Data Definition

              
 Data Definition


                  DDL allows database objects such as schemas,domain,tables,views and indexes to be created and destroyed.

The main sql DDL statements are:

CREATE SCHEMA                                                   DROP

SCHEMA

CREATE DOMAIN          ALTER DOMAIN         DROPDOMAIN

CREATE TABLE             ALTER TABLE             DROP TABLE

CREATE VIEW                                                        DROP VIEW
     
Creating a Database

           
                                                                          This diffres from product.According to the ISO stand relations and other database objets exists in an enviroment .Among other things,Objects exits in an enviroment.Among other things,each enviroment consists  of one or more catalogs , and catalog consists  of a set of schemas.A schemas is a named collection of database  objects there are in some way related to one another.The objects in a character sets.A schema can be created and destroyed  in the following manner.

Create SCHEME[Name |  Authorization creatorindentifier]

Drop SCHEME Name[RESTRICT | CASCADE]

Eg:CREATE SCHEMA Test AuthoRIzation praneeth;



 Pattern match

                Test  whether a strings maches a specified condition.SQL has 2 speacial pattern-matching symbols;

• % - represents any sequence of zero/more characters
• _   - underscore represents any single characters

• LIKE 'k%' means that 1 character must be,but the rest of it can be anythings

•  LIKE 'M_ _ _' means that thare must be extacly 4 characters in the sting,and should start  with m

• LIKE '%w' means any sequence of characterss of length at least 1,with the last character being w
• LIKE '%GLASSGOW%' means a sequens of characters of any length containing Glassgow
• NOT LIKE 'H%' means the 1st character cannot be an H

Example


SELECT WHERE  ClintNo= '' or by any other logical operator such as '<>'


 NULL
                 test whether a column has a null(unknwon) value.In some cases, a value in a table will not cantain any value.Then,you will not be able to refer to that particular row in the following manner.

SELECT...

WHERE clientNO =  ''

or

by any other logical operator such as '<>'

             This is becouse,a null value is considered to have an unknown value,so we cannot test whether it is equal or not equal to another string.instead,we have to test for null explicitly,using the keywords NULL/IS NOTNULL

SELECT...

WHERE clientNO IS NULL