What is SQL ? (01)

 1> What is Data? 
     Data is a raw fact(which cannot be proved) which provides information about attributes of an entity. 

Entity:-  It is nothing but object.
          Ex:- Human Being, Mobile, Employee, student.

Attributes:-  Human Being - Name, age, height, weight, Aadhaar No, PAN, Email, PHONE.
              Building - name, location, no_of_floors, pin code, owner, No_Security, Office No.


2> What is DATABASE? 
   It is a place where we store our data in organized and systematic manner for easy data retrieval(Fetching the       Data). 


3> What is DBMS? 
   - Database Management System Software.
   - It is a software which is used to interact with the data present in database for data manipulation
      using the Query Language. 

DISADVANTAGES:- 1. Data is stored in flat files (.txt). 
                2. Security & Authorization.
                3. We can store the Duplicates (Redundancy).
                4. Identification of redundancy difficult. 
                5. Removing the Redundancy (Normalization) is not possible. 

4> What is RDBMS? 
   - It is a software which is used to interact with the data present in database for data manipulation
      using the Query Language and RELATIONAL MODEL.

            DBMS
         + Relational Model
         ---------------------
           R DBMS

 

5> What are rules of Relational Model ? 
   1. Data is stored in Tables.
   2. Data can be stored in multiple tables in a database/ A database can have multiple tables in it. 
   3. Data should be atomic in nature / We should store the data in singular form
        Each cell should have a single data only. 
   4. The relation between 2 tables is established using Relational Model. 


6> What is Primary Key? 
   - It is a key attribute(Any Attribute which don't have duplicates & null values in it) which is used to        identify the records from the table uniquely. 

  Characteristics of Primary key:- 
    1. It doesn't accept duplicates.
    2. It doesn't accept null values. 
    3. It is a combination of unique and not null constraint. 
    4. We can have only 1 primary key in a table.
    5. It is not mandatory, but it is recommended. 


7> What is Foreign Key? 
   - It is an attribute which is used to establish the relationship between 2 tables. 


Characteristics of Foreign key:- 
    1. It will accept duplicates.
    2. It can accept null values. 
    3. We can have any no. of foreign keys in a table.  (if 4 tables, then foreign key 3)
    4. The Table from which the key is taken is called Parent Table. 
    5. The table where it is used is called as Child Table. 
    6. We can have only that data in foreign key which is present in its primary key in  its own table.
    7. It should be a Primary key in its own table.