Primary Key vs. Foreign Key vs. Unique Key

Primary Key :

• The primary key uniquely identifies each record in the table.
• The primary key cannot have a NULL value. If the column(s) contain NULL values, the system will not add the primary key constraint.
• The primary key cannot have duplicate values.
• A table can have only one Primary Key.

Example : 

In the below STUDENT table "Roll_Number" is the Primary Key of the table. This Roll_Number can be used to uniquely identify the rows of a STUDENT table.

Unique Key : 

• Unique Key is a key which has a unique value and is used to prevent duplicate values in a column.
• Primary key also has a unique value for each column.
• A unique key can have a NULL value which is not allowed in a primary key .
• If we have a value present then either it would be unique or no value would be present at all.
• Main motive of a unique key in a table is to prevent duplicate values.
• A table can have multiple Unique Key.

Example : In the above table, we have Phone_Number as the Unique Key and Roll_Number as the Primary Key. If any student has a Phone_Number then it should be unique. Also, there might be some student who doesn't have any Phone_Number. In the table the Student Scott do not have a Phone Number. 

Note : 

• Primary Key is used to uniquely identify a row but a Unique Key is used to prevent duplicate values in a column.
• A Table can have only one primary key but it can have multiple unique keys. In the above Student Table, we have only one Primary Key i.e Roll_Number but we have two Unique Key i.e Phone_Number and Reg_Number. 

Foreign Key : 

• A foreign key establishes a relationship between tables by referencing the primary key of another table. 
• A table may have multiple foreign keys
• Foreign keys establish relationships between tables, ensuring that data matches up correctly and maintains order.

Primary Key :

• The primary key uniquely identifies each record in the table.
• The primary key cannot have a NULL value. If the column(s) contain NULL values, the system will not add the primary key constraint.
• The primary key cannot have duplicate values.
• A table can have only one Primary Key.

Example : 

In the below STUDENT table "Roll_Number" is the Primary Key of the table. This Roll_Number can be used to uniquely identify the rows of a STUDENT table.

Unique Key : 

• Unique Key is a key which has a unique value and is used to prevent duplicate values in a column.
• Primary key also has a unique value for each column.
• A unique key can have a NULL value which is not allowed in a primary key .
• If we have a value present then either it would be unique or no value would be present at all.
• Main motive of a unique key in a table is to prevent duplicate values.
• A table can have multiple Unique Key.

Example : In the above table, we have Phone_Number as the Unique Key and Roll_Number as the Primary Key. If any student has a Phone_Number then it should be unique. Also, there might be some student who doesn't have any Phone_Number. In the table the Student Scott do not have a Phone Number. 

Note : 

• Primary Key is used to uniquely identify a row but a Unique Key is used to prevent duplicate values in a column.
• A Table can have only one primary key but it can have multiple unique keys. In the above Student Table, we have only one Primary Key i.e Roll_Number but we have two Unique Key i.e Phone_Number and Reg_Number. 

Foreign Key : 

• A foreign key establishes a relationship between tables by referencing the primary key of another table. 
• A table may have multiple foreign keys
• Foreign keys establish relationships between tables, ensuring that data matches up correctly and maintains order.