add data into primary key

Question

this is my table.In this table i want to add a primary key column name "emp_id" as the first column .I don't know how to do it .So,can you please help me!

EMP_NAME             EMP_POS            SALARY           GENDER 
----------------- ----------------- --------------      ------ 
anand              worker             10000                 M      
balu               manager            50000                 M      
carl               manager            50000                 M      
riya               md                 60000                 F      
prabhu             owner              99999999              M      

Answer 1

The old way of doing this is a multi-step process:

• add the column which will be the primary key
• update the column
• enforce the primary key.

Something like this:

create sequence t23_id;

alter table t23 add id number;

update t23 
set id = t23_id.nextval
;
     
alter table t23 add constraint t23_pk primary key (id);

In 12c Oracle added Identity columns (like SQL Server auto-incrementing columns). This reduces the number of steps to two:

alter table t23i add id number GENERATED ALWAYS AS IDENTITY primary key;

alter table t23i add constraint t23i_pk primary key (id);

Unfortunately it can't be done in one step. This ...

alter table t23i add id number GENERATED ALWAYS AS IDENTITY primary key;  

...hurls ...

ORA-01758: table must be empty to add mandatory (NOT NULL) column

Livesql demo