The developers have already discussed (internally) adding these features
for 3.33, using the PostgreSQL syntax.
The way you do this now is:
~~~~
REPLACE INTO TableA SELECT ... FROM ...;
~~~~
See for example the Fossil code at
<https://fossil-scm.org/fossil/artifact/414c5b8e0150?ln=983-990>.
This code seems like it would be easier to read and understand if
it were written like this:
~~~~~
UPDATE description
SET summary=CASE WHEN plink.isprim THEN ''
ELSE 'merge '
END || 'parent of checkin',
ref=blob.uuid
FROM plink, blob
WHERE description.summary='unknown'
AND plink.cid=description.rid
AND blob.rid=plink.pid;
~~~~~