Classic SQL constructs not supported yet

[NOT] [EXISTS|INTERSECT|EXCEPT]

A syntactically available alternative is EXISTS, but it's not very useful as it doesn't support correlated subqueries. You can also rewrite it using JOIN.

UNION

An available alternative is SELECT DISTINCT by explicitly listed columns + UNION ALL with subqueries (if needed).

NATURAL JOIN

An alternative is to explicitly list the matching columns on both sides.

NOW() / CURRENT_TIME()

An alternative is to use the functions CurrentUtcDate, CurrentUtcDatetime and CurrentUtcTimestamp.