ydb/yql/essentials/docs/en/syntax/not_yet_supported.md

Classic SQL constructs not supported yet

[NOT] [EXISTS|INTERSECT|EXCEPT] {#not-exists}

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.

NATURAL JOIN {#natural-join}

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

NOW() / CURRENT_TIME() {#now}

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