SUBQUERY:
A query may contain more than one sub query. The inner most query (bottem) will get executed first, next the higher,so on and at last the first query (top) will be execuited. A non-correlated subquery is the one in which each query is independent of eachother.
SELECT EMP_NAME, DEPTNO FROM EMP
WHERE EMP_NAME IN (SELECT EMP_NAME FROM DEPT)
CORRELATED SUBQUERY:
Here for each row of the outer table the inner table will be evaluated and the result if the inner table will be sent one row at a time. That means, there exists corelation between the two tables.
See the below example:
SELECT EMP_NAME, DEPTNO FROM EMP
WHERE EMP_NAME IN (SELECT EMP_NAME FROM DEPT WHERE EMP.DEPTNO = DEPT.DEPTNO)
