9. Presence Check
There might be an important piece of data that you want to make sure is always stored.
For example, a school will always want to know an emergency contact number, a video rental store might always want to know a customer's address, a wedding dress shop might always want a record of the brides wedding date.
A presence check makes sure that a critical field cannot be left blank, it must be filled in. If someone tries to leave the field blank then an error message will appear and you won't be able to progress to another record or save any other data which you have entered.
The image above shows a typical online data entry form. The asterisk to the left of the field usually means that you must fill it in, whereas the absence of the asterisk in the mobile field indicates that this is optional.
challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Database data integrity, referential integrity