create table test(id varchar );
insert into test values('1');
insert into test values('11');
insert into test values('12');
select * from test
--Result--
id
character varying
--------------------------
1
11
12
You can see from the above table that I have used the data type – character varying for id
column. But it was a mistake because I am always giving integers as id. So using varchar here is a bad practice. So let’s try to change the column type to integer.
ALTER TABLE test ALTER COLUMN id TYPE integer;
But it returns:
ERROR: column “id” cannot be cast automatically to type integer SQL
state: 42804 Hint: Specify a USING expression to perform the
conversion
That means we can’t simply change the data type because data is already there in the column. Since the data is of type character varying Postgres can't expect it as integer though we entered integers only. So now, as Postgres suggested we can use the USING expression to cast our data into integers.
ALTER TABLE test ALTER COLUMN id TYPE integer USING (id::integer);
It Works.
So you should use
alter table a.attend alter column terminal TYPE INTEGER USING (terminal::integer) ;
