alter table emp1 add constraint pk_emp1_key primary key(empno) ;

select index_name, last_analyzed, num_rows, sample_size from user_indexes where table_name = 'EMP1' ;

INDEX_NAME                     LAST_ANALY   NUM_ROWS SAMPLE_SIZE
------------------------------ ---------- ---------- -----------
NUI_EMP1_EMPNO_ENAME           11/02/2007        107         107

select constraint_name, constraint_type from user_constraints where table_name = 'EMP1' ;

CONSTRAINT_NAME                C
------------------------------ -
PK_EMP1_KEY                    P

DROP TABLE NEW_EMP PURGE ;
CREATE TABLE NEW_EMP
(employee_id NUMBER(6)
	PRIMARY KEY USING INDEX (CREATE INDEX emp_id_idx ON NEW_EMP(employee_id)),
 first_name  VARCHAR2(20),
 last_name   VARCHAR2(25));

select index_name, last_analyzed, num_rows, sample_size from user_indexes where table_name = 'NEW_EMP' ;
 
INDEX_NAME                     LAST_ANALY   NUM_ROWS SAMPLE_SIZE
------------------------------ ---------- ---------- -----------
EMP_ID_IDX