Скачать или смотреть Lecture 10: How to Use Primary Key and Foreign key || by Ashish Asthana

Primary Key Used to identify a row uniquely.
Foreign Key Used to ensure Referential Integrity of the data.
UNIQUE v/s PRIMARY KEY
UNIQUE allows NULL values but PRIMARY KEY does not. Multiple columns may have UNIQUE constraints, but there is only one PRIMARY KEY constraints in a table.
Implementing Primary Key Constraints:

Defining Primary Key at Column Level:
CREATE TABLE Student (
StCode char(3) PRIMARY KEY,
Stname char(20) NOT NULL,
.............................);
Defining Primary Key at Table Level:
CREATE TABLE Student (
StCode char(3) NOT NULL,
Stname char(20) NOT NULL,
.............................PRIMARY KEY (StCode));

Foreign key is a non-key attribute of any table whose value is derived from primary key column of another table. Foreign key column can accept only those values which exists in primary key column of related table. A table can have any number of foreign key in a table
• A Foreign key is non-key column in a table whose value is derived from the Primary key of some other table.
• Each time when record is inserted or updated in the table, the other table is referenced. This constraint is also called Referential Integrity Constraints.
• This constraint requires two tables in which Reference table (having Primary key) called Parent table and table having foreign key is called Child table.

----------------------------------------------------------------------------------------
Like, Share and Subscribe to my channel to get video updates

For more information, Please write back to us at [email protected]