Create Table Examples

Easiest example:

SQL> CREATE TABLE libros( titulo VARCHAR2(20));

Tabla creada.

SQL>

a more complex one, with a primary key definition:

CREATE TABLE news
( news_id            NUMBER PRIMARY KEY,
  news_title          VARCHAR2(30),
  news_text VARCHAR2(254))

Now we can begin populating the data:
insert data oracle

Also we can alter table estructure:

SQL> create table employee (EMP_NO NUMBER(2), JOB_GRADE VARCHAR2(20), EMP_NAME V
ARCHAR2(20), EMP_SNAME VARCHAR2(20) );

Tabla creada.

SQL>
SQL> ALTER table employee ADD ( EMP_DESC VARCHAR2(50) );

Tabla modificada.

SQL>

We have the following table, and you want to avoid null data at least at the ID COL:

SQL> describe employee;
 Nombre                                    ┐Nulo?   Tipo
 ----------------------------------------- -------- -----------------------

 EMP_NO                                             NUMBER(4)
 JOB_GRADE                                          VARCHAR2(20)
 EMP_NAME                                           VARCHAR2(20)
 EMP_SNAME                                          VARCHAR2(20)

SQL> alter table employee modify EMP_NO NUMBER(4) NOT NULL;
alter table employee modify EMP_NO NUMBER(4) NOT NULL
*
ERROR en lÝnea 1:
ORA-02296: no se puede activar (SYSTEM.) - se han encontrado valores nulos

SQL>

You can fix removing the null inserted rows:
SQL> select * from employee where EMP_NO is null;

    EMP_NO JOB_GRADE            EMP_NAME             EMP_SNAME
---------- -------------------- -------------------- --------------------

SQL> DELETE from employee where EMP_NO is null;
1 fila suprimida.

SQL> select * from employee where EMP_NO is null;
ninguna fila seleccionada
SQL>

Now you can alter the table.
SQL> ALTER TABLE employee RENAME COLUMN JOB_GRADE to "EMP_GRADE";

Tabla modificada.

SQL>