You can convert only datatypes that are either implicitly or explicitly convertible to the new datatype, or if there is an explicit conversion function in Transact-SQL. See Chapter 1, “System and User-Defined Datatypes,” in the Reference Manual: Building Blocks for a list of the supported datatype conversions. If you attempt an illegal datatype modification, Adaptive Server raises an error message and the operation is aborted.
You cannot convert an existing column datatype to the timestamp datatype,
nor can you modify a column that uses the timestamp datatype
to any other datatype.
If you issue the same alter table...modify command more than once, Adaptive Server issues a message similar to this:
Warning: ALTER TABLE operation did not affect column ‘au_lname’. Msg 13905, Level 16, State 1: Server ‘SYBASE1’, Line 1: Warning: no columns to drop, add or modify. ALTER TABLE ‘authors’ was aborted.
