Different Keys in SQL

Types of Keys

Primary Key:

The values in the column should Uniquely define the Record/Tuple in that Table.

ID

STUDENT_NAME

CLASS

MARKS

PHONE

Consider the above Table.Here the column ID can Uniquely Define each student in the system.Hence ID is the Primary Key.

Foreign Keys:

When the values in the column of the Referencing table contains a value of the Primary Key of the Reference Table, that column is known as Foreign Key of the table.

eg.

Consider that the Reference Table is:

Supplier_ID	Name	Phone
1	ABC	7846
2	DEF	7585
3	GHI	7886

And Referencing Table is:

Item_id	Item_name	Supplier
45	pencil	1
76	eraser	1
89	scale	2

Then the Supplier in the Referencing Table is the Foreign Key as it refers to the Supplier_ID in the Reference Table.

Note: Foreign Key can have repeated values as it refers to Primary Key of  reference table.

Candidate Key: It is defined as the minimal number of attributes required to identify a Record/Tuple.

eg.Consider the following table where the Primary key is Reg_no. and exam_seat_no.

Reg_no

Student_name

class

Exam_seat_no

DOB

Now, the Reg_no , Student_name and Eaxm_seat_no can uniquely define a student.

Similarly,  Reg_no and Exam_seat_no can uniquely define a student but it's not minimal .

Only  Reg_no or Exam_seat_No can Uniquely define a student hence Reg_no  and Exam_seat_no separately are Candidate keys.

Super Keys:

In above example, Reg_no and Student_name together or Reg_no and Exam_seat_no together or all the three together are called Super key.

So, Super Key  is basically the  group of columns  which can Uniquely define a Record/Tuple ,and not necessarily be minimal.

Note:

           EVERY CANDIDATE KEY IS A SUPER KEY  BUT A SUPER KEY MAY BE / MAY NOT             BE   A  CANDIDATE KEY.