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docs/MQL.md

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+# 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"``