If i defined a database schema where some field cannot contain a NULL field
But i enter a NULL value, would the validation occur on the database software?
If that database server where run on a different machine, would be sent through the network before receiving an error response?
is this what the mean by Database validations and application validations?
Where application validations are enforced before data transmission?
It is better to have the database ensure data integrity.  After all, your application layer is not be the only way to change data -- someone could run explicit INSERT and UPDATE statements within the database.
In addition, the optimizers in some databases can make use of NOT NULL constraints for query optimization.
Difference between applcation and database validations