mql.md
# 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:
```javascript
[{
"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.
```javascript
[{
"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,
```javascript
[{
"id": "A",
"some_predicate": "B"
}]
```
will only match if the quad
```text
A some_predicate B .
```
exists. In order to reverse the directions, "!predicates" are used. So that:
```javascript
[{
"id": "A",
"!some_predicate": "B"
}]
```
will only match if the quad
```text
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.
```javascript
[{
"id": "A",
"@x:some_predicate": "B",
"@y:some_predicate": "C"
}]
```
Will only match if _both_
```text
A some_predicate B .
A some_predicate C .
```
exist.
This combines with the reversal rule to create paths like `"@a:!some_predicate"`