I want to know the difference between a clustered Index and a non-clustered index
Let's make it little simpler, Clustered Index is Primary Index and Non-clustered Index is Secondary Index.
Now suppose an index "EMP_IDX" has been defined on a column say "DEPT" on a TABLE "EMP_TBL", so in case of a clustered index all the values for the same "DEPT" would be stored in the same block/area of database memory else its a Non-clustered index.
And also why we need a clustered Index.
Now how do you expect for a Select query to behave where you want to have all EMP_NAME for any specific "DEPT".