What is the difference between related subquery and unrelated subquery in database

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Subqueries can be divided into related subqueries and non-related subqueries, and the execution order and relationship between them and the outer query are different.

Associated subquery:

In the associated subquery, the internal query is executed once for each row of data returned by the external query. In addition, the information flow of the associated subquery is two-way, each row of data from the external query passes a value to the subquery, and then the subquery executes for each row of data once and returns its records, and then the external query makes a decision based on the returned records.

Select * from dept d where exists (select * from emp e where e.deptno = d.deptno)

Features:

1. Execute the outer query first

2. Execute the inner layer query again

Unrelated subquery:

An unrelated subquery is a subquery independent of the external query, and the value is passed to the external query after the subquery is executed.

Select * from emp where sal = (select max (sal) from emp)

Features:

1. Execute the inner layer query first

2. Execute the outer query again

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