I'm using the following query:
alter table hlt_citas drop constraint Ref_hlt_citas_to_hlt_atencion;
When I query the structure of the table, the part of the foreign key is the following:
"Ref_hlt_citas_to_hlt_atencion" FOREIGN KEY (cod_atencion)
REFERENCES hlt_atencion(cod_atencion) ON UPDATE CASCADE ON DELETE RESTRICT
But when I run the query it tells me that it does not exist.
When identifiers like column or constraint names are given in double quotes, the identifiers become case-sensitive. You should then also use double-quotes in queries to maintain the case-sensitivity:
alter table hlt_citas drop constraint "Ref_hlt_citas_to_hlt_atencion";
Related
I have an ALTER TABLE statement, written in T-SQL (SQL Server):
ALTER TABLE myTable WITH CHECK ADD CONSTRAINT [FK_myTable_myColumn] FOREIGN KEY(myColumn) REFERENCES otherTable (Column)
If I want to translate this statement in Postgresql, how can I make this? Paying attention to WITH CHECK ADD CONSTRAINT
You need to
remove WITH CHECK - I don't know what this is supposed to do, but you can't have a "check constraint" together with a foreign key constraint in Postgres
use standard compliant identifiers (without the square brackets)
ALTER TABLE my_table
ADD CONSTRAINT fk_mytable_mycolumn
FOREIGN KEY(my_column) REFERENCES other_table (column)
I need to create a migration for an already existing table to make it's foreign key have a UNIQUE constraint. How do I do this?
From the examples I found in the documentation, it is mostly done when the table is created. The issue is I need to add this onto a column that already exists and is already set as a foreign key. This is what the table looks like at it's creation:
CREATE TABLE IF NOT EXISTS "myTable" (
"_id" SERIAL NOT NULL,
"myForeignKeyId" INTEGER NOT NULL,
"name" VARCHAR(255) NOT NULL,
CONSTRAINT "pk_myTable" PRIMARY KEY ("_id"),
CONSTRAINT "fk_myTable_myForeignKeyId" FOREIGN KEY ("myForeignKeyId") REFERENCES "myOtherTable" ("_id")
);
What I want to do is on a migration make myForeignKeyId unique. How do I do that?
I have tried to following:
CREATE UNIQUE INDEX CONCURRENTLY "myTable_myForeignKeyId"
ON province ("myForeignKeyId");
ALTER TABLE IF EXISTS "myTable"
ADD CONSTRAINT "myForeignKeyId"
UNIQUE USING INDEX "myTable_myForeignKeyId";
First off, when I try this in a migration I get the error:
CREATE INDEX CONCURRENTLY cannot run inside a transaction block
So that part cannot be done, but even just doing it through SQL, the second part doesn't work either as it claims myForeignKeyId already exists. Even if I add an ALTER COLUMN myForeignKeyId it just says there is an error on that line.
This seems like it should be a simple enough operation, how can I do this?
After digging some more found quite a simple way to do this, was clearly originally off target.
To add a unique constraint to a column:
ALTER TABLE "myTable"
ADD CONSTRAINT "myUniqueKeyNameOfChoice" UNIQUE ("myColumn");
To remove it:
ALTER TABLE "myTable"
DROP CONSTRAINT "myUniqueKeyNameOfChoice";
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
When trying to ALTER TABLE in Postgres 9.5 to create foreign key constraint: from product_template.product_brand_id to product_brand.id
ALTER TABLE public.product_template
ADD CONSTRAINT product_template_product_brand_id_fkey
FOREIGN KEY (product_brand_id)
REFERENCES public.product_brand (id) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE SET NULL;
Returns error
ERROR: insert or update on table "product_template" violates foreign key constraint "product_template_product_brand_id_fkey"
DETAIL: Key (product_brand_id)=(12) is not present in table "product_brand".
STATEMENT: ALTER TABLE "product_template" ADD FOREIGN KEY ("product_brand_id") REFERENCES "product_brand" ON DELETE set null
Im confused why postgres is trying to find product_brand.product_brand_id, when the fkey is from product_template.product_brand_id to product_brand.id
Any ideas?
The error message simply states that there is at least one row in the table product_template that contains the value 12 in the column product_brand_id
But there is no corresponding row in the table product_brand where the column id contains the value 12
Key (product_brand_id)=(12) relates the source column of the foreign key, not the target column.
In simple terms, the value of FOREIGN KEY(product_brand_id) provided in your ALTER statement is not present in the source (product_brand) table.
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