deleting referenced data

Hi,

I have The following situation:

CREATE TABLE data1 (
    id SERIAL PRIMARY KEY,
    data INTEGER
);

CREATE TABLE data2 (
    id SERIAL PRIMARY KEY,
    data INTEGER
);

CREATE TABLE data3 (
    id SERIAL PRIMARY KEY,
    data INTEGER
);

CREATE TABLE relations (
    id SERIAL PRIMARY KEY,
    data1 INTEGER REFERENCES data1,
    data2 INTEGER REFERENCES data2,
    data3 INTEGER REFERENCES data3
);

When I delete a record of the table realtions I also want to delete the
record on the data-tables if there is no other record referencing them.
How can I do this?

Regards

Andreas Fromm