About ad hoc query in DSP?

Question

in the edocs of dsp,i read these:
An ad hoc query is an XQuery function that is not stored in a data service, but is instead defined by the client application. You can use an ad hoc query to execute any XQuery function, possibly against a data source defined on a remote DSP server or "even with no back-end data source at all."
what's meaning of "even with no back-end data source at all"??
Can somebody show me some samples??thx a lot.
BTW:
I am looking for some ways to execute SQL dynamic,
like this: SQL1:select * from table1,SQL2:select * from table2,
any body give me some good idea about it ? Thx in advance.
my mail: nizhigang2000(at)gmail(dot).com

Answer 1

A query with no back-end data would return data from it's input arguments or constructed in the query. For example the query :
<result/>
has no backend sources.
You can execute sql through the LD JDBC driver - the sql is converted to equivalent XQuery, and then to sql - which maybe executed against multiple databases depending on your query (that's why there is no direct pass-through). You can use the Relational wrapper if there is exact sql that you would like to have executed. (Import Source Metadata -> Relational)
- Mike