8. SQL Statements : ALTER TABLE : Alter Table... Alter Column Restrictions
 
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Alter Table... Alter Column Restrictions
It is not possible to ALTER TABLE…ALTER COLUMN...if any of the following conditions are met:
The column is or is part of the primary table structure
The column is or is part of an index
The column is used in table partitioning
The column is used in any of the following constraints:
Primary
Unique
Foreign Key
To resolve the problem caused by any of the above, it is necessary to:
For constraints: Drop the constraints, then alter the column, then replace the constraints.
For indexes: Drop the indexes, then alter the column, then replace the indexes.
For primary table structure: Modify the table to heap, then alter the column, then modify the table back to the required structure.
Note:  Any indexes and/or constraints on the table need to be handled accordingly.