Difference between primary key and foreign key

Here I'll explain all these with a few examples: 1. Primary Key: Lets take an example of a class, there could be more than one student with the same name, however, no one can have the same roll number in a class. That's why you can identify a student with his/her roll number because that's unique and a must have attribute of the student. If you have to create a student table that will have his/her, name, subjects, age, roll number etc stored. In that case, every row of that table can only be uniquely identified by checking the value of the roll number column. This type of column in any table is called Primary key. Because, this will help you to identify any row in the table. 2. Foreign Key: Lets take an example of an engineer who is working in a foreign country and his family lives in India and he's the only earning member in his family. If you have to get the total earning of that family, you can get it by checking the database of the company on the basis of the employee id of this foreign family member of this family. Explanation: Let's say we've two tables: 1. Family table with member name, employed, age, gender, address, employee id etc 2. Employee table with employee name, employee id, age, gender, emp_address etc. Now if you see here, there is a common column in these two tables, it's employee id. The employee_id can be NULL in the Family table for non-employed members, that's why it can't be the primary key here, however, in one family you will always have unique names of the members and that's why it'll be the primary key. (Read the definition of Primary key) However, the Employee ID column in the employee table will certainly be the primary key. To get the earning of the family, we need to have the employee id as a bridge so that we can link the two tables and fetch the data from both the tables. Therefore in output we can have the columns from both the tables and it's possible only because of the employee id table. Employee_ID column in Family table is a foreign key. (Read Definition of foreign key)

A primary key is a key that "resides" in the entity itself, a foreign key is the primary key of another entity that is placed in this other entity -kind of referenced in it, indicating a relationship. Example of a 1:1 is a man can only have 1 wife and a woman can only have 1 husband. (1:n), a mother can have many children, while a child can only have one mother...Hope it helps.