left-vs-inner.sql

CREATE TABLE upMsg (
    id         int,
    msg        varchar(31)
);
insert into upMsg values (1, "Foo");
insert into upMsg values (2, "Bar");

CREATE TABLE dnMsg (
    ni		int not null auto_increment primary key,
    id      int not null,
    reply   varchar(30)
) ENGINE=InnoDB;
insert into dnMsg values (null, 1, "whatever, OK");

CREATE TABLE drMsg (
    ri		   int not null auto_increment primary key,
    id         int not null,
    error      varchar(63)
) ENGINE=InnoDB;
insert into drMsg values (null, 2, 'no, no, no');
insert into drMsg values (null, 2, null);

select * from upMsg
left join drMsg using(id);