Subquery Expressions

A subquery is a query block wrapped in parentheses and used as a value. Most often it is a FOR loop whose result — always an array — is assigned, returned, or passed to another expression.

example.fql Ferret v2

query.fql

FQL

LET users = [ { name: "Ada", active: true }, { name: "Grace", active: false }, { name: "Linus", active: true } ] LET activeUsers = ( FOR u IN users FILTER u.active RETURN u.name ) RETURN activeUsers

The parentheses are required. A FOR loop written without them is the output of the query itself, not a value you can place inside another expression.

Composing transformations

Because a subquery is just a value, it can be used anywhere a value is expected — including as an argument to a function.

example.fql Ferret v2

query.fql

FQL

RETURN LENGTH( (FOR n IN 1..10 FILTER n % 2 == 0 RETURN n) )

A subquery can also be indexed like any other array.

example.fql Ferret v2

query.fql

FQL

RETURN (FOR n IN 1..5 RETURN n * n)[2]

Nesting

The RETURN of one loop can be another subquery, which produces nested arrays.

example.fql Ferret v2

query.fql

FQL

RETURN ( FOR i IN 1..3 RETURN ( FOR j IN 1..3 RETURN i * j ) )

Each inner subquery is evaluated once per iteration of the outer loop.

Subqueries and query expressions

A subquery composes FQL transformations: it runs a FOR block and hands you the result. This is different from a Query Expression, which delegates a query to a host value such as an HTML document. They share the word “query” but solve different problems — use a subquery to shape data with FQL, and QUERY to extract data through a host capability.