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
.
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