primary and foreign key in sql

-- 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;

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);

Posted by: Guest on April-25-2020

CREATE TABLE Orders ( OrderID int NOT NULL, OrderNumber int NOT NULL, PersonID int, PRIMARY KEY (OrderID), CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID) REFERENCES Persons(PersonID) );

Foreign Key: It is a column that comes from a different table and using Foreign key tables are related each other It is the primary key of another table It can be duplicate or null for another table Primary Key : It is unique column in every table in a database It can ONLY accept; - nonduplicate values - cannot be NULL Unique Key: Only unique value and also can contain NULL

CREATE TABLE Students ( /* Create table with foreign key - Way 1 */ ID INT NOT NULL Name VARCHAR(255) LibraryID INT PRIMARY KEY (ID) FOREIGN KEY (Library_ID) REFERENCES Library(LibraryID) ); CREATE TABLE Students ( /* Create table with foreign key - Way 2 */ ID INT NOT NULL PRIMARY KEY Name VARCHAR(255) LibraryID INT FOREIGN KEY (Library_ID) REFERENCES Library(LibraryID) ); ALTER TABLE Students /* Add a new foreign key */ ADD FOREIGN KEY (LibraryID) REFERENCES Library (LibraryID);

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