PRIMARY KEY Constraint

• A PRIMARY KEY is the candidate key that is used by the database designer for identifying an entity.
• There is no fiex rule for choosing the primary key, but generally it is the key that can be controlled by the user.
• Primary Key columns do not accept null values
• Primary Key columns do not accept duplicate values, that mean uniqueness of data.
• Oracle recommended Primary Key columns should be short and numeric.
• Primary Key constraint is a combination of NOT NULL+UNIQUE.

Restrictions :

• A table or view can have only one primary key.
• A composite primary key cannot have more than 32 columns.
• the same column or combination of columns cannot be declare as  primary key and a unique key.
• Primary key con't be implemented on columns having  LOB, LONG, VARRAY, NESTED TABLE, OBJECT, TIMESTAMP WITH TIME ZONE.

Syntax :

Column Level PRIMARY KEY Constraint :

CREATE TABLE <TABLE_NAME>
(COLUMN_NAME1 <DATA_TYPE>(WIDTH) ,
 COLUMN_NAME2 <DATA_TYPE>(WIDTH) CONSTRAINT Constraint_Name PRIMARY KEY,
 COLUMN_NAMEn <DATA_TYPE>(WIDTH));

Table Level PRIMARY KEY Constraint :

CREATE TABLE <TABLE_NAME>
(COLUMN_NAME1 <DATA_TYPE>(WIDTH) ,
 COLUMN_NAME2 <DATA_TYPE>(WIDTH),
 COLUMN_NAMEn <DATA_TYPE>(WIDTH),

 CONSTRAINT Constraint_Name PRIMARY KEY(Column_name));

Composite PRIMARY KEY Constraint (always declare in Table level):

CREATE TABLE <TABLE_NAME>
(COLUMN_NAME1 <DATA_TYPE>(WIDTH) ,
 COLUMN_NAME2 <DATA_TYPE>(WIDTH),
 COLUMN_NAMEn <DATA_TYPE>(WIDTH),

 CONSTRAINT Constraint_Name PRIMARY KEY(Column_name1,Column_name2));


See The Example:

PRIMARY KEY CONSTRAINT EXAMPLE