How to add column with specific condition for oracle DB

How do add a field – DOC with Date type to the Registration table. Also DOC (Date of Completion) should be greater than DOJ (Date of joining).

I wrote this query but it is showing error : Column check constraint cannot reference other column

ALTER TABLE Registration add DOC date CHECK(DOC > DOJ);

Answer

Add both the DOC column and the CHECK constraint with a single ALTER TABLE statement (see DBfiddle here) :

-- table for testing
create table atable( doj date ) ;


alter table atable
add (
  doc date
, constraint checkdoc check( doc > doj ) -- cannot be coded *inline*
);


-- testing -----------------------------------------------------
-- this insert must succeed
insert into atable( doj, doc ) values ( sysdate, sysdate + 1 );

-- this insert must fail due to the CHECK constraint
insert into atable( doj, doc ) values ( sysdate + 1, sysdate ) ;
-- ORA-02290: check constraint (...CHECKDOC) violated


-- table/data --------------------------------------------------
select * from atable ;
DOJ         DOC
03-JUN-20   04-JUN-20