE,
B_id uuid REFERENCES TABLE_B(id) ON UPDATE CASCADE ON DELETE CASCADE
);
Here is a db<>fiddle that fixed some typos in your code.
In particular, the foreign key reference should be to a primary key. Although allowed to a unique key, the purpose of primary keys is really to identify individual rows -- and one main use is for foreign key references.
EDIT:
If the constraints already exist, then do the following.
First, get their names:
select *
from information_schema.table_constraints
where constraint_type = 'FOREIGN KEY' and table_name = 'table_a_b';
Note: You can assign names to skip this step.
Then drop the existing foreign key constraint:
alter table table_a_b
drop constraint table_a_b_a_id_fkey;
Finally, add a new one:
alter table table_a_b
add constraint fk_table_a_b_a
foreign key (a_id) references table_a(id)
on update cascade
on delete cascade;
I have 2 tables.Table A have columns as (aid, name,depart) where aid is primary key.
Table B has (aid1,aid2,aid3,created_by) where aid1 is the primary_key. aid1, aid2 and aid3 all are primary key of Table A
I want to delete a record in Table B i.e aid1 and simultaneously with delete cascade all three records in TABLE A should be deleted. My doubt here is where should I put the delete cascade constraint. I know that in parent child relationship we need to put delete cascade on the child table so that when parent is deleted, child entities are also deleted but in this scenario I dont understand where I should put delete cascade
Cascading a table will be on the child table. You have to set this on your child table when creating or after creating the child table.
E.g
CREATE TABLE schools (
id int auto_increment primary key not null,
schoolname varchar(191) not null
);
CREATE TABLE students(
id int auto_increment primary key not null,
studentname varchar(191) not null,
school_id int FOREIGN KEY REFERENCES students(id) ONDELETE CASCADE
);
or
You can as well alter the table by running this command.
ALTER TABLE childTable
ADD FOREIGN KEY (childTableParentTableColumn) REFERENCES parentTable(parentTableColumn);
so I have 3 tables, Applicant, Vacancy and a Link table. I am trying to have the applicant data removed when the vacancy is removed. Here is the current SQL code I have, would this remove the applicant?
CREATE TABLE Applicant(
ID INT PRIMARY KEY,
name varchar(20),
address varchar(20),
VacancyID INT,
FOREIGN KEY (VacancyID) REFERENCES Vacancy(ID) ON DELETE CASCADE);
CREATE TABLE AppVac(
ApplicantID INT PRIMARY KEY,
VacancyID INT PRIMARY KEY,
FOREIGN KEY (ApplicantID) REFERENCES Applicant(ID),
FOREIGN KEY (VacancyID) REFERENCES Vacancy(ID);
CREATE TABLE Vacancy(
ID INT PRIMARY KEY,
.....
No. The Applicant has not relationship to the Vacancy. What gets removed are all corresponding rows in AppVac.
If you want to require that applicants have at least one row in AppVac, then you'll need a delete trigger on the table. Cascading foreign keys won't do that for you. That is, cascading deletes delete from the referring table, not from the reference table.
A very basic question I'm confused about, similar to UPDATE and DELETE CASCADE, do foreign keys gets inserted automatically when a referenced table's primary key is inserted?
Or are they simply a constraint that tells to check the primray key that is being reference and if present add, else error.
As a example, I'm asking about something like this:
...
CONSTRAINT idFK
FOREIGN KEY(id)
REFERENCES Users(id)
ON DELETE CASCADE
ON UPDATE CASCADE
...
Here, will id in this table automatically get inserted when an id is inserted in Users table?
when you insert a record in "Users" table , nothing inserted in any other tables.
when you insert a record in a table with foreign key, the value of foreign key field just will check with the primary key value in parent(Here "Users") table.
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.