Transactions:
----------------

- Atomicity:

begin;
   transaction
commit;  --rollback



Constraints:
---------------

Integrity rules for the database.

Constraint types:

- Database level (Assertions)

- Table level:

         - Primary key
	 - Foreign Key
	 - Unique
	 - Not Null
	 - Check (attribute level of table level)


create table abc (
     vala   varchar(10) primary key
     , valb   int
) ;

create table xyz(
    id int 
    , val1  varchar(10) 
    , val2  varchar(10) not null
    , val3  int
    , val4  int
    , primary key (id)    --not null implicitly true
    , foreign key (val1) references abc(vala)
      on delete cascade
    , foreign key (val2) references def(vald)
    , unique (val2, val3)
    , check (val1 > 4)
    , check (val1 <> 0 or val2 < 5)
) ;

-- If any constraint violated: the transaction causing the change is
   aborted, all changes are rolled back.



drop table b;
drop table a;

create table a (id1 int primary key, id3 int) ;
create table b (id2 int primary key
     , id1 int not null
     , foreign key (id1) references a(id1)
       on delete set null
       on update cascade);

insert into a values(1, 41);
insert into a values(2, 42);
insert into a values(4, 44);

insert into b values(21, 1);
insert into b values(22, 1);
insert into b values(24, 2);



--> restrict (default): if a tuple references the updated/deleted tuple, then the update/delete is rolled back.

--> cascade: cascade the change