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)
)
Related
I have this table:
CREATE TABLE cars_info.cars
(
id SERIAL,
owner_id INTEGER,
brand VARCHAR(50) NOT NULL,
model VARCHAR(50) NOT NULL,
color VARCHAR(50) NOT NULL,
register_number VARCHAR(50) NOT NULL,
created DATE NOT NULL,
PRIMARY KEY(id, brand, model, color, register_number, created),
CONSTRAINT fk_owner_id
FOREIGN KEY(owner_id)
REFERENCES persons_info.persons(id)
);
But when I tried create another table like this:
CREATE TABLE cars_info.violations
(
id SERIAL PRIMARY KEY,
car_id INTEGER NOT NULL,
message VARCHAR(100) NOT NULL,
active BOOLEAN NOT NULL,
CONSTRAINT fk_car_id
FOREIGN KEY(car_id)
REFERENCES cars_info.cars(id)
);
I get an error about that
Target external table "cars" does not have a unique constraint corresponding to the given keys
How can I fix that? I'm a beginner in SQL and don't know how to go about googling that
Your primary key definition for cars
PRIMARY KEY(id, brand, model, color, register_number, created)
makes no sense: The id column, being serial, is itself unique and it alone should be the primary key.
Delete your primary key definition and change the id column definition to:
id serial not null primary key
Unrelated, but best practice is to name table in the singular; name your tables car and violation rather than cars and violations
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
);
Given two tables like so
CREATE TABLE participants(
id SERIAL PRIMARY KEY,
Name TEXT NOT NULL,
Title TEXT NOT NULL
);
CREATE TABLE meetings (
id SERIAL PRIMARY KEY,
Organizer TEXT NOT NULL,
StartTime DATE NOT NULL,
EndTime DATE NOT NULL,
Participants INT[],
);
I want Participants column of 'meetings' table to contain set of integers which are all primary keys (specific participant) from 'participants' table.
How do I define this field in 'meetings' table ?
The old fashioned way is to create a many-many table, with a couple of commonsense constraints:
CREATE TABLE meetings_participants(
meeting_id int not null,
participant_id int not null,
primary key (meeting_id, participant_id),
foreign key(meeting_id) references meetings(id),
foreign key(participant_id) references participants(id)
)
Now it is easy to add and remove people to meetings be inserting or deleting rows or query meetings that e.g. have 4 or more participants.
A more common approach is to create a junction table for the meeting participants.
CREATE TABLE participants (
participant_id INT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
Name TEXT NOT NULL,
Title TEXT NOT NULL
);
CREATE TABLE meetings (
meeting_id INT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
Organizer TEXT NOT NULL,
StartTime DATE NOT NULL,
EndTime DATE NOT NULL
);
CREATE TABLE meeting_participants(
meeting_id INT NOT NULL,
participant_id INT NOT NULL,
PRIMARY KEY (meeting_id, participant_id),
FOREIGN KEY (meeting_id) REFERENCES meetings(meeting_id),
FOREIGN KEY (participant_id) REFERENCES participants(participant_id)
);
Which is then used to join the 2 tables.
For example:
SELECT m.*, p.*
FROM meeting_participants mp
JOIN meetings m USING(meeting_id)
JOIN participants p USING(participant_id)
WHERE m.Organizer = 'John Doe';
I have 2 tables in my database:
CREATE TABLE Apartment
(
StreetName char(50) not null,
Number Integer not null,
Door Integer not null,
Type char(30) not null,
SizeSquareMeter Integer not null,
NID Integer not null FOREIGN KEY REFERENCES Neighborhood(NID)
CONSTRAINT Address PRIMARY KEY (StreetName, Number, Door)
);
CREATE TABLE Resident
(
RID Integer not null PRIMARY KEY,
FirstName varchar(30) not null,
LastName varchar(30) not null,
BirthDate date not null,
Address
);
Now the primary key in apartment is Address and is based on constraint. I want to create a foreign key Address in the Resident table referencing the
primary key Address in the Apartment table.
How can I do that?
