"impedance mismatch" I believe typically refers to the following:
* the fact that the records you can create with SQL are an unbounded combination of every field in every table; in addition, any aggregate or functions applied as well as any renamed fields will further enrich the set of row classes you are able to generate.
* the fact that with (standard) SQL you cannot create anything other than lists of records, whereas objects are directed cyclic graphs
* the fact that objects assume unbounded access to the said directed cyclic graph as if it were in memory, which is a mismatch with optimal SQL querying patterns (the n+1 queries problem)
* the fact that the records you can create with SQL are an unbounded combination of every field in every table; in addition, any aggregate or functions applied as well as any renamed fields will further enrich the set of row classes you are able to generate.
* the fact that with (standard) SQL you cannot create anything other than lists of records, whereas objects are directed cyclic graphs
* the fact that objects assume unbounded access to the said directed cyclic graph as if it were in memory, which is a mismatch with optimal SQL querying patterns (the n+1 queries problem)