ppose that we have a table describing test outcomes.  We wish
    to ensure that there is only one <span class="quote">“<span class="quote">successful</span>”</span> entry for
    a given subject and target combination, but there might be any number of
    <span class="quote">“<span class="quote">unsuccessful</span>”</span> entries.  Here is one way to do it:
</p><pre class="programlisting">
CREATE TABLE tests (
    subject text,
    target text,
    success boolean,
    ...
);

CREATE UNIQUE INDEX 