带别名的SQL嵌套查询

Question

Can anyone explain how this query executes and is it a Correlated Nested Query?

select distinct id 
from activity ac3 
where ac3.activity_id = 151 and ac3.id in
    (select ac2.id from activity ac2 where ac2.activity_id = 150)

Answer 1

It is not correlated subquery. There is no link between outer query and nested query. See example of correlated subquery here .

Answer 2

Nested subquery can be nested inside other sub queries, it is a select statement that is nested within another select statement and will return intermediate results. SQL executes innermost sub query first, and then the next level. The results of the sub query are the query conditions of the primary query.

Query Execution goes as primary query -> sub query -> sub sub query and so on.

Correlated sub query , the sub query uses values from the outer query. The sub query is evaluated once for each row processed by the outer query. This means that the sub query is executed repeatedly, once for each row that might be selected by the outer query.

This is not correlated sub query.

Answer 3

This is not a Correlated query. If sub query is dependent on result of outermost query then it is a Correlated query. You need to read about it more.

This is a simple sub query.

Execution plan : select ac2.id from activity ac2 where ac2.activity_id = 150;

This Query will execute first and main query is dependent on this one.