!CHAPTER Control flow functions

AQL offers the following functions to let the user control the flow of operations:

- *NOT_NULL(alternative, ...)*: Returns the first alternative that is not *null*, 
  and *null* if all alternatives are *null* themselves

- *FIRST_LIST(alternative, ...)*: Returns the first alternative that is a list, and
  *null* if none of the alternatives is a list

- *FIRST_DOCUMENT(alternative, ...)*: Returns the first alternative that is a document,
  and *null* if none of the alternatives is a document

!SECTION Miscellaneous functions

Finally, AQL supports the following functions that do not belong to any of the other
function categories:

- *COLLECTIONS()*: Returns a list of collections. Each collection is returned as a document
  with attributes *name* and *_id*

- *CURRENT_USER()*: Returns the name of the current user. The current user is the user 
  account name that was specified in the *Authorization* HTTP header of the request. It will
  only be populated if authentication on the server is turned on, and if the query was executed
  inside a request context. Otherwise, the return value of this function will be *null*.

- *DOCUMENT(collection, id)*: Returns the document which is uniquely identified by
  the *id*. ArangoDB will try to find the document using the *_id* value of the document
  in the specified collection. If there is a mismatch between the *collection* passed and
  the collection specified in *id*, then *null* will be returned. Additionally, if the
  *collection* matches the collection value specified in *id* but the document cannot be
  found, *null* will be returned. This function also allows *id* to be a list of ids.
  In this case, the function will return a list of all documents that could be found. 

*Examples*
  
      DOCUMENT(users, "users/john")
      DOCUMENT(users, "john")

      DOCUMENT(users, [ "users/john", "users/amy" ])
      DOCUMENT(users, [ "john", "amy" ])

  Note: The *DOCUMENT* function is polymorphic since ArangoDB 1.4. It can now be used with
  a single parameter *id* as follows:

- *DOCUMENT(id)*: In this case, *id* must either be a document handle string
  (consisting of collection name and document key) or a list of document handle strings, e.g.

      DOCUMENT("users/john")
      DOCUMENT([ "users/john", "users/amy" ])

- *SKIPLIST(collection, condition, skip, limit)*: Return all documents 
  from a [skiplist index](../Glossary/README.html#skiplist_index) on collection *collection* that match the specified *condition*.
  This is a shortcut method to use a skiplist index for retrieving specific documents in 
  indexed order. The skiplist index supports equality and less than/greater than queries. The
  *skip* and *limit* parameters are optional but can be specified to further limit the
  results:

      SKIPLIST(test, { created: [[ '>', 0 ]] }, 0, 100)
      SKIPLIST(test, { age: [[ '>', 25 ], [ '<=', 65 ]] })
      SKIPLIST(test, { a: [[ '==', 10 ]], b: [[ '==', 25 ]] }

  The *condition* document must contain an entry for each attribute that is contained in the
  index. It is not allowed to specify just a subset of attributes that are present in an index.
  Additionally the attributes in the *condition* document must be specified in the same order 
  as in the index. 
  If no suitable skiplist index is found, an error will be raised and the query will be aborted.