Export command only provides syntax with basic table,
EXPORT [INTO data-file]
and does not support export query result.
Considering performance, they can create a view to replace that query result.
(1) Create view v2 as select * from t1 inner join t2 on t1.c1 = t2.c3;
(2) export into tmp.csv table v2 description tmp.des;
The view is dynamic and will be executed when be used, so exporting CSV format file will not do additional action, so the performance should be better than creating temporary table method.