The ArangoDB Query Language(AQL) was designed to accomplish a few important goals, including:
The goal of this guide is to ensure Read more
Thinking about your data as a highly connected set of information is a powerful way to gain insights, solve problems and bring products faster into the hands of your users.
Unlike other databases, relationships take the first priority in graph databases and with ArangoDBs multi-model approach for graphs, documents and key/value pairs you can even switch between models or combine them in a single query.
The graph concept is booming but still new to many. So we invested a few bazillion coffees and some night shifts to come up with a good plan for a Graph Course:
Explaining AQL queries becomes even easier in ArangoDB 2.8. While previous versions required writing a hard-to-memorize command like
Suppose there is an AQL query that’s executing in the server for a long time already and you want to get rid of it. What can be done to abort that query?
If a connection to the server can still be established, the easiest is to use the ArangoShell to fetch the list of currently executing AQL queries and send a kill command to the server for the correct query. Read more
I recently wrote two recipes about generating AQL query strings. They are contained in the ArangoDB cookbook by now:
After that, Github user tracker1 suggested in Github issue 1457 to take the ES6 template string variant even further, using a generator function for string building, and also using promises and ES7 async/await.
We can’t use ES7 async/await in ArangoDB at the moment due to lacking support in V8, but the suggested template string generator function seemed to be an obvious improvement that deserved inclusion in ArangoDB.
Basically, the suggestion is to use regular JavaScript variables/expressions in the template string and have them substituted safely.
With regular AQL bind parameters, a query looks like this:
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var bindVars = { name: "test" }; var query = `FOR doc IN collection FILTER doc.name == @name RETURN doc._key`; db._query(query, bindVars); |
This is immune to parameter injection, because the query string and the bind parameter value are passed in separately. But it’s not very ES6-y.
ArangoDB’s devel branch recently saw a change that makes writing some AQL queries a bit simpler.
The change introduces an optional shorthand notation for object attributes in the style of ES6’s enhanced object literal notation.
For example, consider the following query that groups values by age attribute and counts the number of documents per distinct age value:
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FOR doc IN collection COLLECT age = doc.age WITH COUNT INTO length RETURN { age: age, length: length } |
The object declaration in the last line of the query is somewhat redundant because one has to type identical attribute names and values:
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RETURN { age: age, length: length } |
In this case, the new shorthand notation simplifies the RETURN to:
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RETURN { age, length } |
In general, the shorthand notation can be used for all object literals when there is an attribute name that refers to a query variable of the same name.
It can also be mixed with the longer notation, e.g.:
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RETURN { age, length, dateCreated: DATE_NOW() } |
Last week saw the addition of the RETURN DISTINCT for AQL queries. This is a new shortcut syntax for making result sets unique.
For this purpose it can be used as an easier-to-memorize alternative for the already existing COLLECT statement. COLLECT is very flexible and can be used for multiple purposes, but it is syntactic overkill for making a result-set unique.
The new RETURN DISTINCT syntax makes queries easier to write and understand.
Here’s a non-scientific proof for this claim:
Compare the following queries, which both return each distinct age attribute value from the collection:
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FOR doc IN collection COLLECT age = doc.age RETURN age |
With RETURN DISTINCT:
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FOR doc IN collection RETURN DISTINCT doc.age |
Clearly, the query using RETURN DISTINCT is more intuitive, especially for AQL beginners. Apart from that, using RETURN DISTINCT will save a bit of typing compared to the longer COLLECT-based query.
Internally both COLLECT and RETURN DISTINCT will work by creating an AggregateNode. The optimizer will try the sorted and the hashed variants for both, so they should perform about the same.
However, the result of a RETURN DISTINCT does not have any guaranteed order, so the optimizer will not insert a post-SORT for it. It may do so for a regular COLLECT.
As mentioned before, COLLECT is more flexible than RETURN DISTINCT. Notably, COLLECT is superior to RETURN DISTINCT when the result set should be made unique using more than one criterion, e.g.
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FOR doc IN collection COLLECT status = doc.status, age = doc.age, RETURN { status, age } |
This is currently not achievable via RETURN DISTINCT, as it only works with a single criterion.
I just stumbled upon a comment in the ArangoDB blog asking how to create a diff of two documents with AQL.
Though there is no built-in AQL function to diff two documents, it is easily possible to build your own like in the following query.
Read more on how to diff two documents in AQL.
