小蹦请进suppose u want to change column a in table t, from varchar(20) to numeric(20) and change its name from a to b, u can use ALTER TABLE t CHANGE a b numeric(20); value in the column will be truncated, if the memory of new type is not big enough to store this value; As for primary key, if u want to change its type from one to another, u should make sure that there won't be two duplicate value in that column after u do the transformation. otherwise, it will refuse to do the job

不行也。。。i use: ALTER TABLE t CHANGE a a numeric(20); got error: All parts of a PRIMARY KEY must be NOT NULL; If you need NULL in a key, use UNIQUE instead.. why? is it because the field 'a' is a primary key?

how come this error appear,...after u do the altering???? do u have any null value which is ur primary key ?

没有啊。。。id就是1, 10, 100, 101共四个。。。

不行也。。。i use: ALTER TABLE t CHANGE a a numeric(20); got error: All parts of a PRIMARY KEY must be NOT NULL; If you need NULL in a key, use UNIQUE instead.. why? is it because the field 'a' is a primary key?