Starting with Nuxeo 5.4.1, it's possible to make CMISQL calls from Nuxeo. One of the advantages of CMISQL compared to NXQL is that CMISQL allows you to do JOINs between several documents.
First, make sure you installed CMIS in your server, see the page CMIS Page for that.
Once CMIS is working, your Nuxeo Java code can do the following:
Note that you can use your own CallContext implementation with an adequate getUsername() method if it's more convenient. The method isObjectInfoRequired() should also return false for better performance. A CallContext.SERVLET_CONTEXT parameter is important only if you use renditions (to return the rendition URL, which depends on the ServletContext).
You will find more information on the CMISQL syntax in the CMIS specifications.