Primary Key vs. Foreign Key vs. Unique Key

primary key : evry row must have unique value. so we can identify row easily. so nulls cant be accepted and duplicate values also not allowed. 

Foreign Key : easily represents the relation b/w tables. 

Unique Key: mainly values has unique but null values allowed and duplicate values not allowed. eg: bank ac number or debit card numbers or vehicle number plates etc.. 

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.

Hi [mention:6be660007d1541aea0083f7ada8d1339:e9ed411860ed4f2ba0265705b8793d05]  

Primary Key: A primary key uniquely identifies each record in a table. Each value in the primary key column must be unique. primary key cannot contain NULL values. Every record must have a value for the primary key. The primary key is defined within a single table.

Foreign Key:  A foreign key is a column or set of columns in one table that refers to the primary key in another table. It establishes a relationship between the two tables. Foreign keys can contain NULL values, depending on the design of the database. The foreign key is defined in a child table and references the primary key of a parent table.