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87 lines
2.2 KiB
87 lines
2.2 KiB
# MQL Guide
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## General
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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.
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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:
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```json
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[{
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"id": null
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}]
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```
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Is equivalent to all nodes in the graph, where "id" is the special keyword for the value of the node.
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Predicates are added to the object to specify constraints.
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```json
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[{
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"id": null,
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"some_predicate": "some value"
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}]
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```
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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.
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## Keywords
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* `id`: The value of the node.
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## Reverse Predicates
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Predicates always assume a forward direction. That is,
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```json
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[{
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"id": "A",
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"some_predicate": "B"
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}]
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```
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will only match if the quad
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```
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A some_predicate B .
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```
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exists. In order to reverse the directions, "!predicates" are used. So that:
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```json
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[{
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"id": "A",
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"!some_predicate": "B"
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}]
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```
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will only match if the quad
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```
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B some_predicate A .
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```
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exists.
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## Multiple Predicates
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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.
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```json
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[{
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"id": "A",
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"@x:some_predicate": "B",
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"@y:some_predicate": "C"
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}]
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```
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Will only match if *both*
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```
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A some_predicate B .
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A some_predicate C .
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```
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exist.
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This combines with the reversal rule to create paths like ``"@a:!some_predicate"``
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