Databases : Field Lengths

Databases : Field Lengths

Each field in a record can be of either fixed or variable length. When you create a file you must decide whether each field will be of fixed or variable length :

Fixed Length Field	A fixed length field contains a set number of characters.
Variable Length Field	A variable length field can contain any number of characters. If a field is of fixed length, you must decide how many characters it can contain.

If a field is of variable length then an End-Of-Field marker must be put after the data stored in the field to show where it ends.

If a record contains some fields which have variable lengths then the record length will be variable. If a record contains only fixed length fields the record length will be fixed.

The main advantages of variable over fixed length fields or records are:

A variable length field can store an item no matter how long it is.
There is less wastage of space. Each item of data is stored in a space just one character longer than the item of data (it is one longer since an end-of-field marker must follow the field).

The main advantages of fixed over variable length fields or records are :

Allocation of storage space is simple - each field / record requires the same amount of space.
Updating files is simpler - if a field is edited the new contents will be the same length as the old contents so can simply be stored back over the old contents.

GCSE ICT Companion 04 - (C) P Meakin 2004