Normally when creating constraint between two tables i used to use following script:
ALTER TABLE userVendor
ADD CONSTRAINT myrelation
FOREIGN KEY (DataSource, userId)
REFERENCES user(DataSource, userId)
so userVendor table references user table
Now let's say userVendor becomes association table between two tables. Therefore we have additionally third table called Vendor which has datasource and vendor id as composite key. All of them by the way having composite keys.
How to modify my script to say like (wrong script so far):
ALTER TABLE userVendor
ADD CONSTRAINT myrelation
FOREIGN KEY (DataSource, userId, vendorid)
REFERENCES user(DataSource, userId)
REFERENCES user(DataSource, vendorid)
You'll have to add two foreign key constraints, each referencing one table. It is no problem if a column appears in both constraints.
You will need two ADD CONSTRAINT clauses in your ALTER TABLE statement.
Related
I created a table in PostgreSQL with a foreign key constraint.
I dropped the table to which the foreign key belongs. Now how to alter the table or how to defer the foreign key present in the table?
To clarify:
I have a table named test. It has a column called subjectName, which is a foreign key of subject Table. Now I dropped subject table. How to remove the FK constaint on table test
Assuming the following tables:
create table subject
(
name varchar(10) primary key
);
create table test
(
some_column integer,
subject_name varchar(10) not null references subject
);
there are two scenarios what could have happened when you dropped the table subject:
1. you didn't actually drop it:
drop table subject;
ERROR: cannot drop table subject because other objects depend on it
Detail: constraint test_subject_name_fkey on table test depends on table subject
Hint: Use DROP ... CASCADE to drop the dependent objects too.
2. you did drop it, then the foreign key is gone as well.
drop table subject cascade;
NOTICE: drop cascades to constraint test_subject_name_fkey on table test
which tells you that the foreign key constraint was automatically dropped.
Perhaps your question in not exactly what you mean. Are you wanting to remove the which was a foreign key from the table. As amply indicated if you dropped the parent table then the FK is also dropped. However the column itself is not dropped from the child table. To remove that you need to alter the table.
alter table test drop column subject_name;
See demo here
I have a requirement of deleting records from the Postgres database tables.
We have a Customer table which is the main table, this table contains a primary key which is used in so many other tables as a FOREIGN KEY, I want to delete one of the customers as well as its reference used in other tables. Is there any way to delete the customer from main table as well as from other tables which contains foreign key.
Thanks in Advance.
In the other tables, you want a cascading delete foreign key reference. You can create one in the database using:
alter table othertable add constraint fk_othertable_customerid
foreign key (customerid) references customers(customerid)
on delete cascade;
Note: This assumes that customerid is the name of the column in both tables and that it is already defined in the other tables.
A cascading foreign key constraint does exactly what you specify. When a row is deleted in the reference table, then all related rows are deleted.
If you already have foreign key constraints on customerid, then drop the existing constraint and add the cascading version.
Table: A
columns names
--------------------------
PK varchar |datasource
PK int |programid
int |workspaceid
composite pk key of: datasource and programid
Table: B
columns names
---------------------------------------
PK varchar | datasource
PK int | quantitycontractid
int | workspaceid
composite pk key of: datasource and quantitycontractid
I need to make relationship between those tables but using workspaceid and datasource. So i try as usual:
ALTER TABLE A
ADD CONSTRAINT fk_relation
FOREIGN KEY (workspaceid, datasource)
REFERENCES B(workspaceid, datasource)
I am getting following error:
there is no unique constraint matching given keys for referenced table
"B"
You need to add UNIQUE key to B(workspaceid, datasource) before you consider that as a foreign key in table A. This is to ensure a correct one to one or one to many relationships between the two tables.
ALTER TABLE B
ADD CONSTRAINT unq_contraint
UNIQUE KEY (workspaceid, datasource)
The error makes perfect sense to me, your table B doesn't have any unique index or primary key linked to workspaceid. Having said that, your table structure for B looks kinda weird to me. Most databases have a primary key that is an autoincrement and one or more foreign keys linking to other tables. You seem to have made your primary key a combination of multiple foreign keys. While this works, you're gonna have issues like you described and have complicated joins when your query your tables.
