!CHAPTER Advanced features

!SUBSECTION Subqueries 

Wherever an expression is allowed in AQL, a subquery can be placed. A subquery
is a query part that can introduce its own local variables without affecting
variables and values in its outer scope(s).

It is required that subqueries be put inside parentheses *(* and *)* to
explicitly mark their start and end points:

```js
FOR u IN users
  LET recommendations = ( 
    FOR r IN recommendations 
      FILTER u.id == r.userId 
      SORT u.rank DESC
      LIMIT 10
      RETURN r
  )
  RETURN { "user" : u, "recommendations" : recommendations }
```

```js
FOR u IN users
  COLLECT city = u.city INTO g
  RETURN { "city" : city, "numUsers" : LENGTH(g), "maxRating": MAX(
    FOR r IN g 
    RETURN r.user.rating
  )}
```

Subqueries may also include other subqueries themselves.

!SUBSECTION Variable expansion

In order to access a named attribute from all elements in an array easily, AQL
offers the shortcut operator <i>[\*]</i> for variable expansion.

Using the <i>[\*]</i> operator with a variable will iterate over all elements in the
variable thus allowing to access a particular attribute of each element.  It is
required that the expanded variable is an array.  The result of the <i>[\*]</i>
operator is again an array.

```js
FOR u IN users
  RETURN { "user" : u, "friendNames" : u.friends[*].name }
```

In the above example, the attribute *name* is accessed for each element in the
array *u.friends*. The result is a flat array of friend names, made available as
the attribute *friendNames*.