Constraints

NOT NULL CONSTRAINT -  Ensures that a column cannot have a null value.

DEFAULT CONSTRAINT -   Provides a default value for a column when none is specified 

UNIQUE CONSTRAINT -  Ensures that all values in columns are different 

CHECK CONSTRAINT -  Makes sure that all values in a column satisfy certain criteria 

PRIMARY KEY CONSTRAINT -  Used to uniquely identify a row in the table 

FOREIGN KEY CONSTRAINT -  Used to ensure referential integrity of the data 

Primary Key - is used uniquely to identify each row in a table . It can consist of one or more columns on a table . When Multiple columns are used on a table it is called composite key. 

Foreign Key - Foreign key is a column or columns that references a column most often primary key of another table . The purpose of foreign key is to maintain referential integrity of the data.

Pg admin 

Data base - training - right click on training - query click 

Always add semi colen to run the query 

Int - integer 

varchar - variable character