1. Agenda
Test next Week!
SI or no SI?
File Update Techniques – Review

2. File Access Techniques

3. Sequential Access Reads one record at a time
Reads records in keyed sequence or arrival sequence

4. Random Access
Records are processed in some other order than the one in which they were physically written to the disk or indexed.
A key field is looked up in an index, the address is retrieved and the record is accessed from the physical file using the address.

5. Dynamic Access
Defined when a file will be used both randomly and sequentially

6. Due to Normalization Practices, we usually have more than one file in our databases.

7. Employee Database Employeepf Provinces K Emp No K Prov Cde Prov Cde
Foreign Key
Empnotpf
K Emp No
K Note No

8. Defining a Sequential Access File
SELECT Employee-File
ASSIGN to DATABASE-EMPPF
ORGANIZATION is INDEXED
ACCESS MODE is SEQUENTIAL
RECORD KEY is
EXTERNALLY-DESCRIBED-KEY.

9. Defining a Random Access File
SELECT Employee-File
ASSIGN to DATABASE-EMPPF
ORGANIZATION is INDEXED
ACCESS MODE is RANDOM
RECORD KEY is
EXTERNALLY-DESCRIBED-KEY
(with duplicates).

10. Defining a Dynamic Access File
SELECT Employee-File
ASSIGN to DATABASE-EMPPF
ORGANIZATION is INDEXED
ACCESS MODE is DYNAMIC
RECORD KEY is
EXTERNALLY-DESCRIBED-KEY
(with duplicates).

11. Beware of File Status Code 95!
Need to add ‘WITH DUPLICATES’ to your select statement or your need to take it out!

12. Randomly Retrieve a Record using a Random Defined File
Update the Key Fields in the File.
READ file-name
INVALID KEY
Perform invalid-logic
NOT INVALID KEY
Perform valid-logic
END-READ.

13. Sequential Read using a Random Defined File?

14. Sequentially Read using a Random Defined Record
Make sure that ACCESS is DYNAMIC!!
READ file-name NEXT RECORD
AT END
Perform perform end-of-file-logic
END-READ.

15. Positioning the File Pointer
Used only with Sequentially defined files.
Initialize the record key
START file-name
Key Condition
INVALID KEY
Perform Invalid-Logic
NOT INVALID KEY
Perform valid-logic
END-START.

16. Other Random Access Verbs

17. REWRITE Updates a record in a file
File must be opened as I-O instead of input
Record must be read first before rewrite
REWRITE record-name (FROM variable-name)
INVALID KEY perform error-rtn
NOT INVALID KEY perform continue-rtn
END-REWRITE.

18. DELETE Deletes a record from a file File must be opened as I-O
The record must be read first
DELETE file-name RECORD
INVALID KEY perform error-rtn
NOT INVALID KEY perform continue-rtn
END-DELETE.

19. WRITE Writes records to a file File must be opened as I-O or OUTPUT
WRITE record-name (FROM variable)
INVALID KEY perform error-rtn
NOT INVALID KEY perform continue-rtn
END-WRITE.

20. CPCH13B