!CHAPTER High-level operations


!SUBSECTION FOR

The *FOR* keyword can be to iterate over all elements of a list.
The general syntax is:

    FOR variable-name IN expression

Each list element returned by *expression* is visited exactly once. It is
required that *expression* returns a list in all cases. The empty list is
allowed, too. The current list element is made available for further processing 
in the variable specified by *variable-name*.

    FOR u IN users
      RETURN u

This will iterate over all elements from the list *users* (note: this list
consists of all documents from the collection named "users" in this case) and
make the current list element available in variable *u*. *u* is not modified in
this example but simply pushed into the result using the *RETURN* keyword.

Note: When iterating over collection-based lists as shown here, the order of
documents is undefined unless an explicit sort order is defined using a *SORT*
statement.

The variable introduced by *FOR* is available until the scope the *FOR* is
placed in is closed.

Another example that uses a statically declared list of values to iterate over:

    FOR year IN [ 2011, 2012, 2013 ]
      RETURN { "year" : year, "isLeapYear" : year % 4 == 0 && (year % 100 != 0 || year % 400 == 0) }

Nesting of multiple *FOR* statements is allowed, too. When *FOR* statements are
nested, a cross product of the list elements returned by the individual *FOR*
statements will be created.

    FOR u IN users
      FOR l IN locations
	RETURN { "user" : u, "location" : l }

In this example, there are two list iterations: an outer iteration over the list
*users* plus an inner iteration over the list *locations*.  The inner list is
traversed as many times as there are elements in the outer list.  For each
iteration, the current values of *users* and *locations* are made available for
further processing in the variable *u* and *l*.

!SUBSECTION RETURN 

The *RETURN* statement can (and must) be used to produce the result of a query.
It is mandatory to specify a *RETURN* statement at the end of each block in a
query, otherwise the query result would be undefined.

The general syntax for *return* is:

    RETURN expression

The *expression* returned by *RETURN* is produced for each iteration the
*RETURN* statement is placed in. That means the result of a *RETURN* statement
is always a list (this includes the empty list).  To return all elements from
the currently iterated list without modification, the following simple form can
be used:

    FOR variable-name IN expression
      RETURN variable-name

As *RETURN* allows specifying an expression, arbitrary computations can be
performed to calculate the result elements. Any of the variables valid in the
scope the *RETURN* is placed in can be used for the computations.

Note: Return will close the current scope and eliminate all local variables in
it.

!SUBSECTION FILTER

The *FILTER* statement can be used to restrict the results to elements that
match an arbitrary logical condition. The general syntax is:

    FILTER condition

*condition* must be a condition that evaluates to either *false* or *true*. If
the condition result is false, the current element is skipped, so it will not be
processed further and not be part of the result. If the condition is true, the
current element is not skipped and can be further processed.

    FOR u IN users
      FILTER u.active == true && u.age < 39
      RETURN u

In the above example, all list elements from *users* will be included that have
an attribute *active* with value *true* and that have an attribute *age* with a
value less than *39*. All other elements from *users* will be skipped and not be
included the result produced by *RETURN*.

It is allowed to specify multiple *FILTER* statements in a query, and even in
the same block. If multiple *FILTER* statements are used, their results will be
combined with a logical and, meaning all filter conditions must be true to
include an element.

    FOR u IN users
      FILTER u.active == true
      FILTER u.age < 39
      RETURN u

!SUBSECTION SORT 

The *SORT* statement will force a sort of the list of already produced
intermediate results in the current block. *SORT* allows specifying one or
multiple sort criteria and directions.  The general syntax is:

    SORT expression direction

Specifying the *direction* is optional. The default (implicit) direction for a
sort is the ascending order. To explicitly specify the sort direction, the
keywords *ASC* (ascending) and *DESC* can be used. Multiple sort criteria can be
separated using commas.

Note: when iterating over collection-based lists, the order of documents is
always undefined unless an explicit sort order is defined using *SORT*.

    FOR u IN users
      SORT u.lastName, u.firstName, u.id DESC
      RETURN u

!SUBSECTION LIMIT

The *LIMIT* statement allows slicing the list of result documents using an
offset and a count. It reduces the number of elements in the result to at most
the specified number.  Two general forms of *LIMIT* are followed:

    LIMIT count
    LIMIT offset, count

The first form allows specifying only the *count* value whereas the second form
allows specifying both *offset* and *count*. The first form is identical using
the second form with an *offset* value of *0*.

The *offset* value specifies how many elements from the result shall be
discarded.  It must be 0 or greater. The *count* value specifies how many
elements should be at most included in the result.

    FOR u IN users
      SORT u.firstName, u.lastName, u.id DESC
      LIMIT 0, 5
      RETURN u

!SUBSECTION LET 

The *LET* statement can be used to assign an arbitrary value to a variable.  The
variable is then introduced in the scope the *LET* statement is placed in.  The
general syntax is:

    LET variable-name = expression

*LET* statements are mostly used to declare complex computations and to avoid
repeated computations of the same value at multiple parts of a query.

    FOR u IN users
      LET numRecommendations = LENGTH(u.recommendations)
      RETURN { "user" : u, "numRecommendations" : numRecommendations, "isPowerUser" : numRecommendations >= 10 } 

In the above example, the computation of the number of recommendations is
factored out using a *LET* statement, thus avoiding computing the value twice in
the *RETURN* statement.

Another use case for *LET* is to declare a complex computation in a subquery,
making the whole query more readable.

    FOR u IN users
      LET friends = (
	FOR f IN friends 
	  FILTER u.id == f.userId
	  RETURN f
      )
      LET memberships = (
	FOR m IN memberships
	  FILTER u.id == m.userId
	  RETURN m
      )
      RETURN { "user" : u, "friends" : friends, "numFriends" : LENGTH(friends), "memberShips" : memberships }

!SUBSECTION COLLECT

The *COLLECT* keyword can be used to group a list by one or multiple group
criteria.  The two general syntaxes for *COLLECT* are:

    COLLECT variable-name = expression
    COLLECT variable-name = expression INTO groups

The first form only groups the result by the defined group criteria defined by
*expression*. In order to further process the results produced by *COLLECT*, a
new variable (specified by *variable-name*) is introduced. This variable
contains the group value.

The second form does the same as the first form, but additionally introduces a
variable (specified by *groups*) that contains all elements that fell into the
group. Specifying the *INTO* clause is optional-

    FOR u IN users
      COLLECT city = u.city INTO g
      RETURN { "city" : city, "users" : g }

In the above example, the list of *users* will be grouped by the attribute
*city*. The result is a new list of documents, with one element per distinct
*city* value. The elements from the original list (here: *users*) per city are
made available in the variable *g*. This is due to the *INTO* clause.

*COLLECT* also allows specifying multiple group criteria. Individual group
criteria can be separated by commas.

    FOR u IN users
      COLLECT first = u.firstName, age = u.age INTO g
      RETURN { "first" : first, "age" : age, "numUsers" : LENGTH(g) }

In the above example, the list of *users* is grouped by first names and ages
first, and for each distinct combination of first name and age, the number of
users found is returned.

Note: The *COLLECT* statement eliminates all local variables in the current
scope. After *COLLECT* only the variables introduced by *COLLECT* itself are
available.