I have two tables, experiment and sample. Samples must be unique within experiments of the same type, but can be shared between experiments that have a different type. I understand how I can add a unique constraint to the samples to make them always unique, but is it possible to create a unique constraint based on the information in the foreign table?
create table experiment (
id integer primary key,
name text,
type text
);
create table sample (
id integer primary key,
name text,
experiment_id integer,
);
alter table sample add constraint exp_fkey foreign key (experiment_id) references experiment(id);
You can create a (redundant) unique constraint on experiment (id, type), add type to sample, create a foreign key constraint from (experiment_id, type) to (id, type) and a unique constraint on sample.type.
Related
I'm trying to define more than one foreign key in the process table. but I am getting the error that the columns I am trying to define as foreign key are not 'unique value'.
For this, I wanted to define id and name columns as primary keys in category and subject tables. However, when I want to create the process table, I still get this error." there is no unique constraint matching given keys for referenced table "category"
I have researched and continue to do so on Stackoverflow and many more. but I couldn't figure it out with solutions or viewpoints of the issues that got the same error I was facing. Maybe there is something I'm not seeing.
first table;
CREATE TABLE category(
category_id INT GENERATED ALWAYS AS IDENTITY,
category_name VARCHAR(210),
category_description TEXT,
CONSTRAINT category_pk PRIMARY KEY(category_id,category_name)
);
second table;
CREATE TABLE subject(
subject_id INT GENERATED ALWAYS AS IDENTITY,
subject_name VARCHAR(210),
subject_description TEXT,
CONSTRAINT subject_pk PRIMARY KEY(subject_id,subject_name)
);
I tried that too but I keep getting the same error
ALTER TABLE category ADD CONSTRAINT some_constraint PRIMARY KEY(category_id,category_name);
third table;
CREATE TABLE process(
process_id INT GENERATED ALWAYS AS IDENTITY,
fk_category_id INTEGER,
fk_subject_id INTEGER,
FOREIGN KEY(fk_category_id) REFERENCES category(category_id) ON DELETE CASCADE ON UPDATE
CASCADE,
FOREIGN KEY(fk_subject_id) REFERENCES subject(subject_id) ON DELETE CASCADE ON UPDATE
CASCADE
);
In your FOREIGN KEY declaration either:
Include both the columns that make up the PRIMARY KEY on category and subject e.g. ... REFERENCES category(category_id, category_name) ...
OR
Do not refer to any column and let the FK pick up the PK automatically e.g. ... REFERENCES category ON DELETE ....
I am going to say that what you are really after is:
CREATE TABLE category(
category_id INT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
category_name VARCHAR(210) UNIQUE,
category_description TEXT
);
CREATE TABLE subject(
subject_id INT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
subject_name VARCHAR(210) UNIQUE,
subject_description TEXT
);
CREATE TABLE process(
process_id INT GENERATED ALWAYS AS IDENTITY,
fk_category_id INTEGER,
fk_subject_id INTEGER,
FOREIGN KEY(fk_category_id) REFERENCES category ON DELETE CASCADE ON UPDATE
CASCADE,
FOREIGN KEY(fk_subject_id) REFERENCES subject ON DELETE CASCADE ON UPDATE
CASCADE
);
An identity column isn't automatically a primary key. So your tables category and subjectdon't have any primary keys and thus can't be referenced by a foreign key.
You need to add PRIMARY KEY to the columns of the tables category and subject
I'm creating a database in PostgreSQL and want to include a many-to-many relationship between the tables. The two tables I want to include are as follows:
CREATE TABLE "meter" (
"id" integer PRIMARY KEY,
"nmi" integer,
"next_scheduled_read_date" timestamp
);
CREATE TABLE "register" (
"id" text PRIMARY KEY,
"value" text
);
The many-to-many relationship I want to have is between meter id and register id. I have then created the junction table below:
CREATE TABLE "meter_registers" (
"meter_id" integer NOT NULL,
"register_id" text NOT NULL,
PRIMARY KEY ("meter_id", "register_id"),
FOREIGN KEY ("meter_id") REFERENCES "meter" ("id") ON UPDATE CASCADE,
FOREIGN KEY ("register_id") REFERENCES "register" ("id") ON UPDATE CASCADE
);
I then want to create a table that references the meter_id and register_id values from the junction table above which is structured as follows:
CREATE TABLE "demand_data" (
"upload_id" integer PRIMARY KEY,
"nmi" integer,
"meter" integer,
"register" text,
"start" timestamp,
"end" timestamp,
"duration" Time,
"demand" double precision
);
Where the meter and register reference the corresponding junction table columns. However, as the values of the junction table will not be unique I can't simply add a foreign key for the meter and register columns so I run into an error when I run the following:
ALTER TABLE "demand_data" ADD FOREIGN KEY ("meter") REFERENCES "meter_registers" ("meter_id");
ALTER TABLE "demand_data" ADD FOREIGN KEY ("register") REFERENCES "meter_registers" ("register_id");
ERROR: there is no unique constraint matching given keys for referenced table "meter_registers"
Is there a way to possibly reference the junction table columns in the demand_data table without the foreign key constraint? I know it's possible to do with a separate query once some data has been added using inner joins however, is it possible to do it through database table set up?
Yes. A compound foreign key.
ALTER TABLE "demand_data"
ADD FOREIGN KEY ("meter","register")
REFERENCES "meter_registers"( "meter_id", "register_id");
Note: Not directly related you should avoid those dreaded double quotes.
