Until the second half of the nineties, nobody would doubt that SQL and Relational Model databases were the de facto standard and a large majority of commercial implementations in use those days were based on this assumption.
Historical examples are IBM, Oracle, SQL Server, Watcom, Gupta SQLBase, and so on. However, with time, some voices started to claim against what was already called impedance mismatch, the different representations of data and source code that happen when programming in object-oriented languages to these databases.
This is something that's clearly revealed when objects or class definitions have to be mapped in some fashion to databases (either tables or relational schemas).
Other problems arose from the different data types supported by both worlds, especially in scalar types and their operation semantics (for example, collations for different string interpretations), although OOP languages only consider this aspect in sort routines and strings...