MySQL Modify field type or length

mysql> ALTER TABLE name modify column field name type;
For example

The Address Table City field in the database is varchar (30)

Modification types can be used (careful modification of types may result in errors in existing data)

mysql> ALTER TABLE address modify column City char (+);

Modified length can be used (modify the length, to ensure that no short and existing data to ensure that the original data is not error)

mysql> ALTER TABLE address modify column city varchar (a);

MySQL Modify field type or length