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Sequential Files

Sequential Files. Chapter 13. Master Files. Set of files used to store companies data in areas like payroll, inventory Usually processed by batch processing Typically stored on magnetic disk Disks can store billions of characters Disk drives read, write data quickly

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Sequential Files

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  1. Sequential Files Chapter 13

  2. Master Files • Set of files used to store companies data in areas like payroll, inventory • Usually processed by batch processing • Typically stored on magnetic disk • Disks can store billions of characters • Disk drives read, write data quickly • Disk records can be any size

  3. Sequential Files • Records always read in sequence • Read first record, process it, then read second record, process it and so on • Payroll records in order by employee number may be processed in sequence for updating or printing reports

  4. Sequential Files • May be sorted into any sequence using any field in record • To distribute checks more easily, records may be sorted by department • Sequential processing then used to print checks, reading first record, then second, etc.

  5. Master File Procedures Typical procedures for sequential processing in batch mode are: • Designing Master File • Creating Master File • Creating Transaction File • Updating Master File • Reporting from Master File

  6. Designing a Master File • Place key fields that uniquely identify record at beginning of record • If possible, choose numeric key fields • Secondary fields after primary key fields • Remaining fields appear in order of importance

  7. Designing a Master File • Choose field size large enough to accommodate data stored in it • Use coded fields where possible to save space • Be sure all date fields include four-digit year

  8. Creating a Master File • Original master file data entered interactively, stored on disk file • Ensure data integrity by using data validation techniques to minimize risk or errors • Control listing or audit trail produced to show data stored in new master file and control totals

  9. Creating a Transaction File • Changes to master file made with separate procedure • Change records stored in file called transaction file

  10. Updating a Master File • Updating is process of making master file current • Update master file by incorporating changes from transaction records

  11. Reporting from a Master File • Scheduled reports prepared on regular basis from data stored in master file • Sales reports, customer bills, checks, etc. • Use detail, exception and group printing techniques • On demand reports produced as need arises • May be in report form or displayed on screen

  12. Sequential File Updating Two input files • Input Master File (Old-Master) • Current through previous updating period • Does not contain changes since previous update • Input Transaction File (Trans-File) • Contains changes since previous update to be applied to Old-Master

  13. Sequential File Updating Two output files • Output Master File (New-Master) • Integrates data from Old-Master with all of changes from Trans-File • Will become Old-Master for next update • Control Listing or Audit Trail • Print file showing changes made to master file, errors during processing and control totals

  14. Sequential File Update Master File Transaction File Sequential Master File Update Updated Master File Update Activity Report

  15. Ordering of Records • Records in Old-Master and Trans-File must be in order by same key field • Compare key fields to determine if given master record is to be updated

  16. Master File 1223 ……….. 1277 ……….. 1456 ……….. 2835 ……….. 2910 ……….. 3006 ……….. 3100 ……….. 3143 ……….. 4372 ……….. ^z Transaction File 1235 ……………………..A 1273 ……………………..C 1456 ……………………..C 1543 ……………………..D 1678 ……………………..B 2910 ……………………..C 3006 ……………………..D 3098 ……………………..C 3099 ……………………..D ^z New Master File 1223 ………………………… 1235 ………………………… 1277 ………………………… 1456 ………………………… 2835 ………………………… 2910 ………………………… 3100 ………………………… 3143 ………………………… 4372 ………………………… ^z

  17. Sequential Update Procedure • Initially, record read from both Old-Master and Trans-File • Comparison of M-Acct-No and T-Acct-No determines next step

  18. 1) T-Acct-No = M-Acct-No • Means transaction record exists with same account number as master record • Perform regular update, adding Trans-File amount to Old-Master amount, storing result in New-Master field • Read next record from Trans-File

  19. 2) T-Acct-No > M-Acct-No • Means master record exists for which there is no corresponding transaction record • Write record from Old-Master to New-Master as is, since no changes need to be made to it • Read next record from Old-Master

  20. 3) T-Acct-No < M-Acct-No • Means transaction record exists for which there is no corresponding master record • Two ways to process this transaction • Create record in New-Master file for this transaction • May assume that T-Acct-No is invalid since no match found in Old-Master file and treat as error condition

  21. Use of HIGH-VALUES • Refers to largest value in computer's collating sequence • Used so all records from both Old-Master and Trans-File are processed • May reach end of Old-Master file before reaching end of Trans-File or vice versa

  22. Use of HIGH-VALUES • When last record read from Old-Master, M-Acct-No set to HIGH-VALUES • For all remaining transaction records T-Acct-No < M-Acct-No • Processed as new accounts and added to New-Master

  23. Use of HIGH-VALUES • When last record read from Trans-File, T-Acct-No set to HIGH-VALUES • For all remaining Old-Master records T-Acct-No > M-Acct-No • Old-Master records added to New-Master without changes

  24. Use of HIGH-VALUES • Loop to process records ends only when both account numbers equal HIGH-VALUES • May be used only with fields defined as alphanumeric • Define account number fields with PIC of Xs even though fields contain numbers

  25. Validity Checking in Updating • Code field may be added to each transaction record to indicate whether transaction is • New account to be added • Update to existing account • Deletion of existing account • Comparison of keys and transaction code type enable program to detect other update errors

  26. Updating Master Disk in Place • May read a master disk record, make changes directly to same record, and rewrite it or update it in place • Only two files needed • Master-File OPEN as I-O • Trans-File OPEN as INPUT • Use REWRITE statement to replace master disk record, currently in storage, that was accessed by preceding READ

  27. Updating Master Disk in Place • To delete a record, establish each record with an activity code field • For example, Code-X = 1 if record is active, or 2 if record is inactive • All master records initially active (1 in Code-X) • Master record deactivated by changing activity code to 2

  28. Updating Master Disk in Place • New records cannot be added in sequence when file open for I-O • Can add records at end of file by opening file in EXTEND mode OPENEXTEND file-name • Disk automatically positioned at end of file, immediately after last record • Sort file after records added to arrange them in sequence

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