Joins.wiki

1. summary How to perform JOINs between IndexedCollections
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Although it is often easier and more performant to de-normalize data, it is possible with CQEngine to perform JOINs and SQL EXISTS-type queries between `IndexedCollection`s at runtime with reasonable performance (at least, outperforming external databases).

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SQL EXISTS

Given a collection of Cars, and a collections of Garages, find cars which are convertible or which have a sunroof, which can be serviced by garages in a particular city. Source code of this example here.

The example above prints:

Ford Focus has a sunroof or is convertible, and can be serviced in Dublin
BMW M3 has a sunroof or is convertible, and can be serviced in Dublin

SQL EXISTS-based JOIN

Given a collection of Cars, and a collections of Garages, find cars which are convertible or which have a sunroof, which can be serviced by garages in a particular city, along with the names of those garages. Source code of this example here.

The example above prints:

Ford Focus has a sunroof or is convertible, and can be serviced in Dublin at the following garages:- 
---> Garage{garageId=4, name='Jill's garage', location='Dublin', brandsServiced=[Ford Focus]}
---> Garage{garageId=3, name='John's garage', location='Dublin', brandsServiced=[Ford Focus, Ford Taurus]}

BMW M3 has a sunroof or is convertible, and can be serviced in Dublin at the following garages:- 
---> Garage{garageId=2, name='Jane's garage', location='Dublin', brandsServiced=[BMW M3]}