Different database key types definition, concepts, features with examples. Basing on the key we can find, identify, and connect the database record. 


Read about the database relationships.


Join_data
Employee_ID
Join_Date
Room_no
3941
20.04.18
204
3940
11.04.18
205
3943
28.05.18
206

Here in the element of the first column second row, we saw data 3941. We failed to understand what is meaning by this data. But if we see the header of this column there is a data name Employee_ID. 
From this heading data, we can understand what the meaning of the data is in the whole column. This is Key.

Database keys types

Below we have discussed the Database key types with simple examples.
Key is basically 3 types-

  1. Primary key
  2. Composite key
  3. Foreign key

Primary key

Employee_Info
Employee
_Name
Employee
_ID
Interview
_date
Join
_Date
Room
_no
Rahman
3941
10.04.18
20.04.18
202
Khan
3940
01.04.18
11.04.18
203
Rahman
3943
18.05.18
28.05.18
203
Jordan
3942
30.04.18
9.05.18
202
Stuart
3944
19.03.18
29.03.18
202
                                      
Here each person cannot be uniquely identified by Employee_Name, Interview_date, Join_Date, and Room_no.

But it can be easily identified by Employee_ID. It works as a unique identifier.

Composite Key

Sometime we will fail to identify data by using only a single field. In those conditions, we will use two or more fields for identifying data. This is called a composite key.

Employee_Info

Employee
_Name
Employee
_ID
Section
Interview
_date
Join
_Date
Room
_no
Rahman
3941
Building
10.04.18
20.04.18
202
Khan
3940
Building
01.04.18
11.04.18
203
Rahman
3941
Updating
10.04.18
20.04.18
202
Khan
3940
Updating
01.04.18
11.04.18
203

In this example, we are seeing that we failed to identify uniquely by Employee_ID. In this case, we have to use another key for identity. We can do one thing. There is a key in the table “Section”. Here we will make a composite key by using two keys “Employee_ID” and “Section”. After that, we can identify easily.

Foreign key

If we use a primary key of a table in another table as a normal key then it will be called a foreign key.

Join_data
Employee_ID
Join_Date
Room_no
3941
20.04.18
204
3940
11.04.18
205
3943
28.05.18
206

Interview_data
Interview_date
Employee_ID
10.04.18
3941
01.04.10
3940
18.05.18
3943

Employee_ID key is the primary key of the Join_data table but in the Interview_data table is a normal key.

What is the difference between primary key and foreign key?

  
Some difference between Database key types ( Primary key and foreign key) is given below.

Primary Key
Foreign Key
By using which field each of the records can be identified individually that is called the primary key.
If the primary key of one table is used as a normal key of another table is called a foreign key.
Used for data identification.
Used for creating a relationship between two tables.
It is the parent.
It is a child.
In a table, there will be a single primary key only.
In a table, multiple foreign keys can be saved.