When you drop a column from a table, the views based on that table remain unchanged. That is, the database server does not automatically drop the corresponding columns from associated views.
The view is not automatically dropped because ALTER TABLE can change the order of columns in a table by dropping a column and then adding a new column with the same name. In this case, views based on the altered table continue to work, but retain their original sequence of columns.
If a view is invalidated when you alter the underlying table, you must rebuild the view by using the DROP VIEW and CREATE VIEW statements.
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