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COBOL Tutorial

COBOL SEQUENCE Clause - Quick Reference

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Overview

The SEQUENCE clause is used to define the ordering of data within COBOL files. It specifies how records are organized and accessed, ensuring consistent data ordering for efficient processing.

Purpose and Usage

  • Data ordering - Define the order of data records
  • File organization - Organize file structure
  • Access optimization - Optimize data access patterns
  • Processing efficiency - Improve processing efficiency
  • Business logic support - Support business ordering requirements

Syntax

The SEQUENCE clause is used in the FILE-CONTROL paragraph of the ENVIRONMENT DIVISION.

Basic Syntax

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* Basic SEQUENCE syntax
SELECT file-name
ASSIGN TO physical-file-name
ORGANIZATION IS SEQUENTIAL
ACCESS MODE IS SEQUENTIAL
SEQUENCE IS ASCENDING KEY key-field.
 
* Complete example
IDENTIFICATION DIVISION.
PROGRAM-ID. SEQUENCE-EXAMPLE.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT SEQUENTIAL-FILE
    ASSIGN TO 'DATA/SEQUENTIAL.DAT'
    ORGANIZATION IS SEQUENTIAL
    ACCESS MODE IS SEQUENTIAL
    SEQUENCE IS ASCENDING KEY CUSTOMER-ID.
 
DATA DIVISION.
FILE SECTION.
FD  SEQUENTIAL-FILE.
01  FILE-RECORD.
    05  CUSTOMER-ID    PIC 9(6).
    05  CUSTOMER-NAME  PIC X(30).
    05  CUSTOMER-BAL   PIC 9(8)V99.
 
WORKING-STORAGE SECTION.
01  WS-FILE-STATUS     PIC XX.
 
PROCEDURE DIVISION.
MAIN-LOGIC.
    OPEN INPUT SEQUENTIAL-FILE
    READ SEQUENTIAL-FILE
    CLOSE SEQUENTIAL-FILE
    STOP RUN.

SEQUENCE defines data ordering in files.

Practical Examples

Examples of using the SEQUENCE clause in different scenarios.

Ascending Sequence

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* Ascending sequence by customer ID
SELECT CUSTOMER-FILE
ASSIGN TO 'CUSTOMER.DAT'
ORGANIZATION IS SEQUENTIAL
ACCESS MODE IS SEQUENTIAL
SEQUENCE IS ASCENDING KEY CUSTOMER-ID.

SEQUENCE for ascending order by customer ID.

Descending Sequence

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* Descending sequence by date
SELECT TRANSACTION-FILE
ASSIGN TO 'TRANSACT.DAT'
ORGANIZATION IS SEQUENTIAL
ACCESS MODE IS SEQUENTIAL
SEQUENCE IS DESCENDING KEY TRANSACTION-DATE.

SEQUENCE for descending order by date.

Multiple Key Sequence

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* Multiple key sequence
SELECT ORDER-FILE
ASSIGN TO 'ORDER.DAT'
ORGANIZATION IS SEQUENTIAL
ACCESS MODE IS SEQUENTIAL
SEQUENCE IS ASCENDING KEY CUSTOMER-ID
    DESCENDING KEY ORDER-DATE.

SEQUENCE with multiple keys.

Best Practices

Understanding best practices ensures effective use of the SEQUENCE clause.

Best Practices

  • Choose appropriate keys - Select meaningful sequence keys
  • Consider access patterns - Match sequence to access requirements
  • Optimize performance - Use sequence for performance optimization
  • Maintain consistency - Keep sequence consistent across programs
  • Document sequence logic - Document the reasoning behind sequence choices

Test Your Knowledge

1. What is the primary purpose of the SEQUENCE clause in COBOL?

  • To sequence program execution
  • To define file organization and data ordering
  • To sequence data types
  • To sequence memory allocation

2. In which COBOL division is the SEQUENCE clause typically used?

  • IDENTIFICATION DIVISION
  • ENVIRONMENT DIVISION
  • DATA DIVISION
  • PROCEDURE DIVISION

3. What types of files can use the SEQUENCE clause?

  • Only sequential files
  • Only indexed files
  • Sequential and indexed files
  • All file types

4. How does SEQUENCE affect file processing?

  • It has no effect
  • It determines the order of data access
  • It only affects file creation
  • It only affects file deletion

5. What is the relationship between SEQUENCE and SORT?

  • They are the same thing
  • SEQUENCE defines order, SORT arranges data
  • SEQUENCE is obsolete, SORT is modern
  • They cannot be used together

Frequently Asked Questions

Related Concepts

File Organization Types

Different file organization types in COBOL.

Sort and Merge Operations

Sort and merge operations in COBOL.

File Processing

File processing techniques in COBOL.

SEPARATE ClauseSEQUENTIAL Clause