# MQL Guide

## General

Cayley's MQL implementation is a work-in-progress clone of [Freebase's MQL API](https://developers.google.com/freebase/mql/). At the moment, it supports very basic queries without some of the extended features. It also aims to be database-agnostic, meaning that the schema inference from Freebase does not (yet) apply.

Every JSON Object can be thought of as a node in the graph, and wrapping an object in a list means there may be several of these, or it may be repeated. A simple query like:

```json
[{
  "id": null
}]
```

Is equivalent to all nodes in the graph, where "id" is the special keyword for the value of the node.

Predicates are added to the object to specify constraints. 

```json
[{
  "id": null,
  "some_predicate": "some value"
}]
```

Predicates can take as values objects or lists of objects (subqueries), strings and numbers (literal IDs that must match -- equivalent to the object {"id": "value"}) or null, which indicates that, while the object must have a predicate that matches, the matching values will replace the null. A single null is one such value, an empty list will be filled with all such values, as strings.

## Keywords

* `id`: The value of the node.

## Reverse Predicates

Predicates always assume a forward direction. That is,

```json
[{
  "id": "A",
  "some_predicate": "B"
}]
```

will only match if the triple
```
A some_predicate B .
```

exists. In order to reverse the directions, "!predicates" are used. So that:

```json
[{
  "id": "A",
  "!some_predicate": "B"
}]
```

will only match if the triple
```
B some_predicate A .
```
exists.

## Multiple Predicates

JSON does not specify the behavior of objects with the same key. In order to have separate constraints for the same predicate, the prefix "@name:" can be applied to any predicate. This is slightly different from traditional MQL in that fully-qualified http paths may be common predicates, so we have an "@name:" prefix instead.


```json
[{ 
  "id": "A",
  "@x:some_predicate": "B",
  "@y:some_predicate": "C"

}]
```

Will only match if *both*

```
A some_predicate B .
A some_predicate C .
```

exist.

This combines with the reversal rule to create paths like ``"@a:!some_predicate"``
