Question

i’m just starting with SQL, i have an assignement to do that involves a relational scheme to SQL tables.

I have a big doubt on how to represent the following:
I have the table “Original” that is represented by the following SQL:

create table Original (
  idB       char(10)      not null unique,
  tituloM   varchar(255)  not null, 
  primary key (idB, tituloM),
  foreign key (idB, tituloM) references Musica on delete cascade on update cascade
);

I now have to represent the table “Live” that has the following relational representation:
Live(idB; tituloM; data; hora; tituloMO), where idB and tituloMO are foreign keys to “Original”. My doubt is, we don’t have a “tituloMO” in the “Original” table. How can i represent this ? Currently my “Original” table looks like this:

create table Live (
  idB      char(10)  not null unique,
  tituloM  varchar(255)  not null,
  /* date goes here */
  /* time goes here */
  tituloMO  varchar(255) not null,
  primary key (idB, tituloM),
  foreign key (idB, tituloM) references Musica on delete cascade on update cascade,
  foreign key (idB, data, hora) references Concerto on delete cascade on update cascade,
  foreign key (idB, tituloMO)
);

How can I represent the tituloMO field correctly?

Answer 1

When you make a FOREIGN KEY reference, you don’t need for the corresponding columns to have same names, only compatible datatypes.

So, your constraints would be:

create table Original (
  idB       char(10)      not null unique,
  tituloM   varchar(255)  not null, 
  primary key (idB, tituloM),
  foreign key (idB, tituloM) 
    references Musica (idB, tituloM)         --- you need these, too
      on delete cascade 
      on update cascade
);

create table Live (
  idB      char(10)  not null unique,
  tituloM  varchar(255)  not null,
  /* date goes here */
  /* time goes here */
  tituloMO  varchar(255) not null,
  primary key (idB, tituloM),
  foreign key (idB, tituloM) 
    references Musica (idB, tituloM)         --- and these
      on delete cascade 
      on update cascade,
  foreign key (idB, data, hora) 
    references Concerto (idB, data, hora)    --- and these
      on delete cascade 
      on update cascade,
  foreign key (idB, tituloMO)
    references Original (idB, tituloM)       --- this is an assumption
      on delete cascade                      --- based on the datatypes
      on update cascade,
);