They are used when you need to store a lot of data
data is organised into fields and records
the key field contains an item that is unique to that record - no records are the same in the key field
they are good becasur if it is really large then you can search for that person - less than <, more than >, equal to =, not equal to <>
they can also be sorted - you choose a field and then the records are sorted into order using the entries of the field
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