Grouping

To group results by arbitrary criteria, AQL provides the COLLECT keyword. COLLECT will perform a grouping, but no aggregation. Aggregation can still be added in the query if required.

Ensuring uniqueness

COLLECT can be used to make a result set unique. The following query will return each distinct age attribute value only once:

FOR u IN users
    COLLECT age = u.age
    RETURN age

This is grouping without tracking the group values, but just the group criterion (age) value.

Grouping can also be done on multiple levels using COLLECT:

FOR u IN users
    COLLECT status = u.status, age = u.age
    RETURN { status, age }

Alternatively RETURN DISTINCT can be used to make a result set unique. RETURN DISTINCT supports a single criterion only:

FOR u IN users
    RETURN DISTINCT u.age

RETURN DISTINCT does not change the order of results. For above query that means the order is undefined because no particular order is guaranteed when iterating over a collection without explicit SORT operation.

Fetching group values

To group users by age, and return the names of the users with the highest ages, we’ll issue a query like this:

FOR u IN users
    FILTER u.active == true
    COLLECT age = u.age INTO usersByAge
    SORT age DESC LIMIT 0, 5
    RETURN {
        age,
        users: usersByAge[*].u.name
    }

[
  { "age": 37, "users": [ "John", "Sophia" ] },
  { "age": 36, "users": [ "Fred", "Emma" ] },
  { "age": 34, "users": [ "Madison" ] },
  { "age": 33, "users": [ "Chloe", "Michael" ] },
  { "age": 32, "users": [ "Alexander" ] }
]

The query will put all users together by their age attribute. There will be one result document per distinct age value (let aside the LIMIT). For each group, we have access to the matching document via the usersByAge variable introduced in the COLLECT statement.

Variable Expansion

The usersByAge variable contains the full documents found, and as we’re only interested in user names, we’ll use the expansion operator [*] to extract just the name attribute of all user documents in each group:

usersByAge[*].u.name

The [*] expansion operator is just a handy short-cut. We could also write a subquery:

( FOR temp IN usersByAge RETURN temp.u.name )

Grouping by multiple criteria

To group by multiple criteria, we’ll use multiple arguments in the COLLECT clause. For example, to group users by ageGroup (a derived value we need to calculate first) and then by gender, we’ll do:

FOR u IN users
    FILTER u.active == true
    COLLECT ageGroup = FLOOR(u.age / 5) * 5,
            gender = u.gender INTO group
    SORT ageGroup DESC
    RETURN {
        ageGroup,
        gender
    }

[
  { "ageGroup": 35, "gender": "f" },
  { "ageGroup": 35, "gender": "m" },
  { "ageGroup": 30, "gender": "f" },
  { "ageGroup": 30, "gender": "m" },
  { "ageGroup": 25, "gender": "f" },
  { "ageGroup": 25, "gender": "m" }
]

Counting group values

If the goal is to count the number of values in each group, AQL provides the special COLLECT WITH COUNT INTO syntax. This is a simple variant for grouping with an additional group length calculation:

FOR u IN users
    FILTER u.active == true
    COLLECT ageGroup = FLOOR(u.age / 5) * 5,
            gender = u.gender WITH COUNT INTO numUsers
    SORT ageGroup DESC
    RETURN {
        ageGroup,
        gender,
        numUsers
    }

[
  { "ageGroup": 35, "gender": "f", "numUsers": 2 },
  { "ageGroup": 35, "gender": "m", "numUsers": 2 },
  { "ageGroup": 30, "gender": "f", "numUsers": 4 },
  { "ageGroup": 30, "gender": "m", "numUsers": 4 },
  { "ageGroup": 25, "gender": "f", "numUsers": 2 },
  { "ageGroup": 25, "gender": "m", "numUsers": 2 }
]

Aggregation

Adding further aggregation is also simple in AQL by using an AGGREGATE clause in the COLLECT:

