Matching documents

So far, we either looked up a single document, or returned the entire character collection. For the lookup, we used the DOCUMENT() function, which means we can only find documents by their key or ID.

To find documents that fulfill certain criteria more complex than key equality, there is the FILTER operation in AQL, which enables us to formulate arbitrary conditions for documents to match.

Equality condition

FOR c IN Characters
    FILTER c.name == "Ned"
    RETURN c

The filter condition reads like: “the attribute name of a character document must be equal to the string Ned”. If the condition applies, character document gets returned. This works with any attribute likewise:

FOR c IN Characters
    FILTER c.surname == "Stark"
    RETURN c

Range conditions

Strict equality is one possible condition we can state. There are plenty of other conditions we can formulate however. For example, we could ask for all young characters:

FOR c IN Characters
    FILTER c.age >= 13
    RETURN c.name
[
  "Joffrey",
  "Tyrion",
  "Samwell",
  "Ned",
  "Catelyn",
  "Cersei",
  "Jon",
  "Sansa",
  "Brienne",
  "Theon",
  "Davos",
  "Jaime",
  "Daenerys"
]

The operator >= stands for greater-or-equal, so every character of age 13 or older is returned (only their name in the example). We can return names and age of all characters younger than 13 by changing the operator to less-than and using the object syntax to define a subset of attributes to return:

FOR c IN Characters
    FILTER c.age < 13
    RETURN { name: c.name, age: c.age }
[
  { "name": "Tommen", "age": null },
  { "name": "Arya", "age": 11 },
  { "name": "Roose", "age": null },
  ...
]

You may notice that it returns name and age of 30 characters, most with an age of null. The reason for this is, that null is the fallback value if an attribute is requested by the query, but no such attribute exists in the document, and the null is compares to numbers as lower (see Type and value order). Hence, it accidentally fulfills the age criterion c.age < 13 (null < 13).

Multiple conditions

To not let documents pass the filter without an age attribute, we can add a second criterion:

FOR c IN Characters
    FILTER c.age < 13
    FILTER c.age != null
    RETURN { name: c.name, age: c.age }
[
  { "name": "Arya", "age": 11 },
  { "name": "Bran", "age": 10 }
]

This could equally be written with a boolean AND operator as:

FOR c IN Characters
    FILTER c.age < 13 AND c.age != null
    RETURN { name: c.name, age: c.age }

And the second condition could as well be c.age > null.

Alternative conditions

If you want documents to fulfill one or another condition, possibly for different attributes as well, use OR:

FOR c IN Characters
    FILTER c.name == "Jon" OR c.name == "Joffrey"
    RETURN { name: c.name, surname: c.surname }
[
  { "name": "Joffrey", "surname": "Baratheon" },
  { "name": "Jon", "surname": "Snow" }
]

See more details about Filter operations.