different results from query in java code and on oracle db server: but no connection error

Question

I have java code that connects to a remote oracle 11g EE db server. If i run a particular query in sqlplus it returns one result

SQL> SELECT COURSENAME from COURSES where skillID=1;

COURSENAME
--------------------
basic

But if I run the same query from the java code below it returns no results. I can copy the query syntax out of the query variable in the java debugger and running it on oracle so I know there is no syntax issue with the query. Also, it is not SQL exceptions or class not found exceptions so it seems to be running the query successfully -- just returning zero results.

What might be going on?

    private String getCourseForSkill(int skillID){
    try{
        Class.forName("oracle.jdbc.OracleDriver"); 
        String query="SELECT COURSENAME from COURSES where skillID=" + skillID ; 
        con = DriverManager.getConnection(url, user, password);
        Statement stmt = con.createStatement();
        rs = stmt.executeQuery(query);
        rs.next();
        return rs.getString("COURSENAME");
    }
    catch (ClassNotFoundException ex){
        System.out.println(ex.getMessage());
    }
    catch (SQLException ex) {
         System.out.println(ex.getMessage());
    }
    return null;
}

Answer 1

1. I think you're connecting to different Oracle instances, or more likely, as different Oracle users in the two cases

   @GreyBeardedGeek the URL looks like "jdbc:oracle:thin:@website:port:orcl I get to the manual query by doing ssh@website, authenticating and then running command=sqlplus

   Safer to run sqlplus <username>/<password>@<orainstancename>, because you can explicitly specify the oracle instance ID. In your case, it seems your program is using jdbc connection jdbc:oracle:thin:@website:port:orcl, so your orainstancename would be 'orcl' - just ensure that your tnsnames.ora file has the instance 'orcl' with the same 'port' as used by the jdbc connection

2. How to debug a little more

   Run the following code:

   con = DriverManager.getConnection(url, user, password);
   con.setAutoCommit(false);
   String insert="INSERT INTO COURSES (SKILLID, COURSE)"+ // can add other columns
                "values (?, ?) );"                       // add ? for other columns    
   PreparedStatement ps = con.createPreparedStatement();
   ps.setInt(1, 999);
   ps.setString(2, "Complete Bullwarks");
   // can set other columns
   ps.executeUpdate(insert);
   con.commit();
   

   NOW connect manually, re-run your original select statement & see if the added row is there. If no error in java and no new row in Oracle: extremely likely you're using 2 different Oracle instances/schemas.

   ALSO rerun your original java select code, but with SkillID=999 - extremely likely it will work.

Cheers

Answer 2

I had to do a commit to add the rows. When I typed commit; into the sql plus terminal then the remote jdbc connection could 'see' the rows. I am used to SQL server where you don't have to explicitly do these kinds of commits when using linq-to-sql or sql management studio.

Answer 3

It can be three issues.

1) skillID <> 1 in your Java code. Add debug and check.

2a) You are connecting to another database.

2b) You are connecting to the same database but SELECTING from a table in another schema.

To check 2a and 2b:

select user from dual; -- connect username

select name from v$database; -- database name

select host_name from v$instance; -- host name database is running on

This query returns all three into one result.

select user || '' || d.name || '' || i.host_name from v$database d, v$instance i;

Answer 4

Assuming you are actually connecting to the same database this is caused by not committing the INSERT in the sql*plus connection.

Oracle by default does not run in auto-commit mode when connecting via OCI (which sql*plus uses to connect). Any DML(INSERT ...) executed in sql*plus will not be visible to any other session until it is committed. This is because Oracle provides a read committed isolation level by default. The only thing visible to other users across sessions are write locks.

It doesn't matter if you connect the second connection via JDBC or OCI, it won't see the changes till you commit the first connection.

---

To test this out try opening 2 sql*plus connections and run the following:

-- Executing DDL in Oracle causes an implicit commit before and after the
-- command so the second connection will see the existence of this table:
CREATE TABLE foobar ( x VARCHAR(1) );

Execute this in connection #1 - you should get zero (we haven't inserted anything yet):

SELECT COUNT(*) FROM foobar;

Execute this in connection #2:

INSERT INTO foobar ( x ) VALUES ( 'A' );

Execute this in connection #1 - you should still get zero (INSERT is not committed so connection #1 cannot see it):

SELECT COUNT(*) FROM foobar;

Execute this in connection #2:

COMMIT;

Execute this in connection #1 - you should get 1 (it's committed now):

SELECT COUNT(*) FROM foobar;