Usage of using clause in kingbases alter table

using Clause is used when modifying the table field type , Type of conversion for display .

1. Build table

create table t(id integer);

2. insert data

insert into t select generate_series(1,10);

3. hold id Change the column type to varchar

test=# alter table t alter id type varchar;
ALTER TABLE

 because integer turn varchar There are implicit conversions , So it can be automatically converted .

4. hold id Change column type back integer

test=#  alter table t alter id type integer;
 error :   Field  "id"  Cannot be automatically converted to type  integer
 Tips :   You may need to specify "USING id::integer".

 stay oracle Under Mode varchar To integer Of cast function , So there's no mistake . The above error is in pg Generated in mode .

5. Use Using Clause to cast

test=# alter table t alter id type integer using id::integer;
ALTER TABLE

When converting types, there is implicit type conversion , It will automatically switch , without , Then you have to use using Specify the conversion rules .