Vishal R already answered on how to fix your problem.
TL;DR
I am seeking clarity on this: does a FOREIGN KEY require a UNIQUE CONSTRAINT on the other side, specifically, in Postgres and, generally, in relational database systems?
Perhaps, I can test this, but I'll ask, if the UNIQUE CONSTRAINT is required by the FOREIGN KEY what would happen if I don't create it? Will the Database create one or will it throw an error?
How I got there
I had earlier on created a table with a column username on which I imposed a unique constraint. I then created another table with a column bearer_name having a FOREIGN KEY referencing the previous table's column username; the one which had a UNIQUE CONSTRAINT.
Now, I want to drop the UNIQUE CONSTRAINT on the username column from the database because I have later on created a UNIQUE INDEX on the same column and intuitively I feel that they serve the same purpose, or don't they? But the database is complaining that the UNIQUE INDEX has some dependent objects and so it can't be dropped unless I provide CASCADE as an option in order to drop even the dependent object. It's identifying the FOREIGN KEY on bearer_name column in the second table as the dependent object.
And is it possible for the FOREIGN KEY to be a point to the UNIQUE INDEX instead of the UNIQUE CONSTRAINT?
I am seeking clarity on this: does a FOREIGN KEY require a UNIQUE CONSTRAINT on the other side
No it does not require only UNIQUE CONSTRAINT. It could be PRIMARY KEY or UNIQUE INDEX.
Perhaps, I can test this, but I'll ask, if the UNIQUE CONSTRAINT is required by the FOREIGN KEY what would happen if I don't create it? Will the Database create one or will it throw an error?
CREATE TABLE tab_a(a_id INT, b_id INT);
CREATE TABLE tab_b(b_id INT);
ALTER TABLE tab_a ADD CONSTRAINT fk_tab_a_tab_b FOREIGN KEY (b_id)
REFERENCES tab_b(b_id);
ERROR: there is no unique constraint matching given keys
for referenced table "tab_b"
DBFiddle Demo
And is it possible for the FOREIGN KEY to be a point to the UNIQUE INDEX instead of the UNIQUE CONSTRAINT?
Yes, it is possible.
CREATE UNIQUE INDEX tab_b_i ON tab_b(b_id);
DBFiddle Demo2
I'm using inheritance and I ended up having a problem.
If I run:
select count(*) from estate_properties where id = 86820;
I get 1.
But when I try to run this:
insert into property_images (binary_image, name, property_id) values (16779, 'IMG_0096.jpg', 86820)
I get:
********** Error **********
ERROR: insert or update on table "property_images" violates foreign
key constraint "property_images_property_id_fkey" SQL state: 23503
Detail: Key (property_id)=(86820) is not present in table
"estate_properties".
Also ID on estate_properties is SERIAL.
Note: Another table apartments inherits from estate_properties, and 86820 was added to it. Would that make a difference? Also why would it I still have the ID in the parent table and I can select if from there.
Edit:
Looking more closely at the documentation:
http://www.postgresql.org/docs/9.5/static/ddl-inherit.html
I want to achieve this:
5.9.1. Caveats
Specifying that another table's column REFERENCES cities(name) would
allow the other table to contain city names, but not capital names.
There is no good workaround for this case.
EDIT2:
Here is the declaration of the foreign key:
CONSTRAINT property_images_property_id_fkey FOREIGN KEY (property_id)
REFERENCES estate_properties (id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION
Apparently the answer is here:
Foreign keys + table inheritance in PostgreSQL?
A foreign key can point to a table that is part of an inheritance hierarchy, but it'll only find rows in that table exactly. Not in any parent or child tables. To see which rows the foreign key sees, do a SELECT * FROM ONLY thetable. The ONLY keyword means "ignoring inheritance" and that's what the foreign key lookup will do