Ans:The capacity to change the schema at one level of a database system without having to change the schema at the next higher level. We can define two types of data independence:

Logical data independence

Physical data independence

Logical data independence:It is the capacity to change the structure of the whole database without having to change the part of thedatabase that a particular user group.some times e may change the structure of database by adding details ,data items and sometimes e reduce like removing details,data items.

Physical data independence:It is the capacity to change the physical storage structure of the database without having to change the structure of the whole database.Changes in the physical storege is needed because physical files had to be reorganized.That means creating additional access structures—to improve the performance of retrieval or update.we should provide new accesspath to retrive the data.

physical data independence exists in most databases and physical details like location of data on disk, and hardware details of storage encoding,placement, compression, splitting, merging of records, and so on are hidden from the user.Where as logical data independence is harder to achieve because it allows structural and constraint changes without affecting application programs.

we have a multiple-level DBMS, its catalog must be expanded to include information on how to map requests and data among the various levels.

The DBMS uses additional software to accomplish these mappings by referring to the mapping information in the catalog. Data independence occurs because when the schema is changed at some level, the schema at the next higher level remains unchanged; only the mapping between the two levels is changed. Hence, application programs referring to the higher-level schema need not be changed.