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Understanding a key preserved table

Oracle Database Tips by Donald BurlesonDecember 27,  2015, Updated 9 June 2015

 

Question:  I'm a student studying key preserved table and I need to understand how a key preserved table works with a view. Can you please explain what a "key preserved" table and view might look like?

Answer:  In simple terms, a table is key preserved if the table key participates in the view as a key.  In short, a key-preserved table has its key columns preserved through a SQL join.  Also see the ORA-01752 error.

ORA-01752: cannot delete from view without exactly one key-preserved table

Cause: The deleted table either had no key preserved tables, had more than one key-preserved table, or the key-preserved table was an unmerged view or a table from a read-only view.

Action: Redefine the view or delete it from the underlying base tables.

For a simple example of a key preserved table, consider the one-to-many relationship between a CUSTOMER and an ORDERS table.

A view could be created CUSTOMER_ORDERS that displays all orders for the customer, using CUST_ID as a primary key and using CUST_ID as the key to find the ODERS for a customer.  In this example, the CUSTOMER table is said to be key preserved because the result of the join has a unique key CUST_ID.

If you want key preserved tables in a view, you must explicitly define the primary and foreign keys in the tables, or define unique indexes.  The docs note that if you want a view to be inherently updatable, it must have special treatment for key preserved tables. 

  • For an UPDATE statement, all columns updated must be extracted from a key-preserved table. If the view has the CHECK OPTION, join columns and columns taken from tables that are referenced more than once in the view must be shielded from UPDATE.  

  • For an INSERT statement, all columns into which values are inserted must come from a key-preserved table, and the view must not have the CHECK_OPTION.

 



 

 

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