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They are Oracle data types
those who knoww any language like C and java then it is easy to understand the concept of Varray, as it is related to ARRAY structure of C or java language.
where as Nested Table concept is related to Struct data type of C or java langauge.
Both are type of collections in Oracle. In Varray because its not stored in database columns so no DML operation can be implemented but on other hand as Nested tables we can perform DML operations
Varrays are generally dense , that mean you cannot delete elements of the varray.But the elements of the Nested Table are generally sparse in nature and hence the elements can be deleted in between using the delete function. Varrays have fixed length (length declared during the declaration time), while Nested table can have unlimited length.