Key-preserved table is fundamental to understanding restrictions on modifying join views

Discussion

• Nirja Shah   -Posted on 29 Sep 15
  - The concept of a key-preserved table is fundamental to understanding the restrictions on modifying join views.
  
  - A table is key-preserved if every key of the table can also be a key of the result of the join.
  
  - So, a key-preserved table has its keys preserved through a join.
  
  - It is not necessary that the key or keys of a table be selected for it to be key preserved.
  
  - It is sufficient that if the key or keys were selected, then they would also be keys of the result of the join.
  
  - The key-preserving property of a table does not depend on the actual data in the table.
  
  - It is, rather, a property of its schema.
  
  - For example, if in the emp table there was at most one employee in each department, then deptno would be unique in the result of a join of emp and dept, but dept would still not be a key-preserved table.

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