If you have LowCardinality(String) column with clickhouse lowcardinality optimization, inserting a NULL value will work fine. It will just treat it like you just have inserted an empty string (''). Let's see an example with clickhouse nullable columns.
Let’s insert some values now.
And now let’s see what has been inserted. As you can see, even though we inserted a NULL value, internally it’s saved as an empty string.
Yes, you can.
Using LowCardinality(Nullable(String)) instead of Nullable(String), apart of the benefits that it can bring LowCardinality if the conditions are right, also allows you to order by that column. See an example here: