Check out example codes for "how to remove foreign key constraint in sql". It will help you in understanding the concepts better.

Code Example 1

ALTER TABLE table_name
DROP CONSTRAINT fk_name;

Code Example 2

USE AdventureWorks2012;  
GO  
ALTER TABLE dbo.DocExe   
DROP CONSTRAINT FK_Column_B;   
GO

Code Example 3

ALTER TABLE Orders

ADD CONSTRAINT FK_PersonOrder

FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID);

Learn ReactJs, React Native from akashmittal.com