If demand_data references meter_registers, it should reference its primary key. So add a single foreign key constraint on both columns.
If you want two separate foreign keys,you should probably reference meter and register directly.
I'm trying to run create_distributed_table for tables which i need to shard and almost all of the tables have self relation ( parent child )
but when I run SELECT create_distributed_table('table-name','id');
it throws error cannot create foreign key constraint
simple steps to reproduce
CREATE TABLE TEST (
ID TEXT NOT NULL,
NAME CHARACTER VARYING(255) NOT NULL,
PARENT_ID TEXT
);
ALTER TABLE TEST ADD CONSTRAINT TEST_PK PRIMARY KEY (ID);
ALTER TABLE TEST ADD CONSTRAINT TEST_PARENT_FK FOREIGN KEY (PARENT_ID) REFERENCES TEST (ID);
ERROR
citus=> SELECT create_distributed_table('test','id');
ERROR: cannot create foreign key constraint
DETAIL: Foreign keys are supported in two cases, either in between two colocated tables including partition column in the same ordinal in the both tables or from distributed to reference tables
For the time being, it is not possible to shard a table on PostgreSQL without dropping the self referencing foreign key constraints, or altering them to include a separate and new distribution column.
Citus places records into shards based on the hash values of the distribution column values. It is most likely the case that the hashes of parent and child id values are different and hence the records should be stored in different shards, and possibly on different worker nodes. PostgreSQL does not have a mechanism to create foreign key constraints that reference records on different PostgreSQL clusters.
Consider adding a new column tenant_id and adding this column to the primary key and foreign key constraints.
CREATE TABLE TEST (
tenant_id INT NOT NULL,
id TEXT NOT NULL,
name CHARACTER VARYING(255) NOT NULL,
parent_id TEXT NOT NULL,
FOREIGN KEY (tenant_id, parent_id) REFERENCES test(tenant_id, id),
PRIMARY KEY (tenant_id, id)
);
SELECT create_distributed_table('test','tenant_id');
Note that parent and child should always be in the same tenant for this to work.
Based on the documentation it's pretty straightforward how to prevent any overlapping reservations in the table at the same time.
CREATE EXTENSION btree_gist;
CREATE TABLE room_reservation (
room text,
during tsrange,
EXCLUDE USING GIST (room WITH =, during WITH &&)
);
However, when you have multiple resources that can be reserved by users, what is the best approach to check for overlappings? You can see below that I want to have users reserve multiple resources. That's why I'm using the junction table Resources_Reservations. Is there any way I can use EXCLUDE in order to check that no resources are reserved at the same time?
CREATE TABLE Users(
id serial primary key,
name text
);
CREATE TABLE Resources(
id serial primary key,
name text
);
CREATE TABLE Reservations(
id serial primary key,
duration tstzrange,
user_id serial,
FOREIGN KEY (user_id) REFERENCES Users(id)
);
CREATE TABLE Resources_Reservations(
resource_id serial,
reservation_id serial,
FOREIGN KEY (resource_id) REFERENCES Resources(id),
FOREIGN KEY (reservation_id) REFERENCES Reservations(id),
PRIMARY KEY (resource_id, reservation_id)
);
I think what you want is doable with a slight model change.
But first let's correct a misconception. You have foreign key columns (user_id, resource_id, etc) defined as SERIAL. This is incorrect, they should be INTEGER. This is because SERIAL is not actually a data type. It is a psuedo-data type that is actually a shortcut for: creating a sequence, creating a column of type integer, and defining the sequence created as the default value. With that out of the way.
I think your Resources_Reservations is redundant. A reservation is by a user, but a reservation without something reserved would just be user information. Bring the resource_id into Reservation. Now a Reservation is by a user for a resource with a duration. Everything your current model contains but less complexity.
Assuming you don't have data that needs saving, then:
create table users(
id serial primary key,
name text
);
create table resources(
id serial primary key,
name text
);
create table reservations(
user_id integer
resource_id integer
duration tstzrange,
foreign key (user_id) references users(id)
foreign key (resource_id) references resources(id),
primary key (resource_id, user_id)
);
You should now be able to create your GIST exclusion.
I have a table where part of the primary key is a foreign key to another table.
create table player_result (
event_id integer not null,
pub_time timestamp not null,
name_key varchar(128) not null,
email_address varchar(128),
withdrawn boolean not null,
place integer,
realized_values hstore,
primary key (event_id, pub_time, name_key),
foreign key (email_address) references email(address),
foreign key (event_id, pub_time) references event_publish(event_id, pub_time));
Will the index generated for the primary key suffice to back the foreign key on event_id and pub_time?
Yes.
Index A,B,C
is good for:
A
A,B
A,B,C (and any other combination of the full 3 fields, if default order is unimportant)
but not good for other combinations (such as B,C, C,A etc.).
It will be useful for the referencing side, such that a DELETE or UPDATE on the referenced table can use the PRIMARY KEY of the referencing side as an index when performing checks for the existence of referencing rows or running cascade update/deletes. PostgreSQL doesn't require this index to exist at all, it just makes foreign key constraint checks faster if it is there.
It is not sufficient to serve as the unique constraint for a reference to those columns. You couldn't create a FOREIGN KEY that REFERENCES player_result(event_id, pub_time) because there is no unique constraint on those columns. That pair can appear multiple times in the table so long as each pair has a different name_key.
As #xagyg accurately notes, the unique b-tree index created by the foreign key reference is also only useful for references to columns from the left of the index. It could not be used for a lookup of pub_time, name_key or just name_key, for example.