create table sql
# Simple table describing a vehicle
CREATE TABLE vehicle(
# vehicleId: Unique ID for Primary Key.
# This is how we will reference a record
vehicleId INT NOT NULL,
make VARCHAR(64), # String 64 chars max
model VARCHAR(128),
derivative VARCHAR(255),
PRIMARY KEY(vehicleId)
);
# Add a record
INSERT INTO vehicle VALUES(1000,'Volkswagen','Golf','1.5 TSI EVO Match Edition 5dr');mysql add foreign key
-- On Create
CREATE TABLE tableName (
ID INT,
SomeEntityID INT,
PRIMARY KEY (ID),
FOREIGN KEY (SomeEntityID)
REFERENCES SomeEntityTable(ID)
ON DELETE CASCADE
);
-- On Alter, if the column already exists but has no FK
ALTER TABLE
tableName
ADD
FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;
-- Add FK with a specific name
-- On Alter, if the column already exists but has no FK
ALTER TABLE
tableName
ADD CONSTRAINT fk_name
FOREIGN KEY (SomeEntityID) REFERENCES SomeEntityTable(ID) ON DELETE CASCADE;add constraint fk
ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);foreign key in sql
A FOREIGN KEY is a key used to link two tables together.
A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.
The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.
Example:
# creating table users
CREATE TABLE users(
user_id INT NOT NULL,
user_name VARCHAR(64) NOT NULL,
user_pass VARCHAR(32) NOT NULL,
PRIMARY KEY(user_id);
);
# adding user data
INSERT INTO users VALUES(1,"Raj","raj@123");
# creating table orders
CREATE TABLE orders(
order_id INT NOT NULL,
order_description VARCHAR(255),
orderer_id INT NOT NULL,
PRIMARY KEY(order_id),
FOREIGN KEY (orderer_id) REFERENCES users(user_id)
);
# adding order data
INSERT INTO orders VALUES(1,"Daily groceries",1);sql foreign key
CREATE TABLE orders (
id int NOT NULL,
user_id int,
product_id int,
PRIMARY KEY (id),
FOREIGN KEY (user_id) REFERENCES users(id),
FOREIGN KEY (product_id) REFERENCES products(id)
);sql foreign key
create table Jobs(
job_id number not null,
job_title varchar(30),
min_salary number,
max_salary number
);
create table job_history(
employee_id number not null,
start_date date,
end_date date,
job_id number not null,
department_id number
);
alter table jobs add constraint pk_jobs primary key(job_id);
alter table job_history add constraint fk_job foreign key(job_id) references jobs(job_id);Copyright © 2021 Codeinu
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