add constraint fk
ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);add constraint fk
ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);foreign key
CREATE TABLE Employee(
EmployeeID int NOT NULL,
LastName varchar(50) NOT NULL,
FirstName varchar(20) NOT NULL,
Age int,
DeptNo int,
PRIMARY KEY (EmployeeID),
FOREIGN KEY (DeptNo) REFERENCES Department(DeptNo)
);foreign key
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 NULLforeign key constraint
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);What is foreign key?
Foreign Key is a non-key attribute which is derived from the primary key
of another table which links those tables together.Copyright © 2021 Codeinu
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