constraint or trigger ?

I want to attach the following kind of constraint to a table which contains 2 external foreign keys 'ext1ID' & 'ext2ID': "There can be at most TWO lines referencing these two keys"

• At first, I wanted to create a BeforeInsert trigger, but I dont' know how to make cancel the INSERT if the condition is not checked: will an exception be enough, or even if an exception occurs in a trigger, is the corresponding request however performed ?
• The other solution would be to create a function which calculates (select count(*)) the number of lines that already have these two keys, and attach a constraint to MyTable which uses this function: Alter table MyTable add constraint MyConstraint check (f(x,y)<2);

I have the Oracle error ORA-00904, which let me think I cant call a function in a constraint...

I would like to avoid the third solution which consists in adding the following column to MyTable:
checking NUMBER, UNIQUE, NOT NULL, IN (1,2);

Could somebody help me ?

--
Serge LUCAS
STERIA ATM/Eurocontrol
BS-012
Tel 01 69 88 74 15
Fax 01 69 88 73 33
Mel lua_at_eurocontrol.fr Received on Tue Mar 30 1999 - 02:55:21 CST