使用单个alter table语句添加doc列和check约束（请参见此处的DBFIDLE）：

-- 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

您需要在表级别而不是列级别创建约束。
例如：

SQL> create table t(x number, doj date);

Table created.

SQL> alter table t add doc date;

Table altered.

SQL> alter table t add constraint check_doc check (doc > doj);

Table altered.

SQL>