Explain DDL Commands

Create table with different attributes like Primary,Not Null, Unique.
Study Alter, Delete, Drop commands
create table employee
(
EID varchar2(10) not null Primary key,
EName varchar2(35) not null,
ENo varchar2(10) unique,
EGender varchar2(1) check( EGender in ('M','F','m','f'))
);
Table created.
---------------
insert into employee values('0431152706','Durgesh Chaudhary','0431152706','M');
1 row(s) inserted
---------------
insert into employee values('0431152706','Abhi Garg','0541152706','M');
ORA-00001: unique constraint (SCOTT.SYS_C004075) violated //because of duplicate EID
---------------
insert into employee values('','Abhi Garg','0541152706','M');
ORA-01400: cannot insert NULL into ("SCOTT"."EMPLOYEE"."EID") // because of NULL EID
---------------
insert into employee values('0541152706','Abhi Garg','0541152706','M');
1 row(s) inserted
---------------
insert into employee values('0291152706','Abhi Garg','','M');
1 row(s) inserted // NULL value accepted
---------------
insert into employee values('0281152706','Ani Tuteja','','G');
ORA-02290: check constraint (SCOTT.SYS_C004074) violated // because of check in EGender
---------------
insert into employee values('0281152706','Ani Tuteja','','F');
1 row(s) inserted.
---------------
select * from employee;
EID ENAME ENO EGENDER
541152706 Abhi Garg 541152706 M
431152706 Durgesh Chaudhary 431152706 M
291152706 Abhi Garg - M
281152706 Ani Tuteja - F
---------------
desc employee;
Column Data Type Length Precision Scale Primary Key Nullable Default Comment
EID Varchar2 10 - - 1 - - -
ENAME Varchar2 35 - - - - - -
ENO Varchar2 10 - - - - Yes
EGENDER Varchar2 1 - - - - Yes
---------------
alter table employee modify(ename varchar2(50));
Table altered.
---------------
alter table employee add(Address varchar2(100) default('C-137, Inder Enclave'));
Table altered.
---------------
alter table employee add(TelNo varchar2(10) default('9899024447'));
Table altered.
---------------
desc employee;
Column Data Type Length Precision Scale Primary Key Nullable Default Comment
EID Varchar2 10 - - 1 - - -
ENAME Varchar2 50 - - - - - -
ENO Varchar2 10 - - - Yes - -
EGENDER Varchar2 1 - - - Yes - -
ADDRESS Varchar2 100 - - - Yes ('C-137, Inder Enclave') -
TELNO Varchar2 10 - - - Yes ('9899024447') -
---------------
select * from employee;
EID ENAME ENO EGENDER ADDRESS TELNO
541152706 Abhi Garg 541152706 M C-137, Inder Enclave 9899024447
431152706 Durgesh Chaudhary 431152706 M C-137, Inder Enclave 9899024447
291152706 Abhi Garg - M C-137, Inder Enclave 9899024447
281152706 Ani Tuteja - F C-137, Inder Enclave 9899024447
---------------
alter table employee drop column address;
Table dropped.
---------------
delete from employee where eid='0281152706';
1 row(s) deleted.
---------------
select * from employee;
EID ENAME ENO EGENDER TELNO
541152706 Abhi Garg 541152706 M 9899024447
431152706 Durgesh Chaudhary 431152706 M 9899024447
291152706 Abhi Garg - M 9899024447
---------------