FOR u IN users
    FILTER u.active == true
    COLLECT ageGroup = FLOOR(u.age / 5) * 5,
            gender = u.gender
    AGGREGATE numUsers = LENGTH(1),
              minAge = MIN(u.age),
              maxAge = MAX(u.age)
    SORT ageGroup DESC
    RETURN {
        ageGroup,
        gender,
        numUsers,
        minAge,
        maxAge
    }

[
  {
    "ageGroup": 35,
    "gender": "f",
    "numUsers": 2,
    "minAge": 36,
    "maxAge": 39,
  },
  {
    "ageGroup": 35,
    "gender": "m",
    "numUsers": 2,
    "minAge": 35,
    "maxAge": 39,
  },
  ...
]

We have used the aggregate functions LENGTH here (it returns the length of an array). This is the equivalent to SQL’s SELECT g, COUNT(*) FROM ... GROUP BY g. In addition to LENGTH, AQL also provides MAX, MIN, SUM and AVERAGE, VARIANCE_POPULATION, VARIANCE_SAMPLE, STDDEV_POPULATION, STDDEV_SAMPLE, UNIQUE, SORTED_UNIQUE and COUNT_UNIQUE as basic aggregation functions.

In AQL all aggregation functions can be run on arrays only. If an aggregation function is run on anything that is not an array, a warning will be produced and the result will be null.

Using an AGGREGATE clause will ensure the aggregation is run while the groups are built in the collect operation. This is normally more efficient than collecting all group values for all groups and then doing a post-aggregation.

Post-aggregation

Aggregation can also be performed after a COLLECT operation using other AQL constructs, though performance-wise this is often inferior to using COLLECT with AGGREGATE.

The same query as before can be turned into a post-aggregation query as shown below. Note that this query will build and pass on all group values for all groups inside the variable g, and perform the aggregation at the latest possible stage:

FOR u IN users
    FILTER u.active == true
    COLLECT ageGroup = FLOOR(u.age / 5) * 5,
            gender = u.gender INTO g
    SORT ageGroup DESC
    RETURN {
        ageGroup,
        gender,
        numUsers: LENGTH(g[*]),
        minAge: MIN(g[*].u.age),
        maxAge: MAX(g[*].u.age)
    }

[
  {
    "ageGroup": 35,
    "gender": "f",
    "numUsers": 2,
    "minAge": 36,
    "maxAge": 39,
  },
  {
    "ageGroup": 35,
    "gender": "m",
    "numUsers": 2,
    "minAge": 35,
    "maxAge": 39,
  },
  ...
]

This is in contrast to the previous query that used an AGGREGATE clause to perform the aggregation during the collect operation, at the earliest possible stage.

Post-filtering aggregated data

To filter the results of a grouping or aggregation operation (i.e. something similar to HAVING in SQL), simply add another FILTER clause after the COLLECT statement.

For example, to get the 3 ageGroups with the most users in them:

FOR u IN users
    FILTER u.active == true
    COLLECT ageGroup = FLOOR(u.age / 5) * 5 INTO group
    LET numUsers = LENGTH(group)
    FILTER numUsers > 2 /* group must contain at least 3 users in order to qualify */
    SORT numUsers DESC
    LIMIT 0, 3
    RETURN {
        "ageGroup": ageGroup,
        "numUsers": numUsers,
        "users": group[*].u.name
    }

[
  {
    "ageGroup": 30,
    "numUsers": 8,
    "users": [
      "Abigail",
      "Madison",
      "Anthony",
      "Alexander",
      "Isabella",
      "Chloe",
      "Daniel",
      "Michael"
    ]
  },
  {
    "ageGroup": 25,
    "numUsers": 4,
    "users": [
      "Mary",
      "Mariah",
      "Jim",
      "Diego"
    ]
  },
  {
    "ageGroup": 35,
    "numUsers": 4,
    "users": [
      "Fred",
      "John",
      "Emma",
      "Sophia"
    ]
  }
]

To increase readability, the repeated expression LENGTH(group) was put into a variable numUsers. The FILTER on numUsers is the equivalent an SQL HAVING clause.