--- In firebird-support@yahoogroups.com, "Maurizio P." <mauriz_po@...>
wrote:

>
> Thanks for Answers
> now i know more about foreign keys ,
> i have a question refering to the two tables of my previous message

(tBanks , tCustomers) :

> i want the user cannot delete a tBanks record if it exists in the

tCustomers table .

>
> No other constraints , e.g. a tCustomer record can be not connected

to any tBanks record .

> ...using foreign keys , i can't leave a tCustomer record without a

tBanks record connected .

OK, you want to define a foreign key on tCustomers.tbanksid
referencing tbanks.tbanksid with ON DELETE NO ACTION

This will achieve two things. Firstly, any tbanksid value that goes
into the tCustomer record must be a valid tbanks record. Secondly, any
attempt to delete a tbanks record will fail if there exists a record
in tCustomers that references that tbank.

The only thing you need to now do is to enforce some value exists, and
a simple not null constraint does this effectively.

Adam