>I have a parent table with multiple child tables.
>Each child row has a foreign key that points to one row in the parent.
>Each parent row must have a child row pointing to it.
>Each parent row can have only child from any of the child tables pointing to it.
>The child is created before the parent.
>
>A unique key in each child table would prevent two rows from one table from pointing to the same parent row, but that would >duplicate the FK index. I don't know if that will cause Firebird problems.
>
>I don't know how to enforce the rule that the parent must have a child, as the FK can't be populated until the parent exists,
>and the parent isn't valid unless a child is pointing to it. Right now we just ignore that requirement, and so far no one
>has inserted a childless parent accidentally.
>
>We've also held back on the third requirement of each parent having only one child, but it looks like each child table would
>have to manage that rather than the parent (which would be preferable).

A few of your requirements are quite simple to solve, Rick.

Let's take an example:

MASTER
PK_M
...

CHILD_A
PK_CA
PK_M
...

CHILD_B
PK_CB
PK_M
...

...

Add another table:

ALL_CHILDREN
PK_M PRIMARY KEY

triggers:

CREATE TRIGGER CHILD_A_AI FOR CHILD_A ACTIVE AFTER INSERT AS
BEGIN
INSERT INTO ALL_CHILDREN(PK_M) VALUES (NEW.PK_M);
END

CREATE TRIGGER CHILD_B_AI FOR CHILD_B ACTIVE AFTER INSERT AS
BEGIN
INSERT INTO ALL_CHILDREN(PK_M) VALUES (NEW.PK_M);
END

CREATE TRIGGER CHILD_A_AD FOR CHILD_A ACTIVE AFTER DELETE AS
BEGIN
DELETE FROM ALL_CHILDREN AC WHERE AC.PK_M = OLD.PK_M;
END

CREATE TRIGGER CHILD_B_AD FOR CHILD_B ACTIVE AFTER DELETE AS
BEGIN
DELETE FROM ALL_CHILDREN AC WHERE AC.PK_M = OLD.PK_M;
END

and a FOREIGN KEY:

ALTER TABLE MASTER ADD CONSTRAINT FK_ALL_CHILDREN
FOREIGN KEY REFERENCES ALL_CHILDREN(PK_M)...

This will prevent the same master from being added to several children (provided they all have the trigger) and prevent a MASTER from being added unless there are children. What isn't prevented is addition of parentless children, but you didn't mention that.

HTH,
Set