Reading further up, the schema concept is
de?¬?ned as an ???Entity?????”a concept speci?¬?ed on the metamodel level. Finally, the metametamodel
is used to specify the concepts used for database design.
We can identify the same layers from some programming languages too. At the
application level we have objects, at the model level we can have classes, and at the
metamodel level we can have metaclasses, like in Smalltalk. In a non-object-oriented
view we can see program execution, program code, and programming language
speci?¬?cation, respectively. In the meta-metamodel level we might see BNF (Backus??“
Naur Form) as it is used widely to specify programming languages.
If we take a modeling language view, the same layers can again be identi?¬?ed.
On the application level, a system is used. Following the digital wrist watch
example of Chapter 9, the user pressing a button on a watch is an application level
operation. On the model level, a speci?¬?c button is de?¬?ned. On the metamodel level,
we have the speci?¬?cation of the modeling language, including the button concept.
Finally on the highest level, we have a metamodeling language used to specify the
modeling language. We usually do this kind of type and instance mapping
intuitively as otherwise it would not be possible to understand the models, schemas,
and code.
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