Difference between Primary Key and Unique+Not NUll

Question

Hi Guys,
Is there any difference the column being declared as primary key or unique+NOT NULL.
Please let me know the internal and application point of view.
Thanks in advance!
Ranjan

Answer 1

957590 wrote:
Ok,Thanks however I donot think Primary key uses unique indexPK uses whatever index you specify - unique or not (as long as index is on proper column(s)). However, if you do not specify any index, PK creates unique index:
SQL> create table tbl(
  2                   id number,
  3                   name varchar2(10)
  4                  )
  5  /
Table created.
SQL> alter table tbl
  2    add constraint tbl_pk
  3      primary key(
  4                  id
  5                 )
  6  /
Table altered.
SQL> select  index_name,
  2          uniqueness
  3    from  user_indexes
  4    where table_name = 'TBL'
  5  /
INDEX_NAME                     UNIQUENES
TBL_PK                         UNIQUE
SQL> select  constraint_name,
  2          index_name
  3    from  user_constraints
  4    where table_name = 'TBL'
  5  /
CONSTRAINT_NAME                INDEX_NAME
TBL_PK                         TBL_PK
SQL>  alter table tbl
  2    drop primary key
  3  /
Table altered.
SQL> create index tbl_non_unique_pk_index
  2    on tbl(
  3           id
  4          )
  5  /
Index created.
SQL> alter table tbl
  2    add constraint tbl_pk
  3      primary key(
  4                  id
  5                 )
  6        using index tbl_non_unique_pk_index
  7  /
Table altered.
SQL> select  index_name,
  2          uniqueness
  3    from  user_indexes
  4    where table_name = 'TBL'
  5  /
INDEX_NAME                     UNIQUENES
TBL_NON_UNIQUE_PK_INDEX        NONUNIQUE
SQL> select  constraint_name,
  2          index_name
  3    from  user_constraints
  4    where table_name = 'TBL'
  5  /
CONSTRAINT_NAME                INDEX_NAME
TBL_PK                         TBL_NON_UNIQUE_PK_INDEX
SQL> insert
  2    into tbl
  3    values(
  4           1,
  5           'A'
  6          )
  7  /
1 row created.
SQL> insert
  2    into tbl
  3    values(
  4           2,
  5           'B'
  6          )
  7  /
1 row created.
SQL> insert
  2    into tbl
  3    values(
  4           1,
  5           'C'
  6          )
  7  /
insert
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.TBL_PK) violated
SQL> SY.