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Adding Columns  

If you use the ADD clause to add a new column to the table, the initial value of each row for the new column is null. You can add a column with a NOT NULL constraint only to a table that contains no rows.       

If you create a view with a query that uses the asterisk (*) in the select list to select all columns from the base table, and you subsequently add columns to the base table, Oracle will not automatically add the new column to the view. To add the new column to the view, you can re-create the view using the CREATE VIEW command with the OR REPLACE option.       

Operations performed by the ALTER TABLE command can cause Oracle to invalidate procedures and stored functions that access the table.

Modifying Column Definitions  

You can use the MODIFY clause to change any of the following parts of a column definition:

  •  Datatype

  •  Size

  •  Default value

  •  NOT NULL column constraint   

The MODIFY clause need only specify the column name and the modified part of the definition, not the entire column definition.

Datatypes and Sizes  

You can change:

  •  A CHAR column to VARCHAR2 (or VARCHAR).

  •  A VARCHAR2 (or VARCHAR) to CHAR, but only if the column contains nulls in all rows or if you do not attempt to change the column size.

  •  Any column’s datatype, or decrease any column’s size, if all rows for the column contain nulls. You can always increase the size of a character or raw column or the precision of a numeric column.

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