Operations on Files

When working with files in a DBMS, two fundamental types of operations are performed: retrieval (finding and reading records) and update (inserting, deleting, or modifying records). Both use selection conditions to identify target records.

Selection Conditions

A selection condition specifies criteria that records must meet. Conditions can be

• Simple Uses one field (e.g., Ssn = '123-45-6789')
• Complex Combines multiple fields with comparison operators (e.g., Department = 'Sales' AND Salary ≥ 30000)

File Operations

The typical sequence of file operations

1. Open file
2. Find first record matching selection condition
3. Read / Update / Delete the record
4. Find next matching record (repeat as needed)
5. Insert new records (if required)
6. Close file

Operations can be record-at-a-time (process one record) or set-at-a-time (find all matching records at once). Periodic reorganization may be needed to maintain efficiency.

File Organization vs Access Methods

File organization is how data is physically stored and structured. Access methods are the techniques used to find and manipulate data within that structure. Different access methods can work on the same organization, but some methods require specific organizations.

File Organization Methods

The choice of organization depends on the most frequent operations. For example, if employees are usually looked up by SSN, the file should be organized for fast SSN-based access. If another application groups employees by department, a different organization may be needed. Often, a balance must be found between competing access patterns.

Conclusion

File operations in DBMS involve retrieval and update using selection conditions. The choice of file organization (heap, ordered, hashed, indexed) and access method determines how efficiently records can be found and manipulated. Balancing the most frequent operations against organizational constraints is key to efficient file management.