Creation of Document Type Definitions

Document Type Definition:

Specify Structure and Syntax of XML Document

<!ELEMENT DOCUMENT (CUSTOMER)*>                 
<!ELEMENT CUSTOMER (NAME,DATE,ORDERS)>          
<!ELEMENT NAME (LAST_NAME,FIRST_NAME)>            
<!ELEMENT LAST_NAME (#PCDATA)>                   
<!ELEMENT FIRST_NAME (#PCDATA)>                  
<!ELEMENT DATE (#PCDATA)>                       
<!ELEMENT ORDERS (ITEM)*>                       
<!ELEMENT ITEM (PRODUCT,NUMBER,PRICE)>          
<!ELEMENT PRODUCT (#PCDATA)>                    
<!ELEMENT NUMBER (#PCDATA)>                     
<!ELEMENT PRICE (#PCDATA)>