I'm currently trying the inheritance system with PostgreSQL but I have a problem with the auto-increment index in my child tables.
I have three tables: "Currency", "Crypto" and "Stable"
CREATE TABLE IF NOT EXISTS public.currency
(
id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
name VARCHAR(30) UNIQUE NOT NULL,
symbol VARCHAR(10) UNIQUE NOT NULL,
);
CREATE TABLE IF NOT EXISTS public.stable (id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY) INHERITS (public.currency);
CREATE TABLE IF NOT EXISTS public.crypto (id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY) INHERITS (public.currency);
I insered my data like this:
INSERT INTO public.stable (name, symbol) VALUES ('Euro', '€'), ('Dollar', '$'), ('Tether', 'USDT');
INSERT INTO public.crypto (name, symbol) VALUES ('Bitcoin', 'BTC'), ('Ethereum', 'ETH'), ('Litcoin', 'LTC');
But this is my problem: I would like to have a unique identifier that increments itself through my parent table "Currency".
When I select, I have (take a look in my id: 1,2,3,1,2,3):
But, Is it possible to have something like this instead (1,2,3,4,5,6):
Is it a problem in my primary key?
Thank you
We can try to use create sequence to set row numbers for sharing between multiple tables.
define a new sequence generator
create sequence n_id;
Then we can use this sequence as below, sharing this sequence for those three tables.
create sequence n_id;
CREATE TABLE IF NOT EXISTS currency
(
id INT default nextval('n_id') PRIMARY KEY,
name VARCHAR(30) UNIQUE NOT NULL,
symbol VARCHAR(10) UNIQUE NOT NULL
);
CREATE TABLE IF NOT EXISTS stable (id INT default nextval('n_id') PRIMARY KEY) INHERITS (currency);
CREATE TABLE IF NOT EXISTS crypto (id INT default nextval('n_id') PRIMARY KEY) INHERITS (currency);
sqlfiddle
Related
I can not add a referential constraint, both tables has the two columns id and version as primary key. I will only check the integrity that the row is existing without checking version.
CREATE TABLE TABLEE
(ID INTEGER NOT NULL,
VERSION INTEGER NOT NULL,
PRIMARY KEY (ID , VERSION);
CREATE TABLE CHAIR
(ID INTEGER NOT NULL,
VERSION INTEGER NOT NULL,
ID_TABLEE INTEGER,
PRIMARY KEY (ID , VERSION);
ALTER Chair
ADD constraint MYC FOREIGN KEY (ID)
REFERENCES TABLEE (ID)
ON DELETE RESTRICT
Got this error
[Code: -573, SQL State: 42890] A column list specified in the references clause of constraint "MYC " does not identify a unique constraint of the parent table or nickname "TABLEE".. SQLCODE=-573, SQLSTATE=42890, DRIVER=4.28.11
The problem here is that Tablee its primary key consists of two columns id and version. But the integrity check should only be made on the referenced id, not version.
To establish a foreign key on a table the referenced column needs to:
Be a primary key on the other table.
...or at least act as one.
Since you already have a primary key on the referenced table, you can use the second approach and add a UNIQUE and NOT NULL constraints on ID.
For example:
CREATE TABLE TABLEE (
ID INTEGER NOT NULL,
VERSION INTEGER NOT NULL,
PRIMARY KEY (ID , VERSION),
constraint uq1 unique (id) -- added UNIQUE constraint on ID
);
CREATE TABLE CHAIR (
ID INTEGER NOT NULL,
VERSION INTEGER NOT NULL,
ID_TABLEE INTEGER,
PRIMARY KEY (ID , VERSION)
);
ALTER table Chair
ADD constraint MYC FOREIGN KEY (ID)
REFERENCES TABLEE (ID)
ON DELETE RESTRICT;
See running example at db<>fiddle.
Note: If you want ID to have repeated values over the table, then ID is not a key, and cannot be referenced as one.
I have an entity table with multiple entity types and a table of entity relations which needs to enforce that it only contains relations between entities of the same type.
Currently, I have two approaches to this:
CREATE TABLE entity (
id uuid PRIMARY KEY,
type my_enum_type NOT NULL,
-- … more
);
CREATE TABLE relation (
id uuid PRIMARY KEY,
x uuid REFERENCES entity NOT NULL,
y uuid REFERENCES entity NOT NULL,
CHECK(entity(x).type = entity(y).type)
-- Doesn't work because CHECK cannot reference other tables.
);
Fortunately I currently have only two types, and don't expect that to change soon. But inheritance does not mix well with foreign keys, so it gets quite verbose:
CREATE TABLE entity (
id uuid PRIMARY KEY,
-- … more fields
);
CREATE TABLE entity_a (
PRIMARY KEY (id)
) INHERITS (entity);
CREATE TABLE entity_b (
PRIMARY KEY (id)
) INHERITS (entity);
CREATE TABLE relation (
id uuid PRIMARY KEY,
x uuid NOT NULL,
y uuid NOT NULL,
-- … more fields
);
CREATE TABLE relation_a (
PRIMARY KEY (id),
FOREIGN KEY (x) REFERENCES entity_a (id),
FOREIGN KEY (y) REFERENCES entity_a (id)
) INHERITS (relation);
CREATE TABLE relation_b (
PRIMARY KEY (id),
FOREIGN KEY (x) REFERENCES entity_b (id),
FOREIGN KEY (y) REFERENCES entity_b (id)
) INHERITS (relation);
The second approach definitely has the advantage that it works, but it's verbose, not extensible, and the only advantage over completely separate table definitions is that it avoids copying all additional fields (and then maybe forgetting to update them in both places).
