To define the relationship between two tables (one is called parent and the other one is the child table) connected by columns, a foreign key constraint is used. In this constraint the values of the child table must appear in the parent table, which means that for a foreign key, one table should point to a Primary Key in another table. A table can have multiple foreign keys and each foreign key can have a different referenced table.
Example: To understand the foreign key clearly let's assume the following two tables:
CUSTOMER {Cust_ID, Cust_Name, Age, ContactNo, Gender, Address} VENDOR {Vend_ID, Vend_Name, Cust_ID}
Example: Foreign Key Constraint while using CREATE TABLE statement.
Syntax: CREATE TABLE table_name(Col1 datatype NOT NULL, Col2 datatype NOT NULL, Col3 datatype NOT NULL, CONSTRAINT FK_Column FOREIGN KEY(Col1, Col2, Col3) REFERENCES parent _table(Col1, Col2, Col3) );
AT SINGLE COLUMN LEVEL
