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)
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
);
Let's say I have a table tree and a table special_tree
CREATE TABLE tree VALUES (name VARCHAR(32) UNIQUE NOT NULL PRIMARY KEY,
type VARCHAR(32) NOT NULL);
CREATE TABLE special_tree VALUES (name NOT NULL REFERENCES tree(name),
treat_date DATE,
id INT NOT NULL PRIMARY KEY);
So I have a table containing a list of trees with unique names BUT I want to say that a tree can have multiple 'treat_date' (for various reasons).
Since tree(name) is unique I can't add 2 same name in special_tree.
Is the only solution is to remove unique from tree and then add everywhere i handle the tree table an IF statement to check if name isn't already there? (IF EXISTS (SELECT name FROM tree where tree.name = ...))
If you consider the column name of the table tree it doesn't mean that all referenced columns also should have unique values. Take a look at this example
the best solution for this is (for MySQL):
CREATE TABLE tree VALUES (
id_t int(11) NOT NULL auto_increment,
name VARCHAR(32) NOT NULL,
type VARCHAR(32) NOT NULL,
CONSTRAINT tree_pk PRIMARY KEY (id_t));
CREATE TABLE special_tree VALUES (
id_st int(11) NOT NULL auto_increment,
id_t NOT NULL REFERENCES tree(id_t),
treat_date DATE,
CONSTRAINT special_tree_pk PRIMARY KEY (id_st));
for PostgreSQL:
CREATE TABLE tree VALUES (
id_t serial primary key,
name VARCHAR(32) NOT NULL,
type VARCHAR(32) NOT NULL);
CREATE TABLE special_tree VALUES (
id_st serial primary key,
id_t NOT NULL REFERENCES tree(id_t),
treat_date DATE);
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 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"
I have a one-to-many relationship between two classes for this situation. I have a swimming competition and that competition can have x swimmers.
How can I create an SQL table for this, I know I will have to use the Primary Key of Swimmers as a foreign key in the Swimming competition but I have no idea how to represent the correct number of attributes since it's unknown.
This is called a m:n relationship and usually solved with a mapping table.
Something like this:
create table swimmer
(
id integer not null primary key,
lastname varchar(100) not null,
firstname varchar(100)
)
create table competition
(
id integer not null primary key,
name varchar(50) not null,
comp_date date not null
)
create table participant
(
swimmer_id integer not null,
competition_id integer not null,
rank_in_competetion integer,
primary key (swimmer_id, competition_id),
constraint fk_participant_swimmer (swimmer_id) references swimmer(id),
constraint fk_participant_competition (competition_id) references competition(id)
)