I'm trying to create a hireiarchial table in postgres. I'm using the adjacent list-approach. My question is, how should I reference the ID of the same table when creating the data-model?
create table nodes (
id serial primary key,
parentid WHAT GOES HERE?
name varchar,
);
You can do:
create table nodes (
id serial primary key,
parentid integer references nodes(id),
name varchar
);
These are called "self-referencing tables"
Related
I want to create an application for rotating pairs in a team every day. I need to store this in the database. Requirments are:
A team should be assigned to one ore more members.
Each team can have multiple tabs and different members allocate in them.(If team consist of 4 members for the particular tab only 3 should be part of it)
Each tab will have a pair of members or list of pairs per day stored.
I have ended up designing something like the example below:
create table if not exists team (
id serial not null primary key,
name text not null
);
create table if not exists member (
id serial not null primary key,
team_id integer references team(id),
nickname text
);
create table if not exists team_tab (
id bigserial not null primary key,
team_id integer references team(id) on delete cascade,
name text not null,
member_ids integer[],
);
create table if not exists team_tab_pairs (
id bigserial not null primary key,
team_tab_id integer not null references team_tab(id) on delete cascade,
tab_date date not null,
pair_ids integer[][],
);
I need an advice and suggestions how could I achieve this without having a list of references ids stored in the 2 tables below.
You need an extra table to design an M:N relationship. This is the case, for example, between "team tab" and "member". In addition to both main entities:
create table member (
id serial not null primary key,
team_id integer references team(id),
nickname text
);
create table team_tab (
id bigserial not null primary key,
team_id integer references team(id) on delete cascade,
name text not null
);
...you'll need to create a table to represent the M:N relationship, as in:
create table team_tab_member (
team_tab_id bigint not null,
member_id int not null,
primary key (team_tab_id, member_id) -- optional depending on the model
);
I am new in Posgresql. I have 5 tables and I am trying to INSERT properties to tables. When I tried to Insert 2nd time, I have this error in 'pgadmin'.
ERROR: insert or update on table "question" violates foreign key constraint "question_id_difficulty_fkey" DETAIL: Key (id_difficulty)=(9) is not present in table "difficulty". SQL state: 23503.
my schema is here
id SERIAL PRIMARY KEY,
name varchar
);
CREATE TABLE question (
id SERIAL PRIMARY KEY,
text varchar,
correct_answer varchar,
incorrect_answer1 varchar,
incorrect_answer2 varchar,
incorrect_answer3 varchar,
id_difficulty SERIAL REFERENCES difficulty(id),
id_category SERIAL REFERENCES category (id),
id_creator SERIAL REFERENCES game (id)
);
CREATE TABLE difficulty (
id SERIAL PRIMARY KEY,
name varchar
);
CREATE TABLE category (
id SERIAL PRIAMRY KEY,
name varchar
);
CREATE TABLE user (
id SERIAL PRIMARY KEY,
name varchar
)
You would need a corresponding entry in the difficulty table with an id of 9, so that a referencing id_difficulty column in the question table.
For example, if your difficulty table contained:
id | name
----+----------------
1 | easy
2 | reasonable
3 | difficult
4 | very difficult
5 | impossible
You could only set id_difficulty for rows in the question table to one of those id values. If you set 6, or 12 or anything other than 1 to 5, it would fail because the values are constrained by the values in the foreign key.
The id_difficulty, id_category and id_creator columns shouldn't be using serial, so these should have their defaults dropped:
ALTER TABLE question ALTER COLUMN id_difficulty DROP DEFAULT;
ALTER TABLE question ALTER COLUMN id_category DROP DEFAULT;
ALTER TABLE question ALTER COLUMN id_creator DROP DEFAULT;
Postgres now recommends using generated always as instead of serial. If you do this, then the types will align much more simply:
CREATE TABLE question (
id int generated always as identity PRIMARY KEY,
text varchar,
correct_answer varchar,
incorrect_answer1 varchar,
incorrect_answer2 varchar,
incorrect_answer3 varchar,
id_difficulty int REFERENCES difficulty(id),
id_category int REFERENCES category (id),
id_creator int REFERENCES game (id)
);
CREATE TABLE difficulty (
id int generated always as identity PRIMARY KEY,
name varchar
);
This makes what is happening much clearer. The data type for a foreign key reference needs to match the data type of the primary key. Postgres knows that serial is really int. But using generated always, it is obvious that they are the same.
In addition, generated always as is more consistent with standard SQL.
I use ms sql and i need to create a table with a array column of nvarchar. What is the correct query ?
You don't want an array column. You want a separate table (or perhaps a JSON/XML column, but I won't focus on that).
The normal method would be:
create table main (
main_id int identity primary key,
. . .
);
create table element (
element_id int identity primary key,
position int,
value varchar(255),
. . .
);
create table main_elements (
main_element_id int identity primary key,
main_id int references main(main_id),
element_id int references elements(element_id)
);
I have 2 tables and I am trying to create a Foreign Key between the two. Here is the structure of my tables:
create table users (
id serial,
user_name varchar(50)
);
create table playlists (
id serial,
user_id integer references users(id)
);
I keep getting this error:
ERROR: there is no unique constraint matching given keys for referenced table "users"
Why is there not a unique constraint? If I create the id in the users table as integer PRIMARY KEY, then everything works fine. How do I fix this where the users id auto increments and can be the FK in the playlists table?
Creating a column of type serial doesn't make it the primary key or constraint it in any way. serial just creates an integer column, creates a sequence, and attaches the sequence to the column to provide default values. From the fine manual:
In the current implementation, specifying:
CREATE TABLE tablename (
colname SERIAL
);
is equivalent to specifying:
CREATE SEQUENCE tablename_colname_seq;
CREATE TABLE tablename (
colname integer NOT NULL DEFAULT nextval('tablename_colname_seq')
);
ALTER SEQUENCE tablename_colname_seq OWNED BY tablename.colname;
If you want your id serial columns to be primary keys (which you almost certainly do), then say so:
create table users (
id serial not null primary key,
user_name varchar(50)
);
create table playlists (
id serial not null primary key,
user_id integer references users(id)
);
I'm trying to create 3 tables, but the table Award cannot be created. Within the brackets (Child, Present) I get an error. What should I do? Table Present and table Child are created but not table Award. I'm not able to create table Award, why?
create table Child
(
ID integer primary key,
name varchar(20) unique not null,
age integer,
kind bit
);
create table Presents
(
ID integer primary key,
name varchar(20) unique,
weight float
);
create table Award
(
ID integer primary key,
childID integer references Barn(ID),
presentID integer references Presenter(ID),
unique(Child, Present),
antal int
);
Child and Present are not columns in Award. I believe you want unique(childID, presentID)