SQL中的关联引用和跳过的块

Question

我无法理解http://docs.aws.amazon.com/redshift/latest/dg/r_correlated_subqueries.html中的以下短语。

the block containing the correlation reference and the skipped block are connected by a NOT EXISTS predicate

那么以下SQL中的“关联引用”和“跳过块”是什么？

select event.eventname from event
where not exists
(select * from listing
where not exists
(select * from sales where event.eventid=sales.eventid));