so this is my double primary key table
create table order_mattress
(
order_number int,
mattress_id int ,
primary key (order_number,mattress_id)
);
this is my second table
create table mattress
(
mattress_id int IDENTITY(1,1) PRIMARY KEY,
mattress_name varchar(25)
);
i want to mattress_id in the table mattress to be a foreign key to mattress_id in the table order_mattress how is that possible, without having any problem because of the double primary key in the firsttable
I'd change it so that you have 3 tables as opposed to 2:
Simple order table to hold unique order number:
create table orders
(
order_number int IDENTITY(1,1) PRIMARY KEY,
);
Mattress table that hold all unique mattresses:
create table mattress
(
mattress_id int IDENTITY(1,1) PRIMARY KEY,
mattress_name varchar(25)
);
The order mattress table, then links mattresses to orders with foreign keys.
create table order_mattress
(
order_number int FOREIGN KEY REFERENCES orders(order_number),
mattress_id int FOREIGN KEY REFERENCES mattress(mattress_id)
);
You have the direction wrong - if the mattress table describes mattresses, the mattress_order table should have a foreign key to it:
ALTER TABLE order_mattress
ADD FOREIGN KEY (mattress_id)
REFERENCES mattress(mattress_id)
The other direction is indeed impossible as mattress_id is not unique in order_mattress.
Related
Whenever I try to create these two tables in oracle live SQL it gives right parenthesis error.
CREATE TABLE Section_Table (
Status_ID INT Primary Key,
Names varchar(20),
course_id int FOREIGN KEY references Course_Table(course_id),
Schedule_id int FOREIGN KEY references Schedule_Table(Schedule_id),
Instruction_id int
);
CREATE TABLE Enrollment_Table (
ENROLLMENT_ID INT PRIMARY KEY,
STUDENT varchar(20),
course_id INT FOREIGN KEY REFERENCES Course_Table(course_id),
LEVEL_ID INT FOREIGN KEY REFERENCES Level_Table(LEVEL_ID),
Status_id INT FOREIGN KEY REFERENCES Status_Table(Status_ID),
DATE_ENROLLED DATE
);
I have a table containing Employees all of whom have an ID; I'm referencing this ID in two other tables (Salesman, Mechanic) via a foreign key. What I want to ensure is that an employee is either a mechanic or a salesman, but never both. In other words, I want to associate this logic with my foreign keys:
How would I integrate logic like that into a table? I'm a SQL beginner, so I apologize if this is a dumb question.
These are my tables:
CREATE TABLE [dbo].[Employee]
(
Number INT NOT NULL,
-- ...
PRIMARY KEY(Number)
);
CREATE TABLE [dbo].[Salesman]
(
ID INT NOT NULL,
-- ...
PRIMARY KEY(ID),
FOREIGN KEY(ID) REFERENCES [dbo].[Employee](ID)
);
CREATE TABLE [dbo].[Mechanic]
(
ID INT NOT NULL,
-- ...
PRIMARY KEY(ID),
FOREIGN KEY(ID) REFERENCES [dbo].[Employee](ID)
);
This is a one-of relationship and it is tricky to implement in SQL. Here is one method:
CREATE TABLE [dbo].[Employee] (
EmployeeId INT PRIMARY KEY,
EmployeeType VARCHAR(32)
-- ...
CHECK (EmployeeType IN ('Mechanic', 'SalesPerson')),
UNIQUE (EmployeeType, EmployeeId)
);
CREATE TABLE [dbo].[SalesPerson]
(
EmployeeId INT PRIMAY KEY,
-- ...
EmployeeType as (CONVERT(VARCHAR(32), 'Salesperson')) PERSISTED,
FOREIGN KEY (EmployeeType, EmployeeId) REFERENCES [dbo].[Employee](EmployeeType, EmployeeId)
);
CREATE TABLE [dbo].[Mechanic] (
EmployeeId INT PRIMARY KEY,
EmployeeType as (CONVERT(VARCHAR(32), 'Mechanic')) PERSISTED,
-- ...
FOREIGN KEY (EmployeeType, EmployeeId) REFERENCES [dbo].[Employee](EmployeeType, EmployeeId)
);
Here is a SQL Fiddle illustrating the code.
I have a question about foreign keys.
How does it work when I want to add a foreign key to the first table that I make that references to the primary key of the second table I create?
CREATE TABLE table1
(
name_id INT NOT NULL,
team TEXT REFERENCES table2(team_id),
PRIMARY KEY(name_id)
);
CREATE TABLE table2
(
team_id INT NOT NULL,
teamname TEXT,
PRIMARY KEY(team_id)
);
If I try the code above I get the following error:
ERROR: relation "" does not exist
Thanks in advance.
Either create the second table first. Or use alter table. That is, create the first table without the reference and then do:
alter table table1 add constraint fk_table1_team
foreign key (team_id) REFERENCES table2(team_id);
The declaration for table1 would be:
CREATE TABLE table1 (
name_id INT NOT NULL,
team_id INT,
PRIMARY KEY(name_id)
);
The reference between the tables should be on the primary key and certainly not on a character column, if an integer is available.
here's the syntax of creating a table with Foreign key:
CREATE TABLE table11
(
name_id INT NOT NULL,
team INT,
PRIMARY KEY(name_id),
foreign key(team) references table22(team_id)
);
CREATE TABLE table22
(
team_id INT NOT NULL,
teamname TEXT,
PRIMARY KEY(team_id)
);
but there was another problem. a foreign key from a child table cannot reference to a primary key from a parent folder if they do not contain the same type. in your code team was of TEXT and team_id was of INT which cannot be.
