Keys in DBMS

Question

Hi, this is Khalid. I am new in DBMS. I have just started to study. I am having a little bit confusion in the Concept of Keys of DBMS and Attributes. So, I am getting little bit problem to understand the concept of Keys. Somebody, please help me.
Now, i am just going to show a Relation.
Employee EMP_ID EMP_NAME EMP_SALARY
101 Khalid 17000
102 Rajib 16000
103 Rahul 18000
104 Dibakar 17000
105 Akikul 16000
106 Bidyut 16500
107 Pranjal 16500
Somebody is saying that the rows or the tuples within the table are the Keys, whereas somebody's opinion is is the Column Title. So, i am in the confusion that if the column name is the key value, then what is an attribute ? Please explain the very basic concept of every type of Keys. Such as Super Key, Primery Key, Candidate Key, Foreign Key etc.
So, please help me. This is the very first time in any kind of forum. So, I politely request you everybody to give me a proper solution for my problem. Thanks.

Answer 1

Here EMPLOYEE represents a table.
EMP_ID, EMP_NAME, EMP_SALARY are attributes.
In the table mentioned above, EMP_ID is the primary key. Primary key is the key which can uniquely identify the rows of a table. Here u can see that primary key values are unique for each row.
The various types of key with e.g. in SQL are mentioned below, (For examples let suppose we have an Employee Table with attributes ‘ID’ , ‘Name’ ,’Address’ , ‘Department_ID’ ,’Salary’)
(I) Super Key – An attribute or a combination of attribute that is used to identify the records uniquely is known as Super Key. A table can have many Super Keys.
E.g. of Super Key
1 ID
2 ID, Name
3 ID, Address
4 ID, Department_ID
5 ID, Salary
6 Name, Address
7 Name, Address, Department_ID ………… So on as any combination which can identify the records uniquely will be a Super Key.
(II) Candidate Key – It can be defined as minimal Super Key or irreducible Super Key. In other words an attribute or a combination of attribute that identifies the record uniquely but none of its proper subsets can identify the records uniquely.
E.g. of Candidate Key
1 Code
2 Name, Address
For above table we have only two Candidate Keys (i.e. Irreducible Super Key) used to identify the records from the table uniquely. Code Key can identify the record uniquely and similarly combination of Name and Address can identify the record uniquely, but neither Name nor Address can be used to identify the records uniquely as it might be possible that we have two employees with similar name or two employees from the same house.
(III) Primary Key – A Candidate Key that is used by the database designer for unique identification of each row in a table is known as Primary Key. A Primary Key can consist of one or more attributes of a table.
E.g. of Primary Key - Database designer can use one of the Candidate Key as a Primary Key. In this case we have “Code” and “Name, Address” as Candidate Key, we will consider “Code” Key as a Primary Key as the other key is the combination of more than one attribute.
(IV) Foreign Key – A foreign key is an attribute or combination of attribute in one base table that points to the candidate key (generally it is the primary key) of another table. The purpose of the foreign key is to ensure referential integrity of the data i.e. only values that are supposed to appear in the database are permitted.
E.g. of Foreign Key – Let consider we have another table i.e. Department Table with Attributes “Department_ID”, “Department_Name”, “Manager_ID”, ”Location_ID” with Department_ID as an Primary Key. Now the Department_ID attribute of Employee Table (dependent or child table) can be defined as the Foreign Key as it can reference to the Department_ID attribute of the Departments table (the referenced or parent table), a Foreign Key value must match an existing value in the parent table or be NULL.
(V) Composite Key – If we use multiple attributes to create a Primary Key then that Primary Key is called Composite Key (also called a Compound Key or Concatenated Key).
E.g. of Composite Key, if we have used “Name, Address” as a Primary Key then it will be our Composite Key.
(VI) Alternate Key – Alternate Key can be any of the Candidate Keys except for the Primary Key.
E.g. of Alternate Key is “Name, Address” as it is the only other Candidate Key which is not a Primary Key.
(VII) Secondary Key – The attributes that are not even the Super Key but can be still used for identification of records (not unique) are known as Secondary Key.
E.g. of Secondary Key can be Name, Address, Salary, Department_ID etc. as they can identify the records but they might not be unique