java - What is Object/Relational mismatch -

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i newbie java , reading object relational mapping. found term object/relational mismatch on link hibernate

can explain object/relational mismatch in terms of java. read haacked.com not properly.explanation example appreciable.

hibernate orm (object relational mapping) tool. it's primary purpose translate concepts object oriented programming, such classes, inheritance , fields, concepts used in relational databases, such tables, rows , columns.

for example, class corresponds database table, object (instance of class) corresponds database row, , field corresponds database column.

the term "object/relational mismatch" refers fact there not clear way translate concepts object-oriented programming relational database concepts , vice versa. hibernate attempts solve problem.

for example, how translate inheritance relational database concepts? there's no such thing inheritance in relational database, way has invented represent in database. hibernate has different ways this, example having 1 table class hierarchy discriminator column determine subclass row maps, or having table per subclass.

likewise, there concepts exist in relational database cannot translated object oriented programming concepts.


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