Column names in a statement are implicitly qualified by the table referenced in the from clause at the same level. In the following example, the table name publishers implicitly qualifies the pub_id column in the where clause of the outer query. The reference to pub_id in the select list of the subquery is qualified by the subquery’s from clause—that is, by the titles table:
select pub_name from publishers where pub_id in (select pub_id from titles where type = "business")
This is what the query looks like with the implicit assumptions spelled out:
select pub_name from publishers where publishers.pub_id in (select titles.pub_id from titles where type = "business")
It is never wrong to state the table name explicitly, and you can override implicit assumptions about table names by using explicit qualifications.
