Example 1: 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;
Example 2: sql foreign key
# A foreign key is essentially a reference to a primary
# key in another table.
# A Simple table of Users,
CREATE TABLE users(
userId INT NOT NULL,
username VARCHAR(64) NOT NULL,
passwd VARCHAR(32) NOT NULL,
PRIMARY KEY(userId);
);
# Lets add a LEGIT user!
INSERT INTO users VALUES(1000,"Terry","Teabagface$2");
# We will create an order table that holds a reference
# to an order made by our Terry
CREATE TABLE orders(
orderId INT NOT NULL,
orderDescription VARCHAR(255),
ordererId INT NOT NULL,
PRIMARY KEY(orderId),
FOREIGN KEY (ordererId) REFERENCES users(userId)
);
# Now we can add an order from Terry
INSERT INTO orders VALUES(0001,"Goat p0rn Weekly",1000);
# Want to know more about the plight of Goats?
# See the link below
Example 3: 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);
Example 4: alter table add column and foreign key mysql
ALTER TABLE database.table
ADD COLUMN columnname INT DEFAULT(1),
ADD FOREIGN KEY fk_name(fk_column) REFERENCES reftable(refcolumn) ON DELETE CASCADE;
Example 5: sql primary key
A primary key allows each record in a table to be uniquely identified. There can only be one
primary key per table, and you can assign this constraint to any single or combination of columns.
However, this means each value within this column(s) must be unique.
Typically in a table, the primary key is an ID column, and is usually paired with the AUTO_
INCREMENT keyword. This means the value increases automatically as new records are created.
CREATE TABLE users (
id int NOT NULL AUTO_INCREMENT,
first_name varchar(255),
last_name varchar(255) NOT NULL,
address varchar(255),
email varchar(255),
PRIMARY KEY (id)
);
Example 6: 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)
);