Hello Hema,
if u r trying to update the standard tables using Insert query, and if the data is inconsistent then system will not allow you. This is because the while creating standard tables SAP has taken care that if and only if the corresponding record exists in the check table, then only allow the insertion.
this check is applied at application level using foreign key relationship. Checks at Database level is the second thing. See the figure.
But If you are maintaining inconsistent data in your own customized tables (Z-tables) and if you have not maintained the foreign key checks then system will allow you to insert the data. You can make this check required for system to check before inserting the data into the table.
Regards,
Nikhil G.