Under what conditions can the ALTER TABLE command fail to successfully modify a table?

The ALTER TABLE command may not work in the following cases:

• If the table is locked by another transaction or process.
• If you try to change a structure that violates constraints (for example, adding a NOT NULL column without a default value).
• If the syntax of the command is incorrect or not supported by the DBMS.
• When attempting to change a column's type to one incompatible with the current data.
• If there are dependencies in the table (such as foreign keys) that prevent structural changes.
• If the user lacks the permissions to modify the table.

Example: you cannot simply change a column's type from VARCHAR to INT if the column contains data that cannot be converted to a number.