If we want to see the structure of a table we have to use the DESC keyword. Here, structure means column names , column type and each column size of a table. See the below syntax .

Syntax :
DESC table_name;
 
Example :

DESC student ;

Output
Name        Null Type          
-------------- ---- ------------- 
STUDENT_ID          NUMBER(10)    
STUDENT_NAME        VARCHAR2(100) 
STUDENT_MOB_NO      VARCHAR2(13)  
STUDENT_ADREES      VARCHAR2(200) 

Here null column represents whether columns accept null value or not . By default each column accepts null value.