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On 07/07/13 20:51, Nico Williams wrote:
> QBE is nice and simple, but lacks expressive power.
The Mongo query syntax with an example JSON object:
{ "a": 10, "b": {"c": 3}, "d": ["one", "two", "three"]}
To find all a==10:
{"a" : 10 }
To find all a>=10:
{"a": {"$gte": 10}}
To find c==3:
{ "b.c": 3}
To find where element two of d is "three":
{ "d.2": "three" }
To find where "a" is present but "b" is not:
{"a": {"$exists": true}, "b": {"$exists": false} }
The various $operators allow expressing more deep and meaningful things:
http://docs.mongodb.org/manual/reference/operator/
They are also used in updates. eg to add 3 to "a" and 4 to "b.c"
{"$inc": {"a": 3, "b.c": 4}}
The thing you can't do is reference objects (JOIN) in a different
collection (TABLE in SQL semantics). SQL syntax is good at this. However
there is less of a need due to denormalized data, and other approaches
like map/reduce. It is also fairly normal to store multiple "types" in
the same collection (eg songs, artists and albums) which would require
different tables in SQL. I suspect multi-collection references could be
squeezed into the JSON like syntax but would be ugly.
It will be interesting to see who wins the query syntax mindshare. There
does seem to be a desire to stay with SQL like syntax - for example
Cassandra also uses it. Another thing I like about the Mongo syntax is
that it is trivial to build up queries programmatically.
Roger
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