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constraints :  conditions defined on table columns.

                                      ex: course_dur should be either 180 or 240
                                      ex: student age should be minimum 18

Types of constraints:

1. Null    : column can accept null values
Not null : column cannot accept null values

 2. unique : column does not accept duplicates but can accept upto 1 null values

 3. primary key  : column does not accept duplicates and cannot accept null 


4. foreign key : column accept values based on a different column in same/different table for table to table communication (references)

 ex: assume a courses table with few courses. students table should reference above courses table staff table should reference above courses table

5. check constraint : used to specify conditions on column values
 ex: student gender should be either 'm' or 'f'

6. default constraint : used to specify a default value.
                            applicable for inserts when we do not speficy column name & value

7. indentity property : used to generate a sequence of integer values  *

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