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Meta Data

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• Meta data: Information (or Data) about the data
  • Meta information (or meta data):

    Meta information = information about data (and meta data is a form of data, just used for different purpose)

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  • Meta information includes (at the least):

    The name of each data field of the file The type of each data field of the file The size of each data field of the file

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  • Graphically, the format of the Meta information is as follows:

    +------------------------+-------+-------+ | name of field | type | size | +------------------------+-------+-------+ 24 characters 4 chars int

  • Examples:

    Example 1: +----------------------------+-------+-------+ | ID | C | 10 | +----------------------------+-------+-------+ describes the following data field: Field name is "ID" and the type is char[10] Example 2: +----------------------------+-------+-------+ | Salary | I | 4 | +----------------------------+-------+-------+ describes the following data field: Field name is "Salary" and the data type is integer (integer are 4 bytes long)

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  • Meta data may contain other information, but these three pieces of information are essential to help the program access the data stored

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• Crucial fact about Meta Data

  • Important property of meta information/data:

    The structure (= format) of the meta data (meta information) do not change In other words: The structure of the meta data will always be: FieldName FieldType FieldSize

    Note:

    The content of the meta data file can change, but: the structure of the meta data file will not change !!!. In other words, we do not need to add/remove more fields to/from the meta data structure.

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  • Result:

    Programs that use meta datafiles do not need to change (Because the structure of the meta data file do not change)

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• Examples of Meta Data
  • Example 1:

    Consider a file of student records with the following fields:

    Field name | Type ---------------+---------------------- ID | integer Name | character(30) Level | character(4)

    We would use a meta data file containing the following entries to describe the file:

    +----------------------------+-------+-------+ | ID | I | 4 | +----------------------------+-------+-------+ | Name | C | 30 | +----------------------------+-------+-------+ | Level | C | 4 | +----------------------------+-------+-------+

    • The first entry tells the program that the first field has name "ID", its type is integer (I) and it takes 4 bytes to store the value (integer is 4 bytes long)

    • The second entry tells the program that the second field has name "Name", its type is character (C) and it takes 30 bytes to store the value (so the character string has length 30)

    • The third entry tells the program that the third field has name "Level", its type is character (C) and it takes 4 bytes to store the value (so the character string has length 4)

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  • Example 2:

    Consider a file of employee records with the following fields:

    Field name | Type ---------------+---------------------- SSN | character(9) Name | character(30) department | character(10) salary | integer

    We would use a meta data file containing the following entries to describe the file:

    +----------------------------+-------+-------+ | SSN | C | 9 | +----------------------------+-------+-------+ | Name | C | 30 | +----------------------------+-------+-------+ | department | C | 10 | +----------------------------+-------+-------+ | salary | I | 4 | +----------------------------+-------+-------+