drop foreign key
ALTER TABLE table_name
DROP CONSTRAINT fk_name;drop foreign key
ALTER TABLE table_name
DROP CONSTRAINT fk_name;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;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);foreign key constraint in ms sql
/*Creating Foreign Key Constraint*/
ALTER TABLE ForeignKeyTable ADD CONSTRAINT ForeignKeyTable_foreignKeyColumn_FK
FOREIGN KEY (foreignKeyColumn) REFERENCES PrimaryKeyTable (PrimaryKeyColumn)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 server foreign key
CREATE TABLE Orders (
OrderID int NOT NULL PRIMARY KEY,
OrderNumber int NOT NULL,
PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
);Copyright © 2021 Codeinu
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