sql server alter table add foreign key with name code example

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: 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 3: 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 4: foreign key mssql

CREATE TABLE Sales.TempSalesReason 
   (
      TempID int NOT NULL, Name nvarchar(50)
      , CONSTRAINT PK_TempSales PRIMARY KEY NONCLUSTERED (TempID)
      , CONSTRAINT FK_TempSales_SalesReason FOREIGN KEY (TempID)
        REFERENCES Sales.SalesReason (SalesReasonID)
        ON DELETE CASCADE
        ON UPDATE CASCADE
   )
;