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Mid Range Concepts

Mid Range Concepts. Chapter 11 Logical Files. Defining Keys. File Access: Direct Access : the storage device can go directly to the location and read the information stored there. Primary Key: Field whose value is unique for each record.

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Mid Range Concepts

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  1. Mid Range Concepts Chapter 11 Logical Files

  2. Defining Keys • File Access: • Direct Access: the storage device can go directly to the location and read the information stored there. • Primary Key: Field whose value is unique for each record. • Sequential Access: storage device must go through all storage locations that physically precede the storage location being sought. • Relative Record Number: Each record is given a number based on it relative position within the file.

  3. Defining Keys • Key Fields: DDS provides the ability to define key fields. • KSAP (Keyed sequence access path): Provides the programmer the ability to access the records in both arrival sequence and keyed sequence. • Key Fields can be Primary or Secondary • Primary: unique • Secondary: not unique

  4. Defining Keys • Primary Keys: • Place the word UNIQUE prior to any file references in DDS. • AFTER record specifications reenter the field name of the key with a K in position 17. • Secondary Keys: • Same as above but DO NOT specify unique.

  5. Logical Files • Logical Files: usually created to satisfy different user access requirements. • They offer a different view of the data than supplied by the physical file. • They do not contain any data. • They are based on a physical file and cannot exist independently. • Utilize DDS to define the record layout.

  6. Logical Files • Record level keyword PFILE is required to specify the physical file they are based on. • Would you want to perform updates to logical files? • Be careful when not all the fields of the physical file are present.

  7. Logical Files • Can add or delete records of a physical file and the logical file will automatically see the adjustments. • Cannot delete an entire physical file if logical files exist using it. • Command: DSPDBR (Display Database relations) will display all dependent files for a specified file.

  8. Logical files • Logical files are much easier to change than physical files. • Can use the command FNDSTRPDM to find all programs that use the logical file because they will need to be recompiled.

  9. Logical Files • Joining logical file: Can contain information from multiple physical files. • Up to 32 files can be joined in a logical file. • JFILE keyword is used to specify file names. • JOIN keyword is used to specify files joined. • JFLD keyword is used to specify fields used to join the files.

  10. Logical Files

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