I am new in SQL and trying to understand Foreign key syntax. I know this was asked in multiple questions but each question I found did not seem to teach me what I am doing wrong here.
This is my SQL code:
CREATE TABLE Customer
(
id int primary key,
name varchar(30),
age int,
gender bool
);
CREATE TABLE Minor
(
FOREIGN KEY (parent_id) REFERENCES Customer(id)
);
CREATE TABLE Adult
(
FOREIGN KEY (parent_id) REFERENCES Customer(id)
);
CREATE TABLE Shop
(
id int primary key
);
CREATE TABLE Drink
(
name varchar(30) primary key
);
CREATE TABLE AlcoholicDrink
(
FOREIGN KEY (name) REFERENCES Drink(name)
);
CREATE TABLE NonAlcoholicDrink
(
FOREIGN KEY (name) REFERENCES Drink(name)
);
And this is the error I am getting:
ERROR: column "parent_id" referenced in foreign key constraint does not exist
SQL state: 42703
You need to add fields in your tables to make the reference. Something like this :
CREATE TABLE Customer
(
id int primary key,
name varchar(30),
age int,
gender bool
);
CREATE TABLE Minor
(
minor_id serial primary key,
parent_id int,
other_fields text etc.
FOREIGN KEY (parent_id) REFERENCES Customer(id)
);
This is simply the reason why it's not working.
Like this
CREATE TABLE Customer
(
id int primary key,
name varchar(30),
age int,
gender bool
);
CREATE TABLE Minor
(
parent_id int ,
FOREIGN KEY (parent_id) REFERENCES Customer(id)
);
To add an answer without just a code snippet:
You've got the right idea with FOREIGN KEY (parent_id) REFERENCES Customer(id). This bit adds a constraint or a "rule" to your table. This constraint ensures that parent_id holds a real id in your Customer table.
The error message told us that column "parent_id" referenced in foreign key constraint does not exist. The confusion comes, understandably, from mistaking the constraint declaration for a column declaration.
As others have pointed out, we need to both 10 declare the foreign key constraint and 2) declare the column.
CREATE TABLE Customer
(
id int primary key,
name varchar(30),
age int,
gender bool
);
CREATE TABLE Minor
(
parent_id int, -- Declare the column
FOREIGN KEY (parent_id) REFERENCES Customer(id) -- Declare the constraint
);
I have the following queries that I run to create tables in MS SQL Server:
CREATE TABLE menus
(
menu_id int NOT NULL PRIMARY KEY,
menu_name char,
other_details char
)
CREATE TABLE bookings
(
booking_Id int NOT NULL PRIMARY KEY,
date_booked DATE,
date_of_booking DATE,
other_details char,
staff_id int FOREIGN KEY REFERENCES staff(staff_id),
customer_id int FOREIGN KEY REFERENCES customers(customer_id)
)
CREATE TABLE menus_booked
(
menu_id INT NOT NULL,
booking_id INT NOT NULL,
CONSTRAINT PK_menus_booked PRIMARY KEY(menu_id,booking_id),
FOREIGN KEY (menu_id) REFERENCES menus(menu_id),
FOREIGN KEY (booking_id) REFERENCES bookings(booking_id)
)
CREATE TABLE menu_changes
(
change_id int NOT NULL PRIMARY KEY,
menu_id int NOT NULL,
booking_id int NOT NULL,
change_details char,
FOREIGN KEY (menu_id) REFERENCES menus_booked(menu_id),
FOREIGN KEY (booking_id) REFERENCES menus_booked(booking_id)
)
On running the last query I get the error:
There are no primary or candidate keys in the referenced table 'menus_booked' that match the referencing column list in the foreign key 'FK_menu_chan_menu'
I am unsure if my queries are correct and can't resolve this error.
The primary key of menus_booked is a unique combination of menu_id and booking_id. A foreign must point to that combination, not just one of its fields, which is not necessarily unique. Your query currently tries to define two foreign keys, one on each column, instead of one foreign key on the combination of the columns:
CREATE TABLE menu_changes
(
change_id int NOT NULL PRIMARY KEY,
menu_id int NOT NULL,
booking_id int NOT NULL,
change_details char,
FOREIGN KEY (menu_id, booking_id)
REFERENCES menus_booked(menu_id, booking_id) -- Here!
)
A foreign key has to reference a primary key (or unique key but here the PK is the problem), and it has to reference it in it's entirety.
FOREIGN KEY (menu_id) REFERENCES menus_booked(menu_id),
FOREIGN KEY (booking_id) REFERENCES menus_booked(booking_id)
You have two foreign key's referencing part of the primary key of menus_booked. You'll have to alter it to:
FOREIGN KEY (menu_id, booking_id) REFERENCES menus_booked(menu_id, booking_id)