Any suggestions on how to solve this more elegantly?
The following statements will add constraints to your solution 1) so that the condition is always satisfied:
/* we need a (redundant) UNIQUE constraint as target for foreign keys */
ALTER TABLE entity ADD UNIQUE (type, id);
/* add a (redundant) "type" column and fill it from "entity" */
ALTER TABLE relation ADD type my_enum_type;
UPDATE relation SET type = e.type
FROM entity AS e
WHERE relation.x = e.id;
ALTER TABLE relation ALTER type SET NOT NULL;
/* now we can add foreign keys that guarantee your condition */
ALTER TABLE relation ADD FOREIGN KEY (type, x) REFERENCES entity (type, id);
ALTER TABLE relation ADD FOREIGN KEY (type, y) REFERENCES entity (type, id);
/* remove the bloat (optional) */
VACUUM (FULL) relation;
True, it adds a redundant column and an redundant constraint, but I think it is the most elegant and natural way to ensure your condition.
I'm trying to create some Postgres tables, one has a foreign key on anothers' index:
CREATE TABLE Foo (ID SERIAL PRIMARY KEY);
CREATE TABLE File (FooId TEXT NOT NULL REFERENCES Foo (ID));
But it results in ERROR: foreign key constraint file_fooid_fkey cannot be implemented
Appears the types have to "match" or you'll get that error.
This works (change to int to match the size/type of SERIAL KEY which is an INT):
CREATE TABLE Foo (ID SERIAL PRIMARY KEY);
CREATE TABLE File (FooId INT NOT NULL REFERENCES Foo (ID));
Is it possible to assign a foreign key to a json property in PostgreSQL? Here is an example what I would like to achieve, but it doesn't work:
CREATE TABLE Users (Id int NOT NULL PRIMARY KEY);
CREATE TABLE Data (
Id int NOT NULL PRIMARY KEY,
JsonData json NOT NULL, -- [{Id: 1, somedata: null},{Id: 2, somedata: null}, ...]
CONSTRAINT FK_Users_Data FOREIGN KEY (JsonData->Id) REFERENCES Users(Id) -- this constraint will fail
);
It is not possible, and may not ever be possible, to assign a foreign key to a json property. It'd be a major and quite complicated change to PostgreSQL's foreign key enforcement. I don't think it's impossible to do, but would face similar issues to those experienced by the foreign-keys-to-arrays patch.
With 9.4 it'll be possible to make a whole json object a foreign key as jsonb supports equality tests. In 9.3 you can't even do that.
Here's a little SPI function have_ids which I use for an integrity constraint on a one-to-many relationship with a jsonb column
CREATE TABLE foo (
id INTEGER NOT NULL
)
CREATE TABLE bar (
foo_ids pg_catalog.jsonb DEFAULT '[]'::jsonb NOT NULL,
CONSTRAINT bar_fooids_chk CHECK (have_ids ('foo', foo_ids))
)
With a couple of triggers on foo it's almost as good as a foreign key.
The foreign key parameter must be a column name:
http://www.postgresql.org/docs/current/static/sql-createtable.html
You will have to normalize
create table user_data (
id int not null primary key,
user_id int not null,
somedata text,
constraint fk_users_data foreign key (user_id) references Users(Id)
);
Yes it is possible but you will have to store another value. If you change your schema to:
CREATE TABLE Users (Id int NOT NULL PRIMARY KEY);
CREATE TABLE Data (
Id int NOT NULL PRIMARY KEY,
JsonData json NOT NULL,
UserId int generated always as ((JsonData->>'Id')::int) stored references Users(Id)
);
INSERT INTO Users VALUES (1);
Foreign key that doesn't exist:
INSERT INTO Data VALUES (1, '{"Id": 3}');
Returns the error:
ERROR: insert or update on table "data" violates foreign key constraint "data_userid_fkey" DETAIL: Key (userid)=(3) is not present in table "users".
Foreign key that does work:
INSERT INTO Data VALUES (1, '{"Id": 1}');
How to provide primary key for multiple column in a single table using PostgreSQL?
Example:
Create table "Test"
(
"SlNo" int not null primary key,
"EmpID" int not null, /* Want to become primary key */
"Empname" varchar(50) null,
"EmpAddress" varchar(50) null
);
Note: I want to make "EmpID" also a primary key.
There can only be one primary key per table - as indicated by the word "primary".
You can have additional UNIQUE columns like:
CREATE TABLE test(
sl_no int PRIMARY KEY, -- NOT NULL due to PK
emp_id int UNIQUE NOT NULL,
emp_name text,
emp_addr text
);
Columns that are (part of) the PRIMARY KEY are marked NOT NULL automatically.
Or use a table constraint instead of a column constraint to create a single multicolumn primary key. This is semantically different from the above: Now, only the combination of both columns must be unique, each column can hold duplicates on its own.
CREATE TABLE test(
sl_no int, -- NOT NULL due to PK below
emp_id int , -- NOT NULL due to PK below
emp_name text,
emp_addr text,
PRIMARY KEY (sl_no, emp_id)
);
Multicolumn UNIQUE constraints are possible, too.
Aside: Don't use CaMeL-case identifiers in Postgres. Use legal, lower-case identifiers so you never have to use double-quotes. Makes your life easier. See:
Are PostgreSQL column names case-sensitive?
In case you want to specify the name of the primary key constraint:
CREATE TABLE test(
sl_no int not null,
emp_id int not null,
emp_name text,
emp_addr text,
constraint pk_test primary key (sl_no, emp_id)
);
Source: https://www.postgresqltutorial.com/postgresql-primary-key/