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description: Collection data can be accessed by specifying a collection name in a query
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Accessing data from collections
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Collection data can be accessed by specifying a collection name in a query.  A
collection can be understood as an array of documents, and that is how they are
treated in AQL. Documents from collections are normally accessed using the
`FOR` keyword. Note that when iterating over documents from a collection, the
order of documents is undefined. To traverse documents in an explicit and
deterministic order, the `SORT` keyword should be used in addition.

Data in collections is stored in documents, with each document potentially
having different attributes than other documents. This is true even for
documents of the same collection.

It is therefore quite normal to encounter documents that do not have some or all
of the attributes that are queried in an AQL query. In this case, the
non-existing attributes in the document will be treated as if they would exist
with a value of *null*.  That means that comparing a document attribute to
*null* will return true if the document has the particular attribute and the
attribute has a value of *null*, or that the document does not have the
particular attribute at all.

For example, the following query will return all documents from the collection
*users* that have a value of *null* in the attribute *name*, plus all documents
from *users* that do not have the *name* attribute at all:

    FOR u IN users
      FILTER u.name == null
      RETURN u

Furthermore, *null* is less than any other value (excluding *null* itself). That
means documents with non-existing attributes may be included in the result
when comparing attribute values with the less than or less equal operators.

For example, the following query will return all documents from the collection
*users* that have an attribute *age* with a value less than *39*, but also all
documents from the collection that do not have the attribute *age* at all.

    FOR u IN users
      FILTER u.age < 39
      RETURN u

This behavior should always be taken into account when writing queries.