hypothetical.sql

CREATE TABLE A (
    id SERIAL NOT NULL PRIMARY KEY
);

CREATE TABLE B (
    id SERIAL NOT NULL PRIMARY KEY
);

CREATE TABLE Parent (
    id SERIAL NOT NULL PRIMARY KEY,
    aId INTEGER NOT NULL REFERENCES A (id),
    bId INTEGER NOT NULL REFERENCES B (id),
    UNIQUE(aId, bId)
);

CREATE TABLE Child (
    parentId INTEGER NOT NULL REFERENCES Parent (id),
    createdOn TIMESTAMP NOT NULL
);
-- Is it possible to create a unique constraint on Child 
-- such that for all rows in Child at most one references
-- a Parent having some value of aId? Stated another way
-- can I created a unique constraint so that the join of the
-- above tables will have no duplicate aId?