Since your primary key in Apartment is made up from three columns, any table wanting to reference Apartment will also need to have ALL three columns to establish a foreign key link.
So you need to change your Resident table to:
CREATE TABLE Resident
(
RID Integer not null PRIMARY KEY,
FirstName varchar(30) not null,
LastName varchar(30) not null,
BirthDate date not null,
StreetName char(50) not null,
Number Integer not null,
Door Integer not null,
);
and then you can add the foreign key constraint:
ALTER TABLE Resident
ADD CONSTRAINT FK_Resident_Apartment
FOREIGN KEY (StreetName, Number, Door) REFERENCES Apartment (StreetName, Number, Door);
The other option would be to add a surrogate column (an artificial, additional column) to Apartment - either as primary key, or (for SQL Server) at least with a unique constraint - so that you could establish the FK link to that surrogate column (using that one, single column - instead of having to replicate three columns of "real" data)
I'm new to SQL and there are a lot of things going on that I still don't seem to quite understand. I have the following table
CREATE TABLE Person
(
First_Name varchar(20) NOT NULL,
Name varchar(20) NOT NULL,
Address varchar(50) NOT NULL,
PRIMARY KEY (First_Name, Name, Address)
);
I know want to create another table that has the primary key from the table Person as foreign key and also as primary key:
CREATE TABLE Purchase
(
No_Installments int,
Rate int,
Person varchar(50) NOT NULL PRIMARY KEY,
CONSTRAINT PFK
FOREIGN KEY (Person) REFERENCES Person (First_Name, Name, Address)
);
For some reason this doesn't work and I get an error every time. I've already looked up the other threads here on stackoverflow, but they don't really seem to help me. What am I doing wrong?
If you have a compound PK made up from three columns, then any child table that wants to establish a foreign key relationship must ALSO have all those 3 columns and use all 3 columns to establish the FK relationship.
FK-PK relationship is an all or nothing proposal - you cannot reference only parts of a primary key - either you reference all columns - or you don't reference.
CREATE TABLE Purchase
(
No_Installments int,
Rate int,
Person varchar(50) NOT NULL PRIMARY KEY,
First_Name varchar(20) NOT NULL,
Name varchar(20) NOT NULL,
Address varchar(50) NOT NULL,
CONSTRAINT PFK
FOREIGN KEY (First_Name, Name, Address)
REFERENCES Person (First_Name, Name, Address)
);
Have an integer primary key, using identity, auto_increment, serial or whatever for your database:
CREATE TABLE Person (
PersonId int identity PRIMARY KEY
First_Name varchar(20) NOT NULL,
Name varchar(20) NOT NULL,
Address varchar(50) NOT NULL,
CONSTRAINT unq_person_3 UNIQUE (First_Name, Name, Address)
);
Then use the identity column for the reference:
CREATE TABLE Purchase (
PurchaseId int identity PRIMARY KEY,
No_Installments int,
Rate int,
PersonId int,
CONSTRAINT PFK
FOREIGN KEY (PersonId) REFERENCES Person (PersonId)
);
Notes:
You really don't want to have to deal with a composite primary key. Have you thought about what the joins will look like?
You don't want a primary key where the values are subject to change. What happens when someone changes his/her name? When someone moves?
Person should not be the primary key in Purchases. Are you only allowing someone to make one purchase?
As noted initially, how you generate such a column varies by database; identity happens to be the way that SQL Server does this.
You probably want to assign a unique ID to each person, not relying on their name to be unique or for an address to be required. That ID will be your Primary key and foreign key. Your purchase table should also have its own id for its primary key -- otherwise because primary keys must be unique, each person can only have one purchase.
CREATE TABLE Person (
id serial NOT NULL,
First_Name varchar(20) NOT NULL,
Name varchar(20) NOT NULL,
Address varchar(50) NOT NULL,
PRIMARY KEY (id));
CREATE TABLE Purchase (
id serial NOT NULL,
No_Installments int,
Rate int,
Person int NOT NULL,
FOREIGN KEY (Person) REFERENCES Person (id),
PRIMARY KEY (id));