How to add Primary Key in oracle?

What is Primary key?

Primary key is on single field or on multiple fields that is using for uniqueness of the record. One table always has one primary key.

Primary key at the stage of table creation:
CREATE TABLE GRN_MASTER
(
GRN_NUM       NUMBER,
GRN_DATE      DATE,
SUPPLIER_ID   NUMBER,
GATE_PASSS_ID NUMBER,
STORE_ID      NUMBER,
APPROVED_ID   NUMBER,
CONSTRAINTS PK_GRN_NUM PRIMARY KEY (GRN_NUM)
);

Primary key at the by using alter table:
ALTER TABLE GRN_MASTER
ADD CONSTRAINTS PK_GRN_NUM PRIMARY KEY (GRN_NUM);

Drop a Primary key:
ALTER TABLE GRN_MASTER
DROP CONSTRAINTS PK_GRN_NUM ;

Disabled a Primary key:
ALTER TABLE GRN_MASTER
DISABLE CONSTRAINTS PK_GRN_NUM;

Enabled a Primary key:
ALTER TABLE GRN_MASTER
ENABLE CONSTRAINTS PK_GRN_NUM;