IN Clause (SQL)

The comparison operator IN compares a value $v$ with a set (or multi-set) of values $V$ and evaluates to TRUE if $v$ is one of the elements in $V$:

SELECT NAME, ADDRESS
FROM   EMPLOYEE
WHERE  DNO IN (
	SELECT DNUMBER
	FROM   DEPARTMENT
	WHERE  DNAME = 'Research'
)

The nested query selects the number of the ‘Research’ department, the other query selects an employee if its department number is in the result of the nested query.

In this example, the nested query is not correlated with the outer query.