将Oracle SQL查询迁移到PostgreSql

Question

I have a simple Oracle query:

SELECT SEQ_01.NEXTVAL FROM DUAL

I want write this query in SQL standard for run it in both DB

Answer 1

The equivalent of Oracle's select seq_01.nextval from dual is:

select nextval('seq_01');

More details in the manual: http://www.postgresql.org/docs/current/static/functions-sequence.html

There is no way you can write a SQL statement that works in both DBMS equally. Neither of them implements the syntax defined by SQL standard for sequences

---

Using your own functions you can actually make the same SQL work in both DBMS.

For Oracle

create or replace function seq_nextval(p_sequence_name varchar)
  return integer
as
  l_statement varchar(4000);
  l_value     integer;
begin 
  l_statement := 'select '||upper(p_sequence_name)||'.nextval from dual';
  execute immediate l_statement
    into l_value;
  return l_value;
end;
/  

select seq_nextval('seq_01')
from dual;

For Postgres

create table dual (dummy varchar(1));
insert into dual values ('X');

create function seq_nextval(p_sequence_name text)
  returns bigint
as
$$
   select nextval(p_sequence_name);
$$
language sql;

select seq_nextval('seq_01')
from dual;

---

Note that you pay the usual price for DBMS independent SQL: it runs equally slow on all platforms. Especially the Oracle workaround with PL/SQL and dynamic SQL is going to be massively slower than a plain seq_01.nextval call.