Question

1. Explain the problems that may occur when the DBMS does not support the concept of null.

2. What does it mean that a catalog is self-describing? What does the following statement mean?
(( TABLES JOIN COLUMNS ) WHERE COLCOUNT < 3 ) [TABNAME, COLNAME]

Answer 1

1.Answer: Null means no entry. When data not present in database we used null concept. If DBMS does not support the null concept then there is difficult to distinguish the weather the data is present into database or not. If data present into database its show the value or not null . If data does not present into database its show the Null.

2.Answer: Catlog is set tables in database that contain the information about database. Catlog contain itself information of database. for example there is "tables " table in the catlog that contain one row for every table in database. Catlog is self-describing means its meta data , data about data. In DBMS catlog store the particular information. The "self describing" nature of the database refers to the database's information of the entity within the database as reflected in the contents of its catalog.

(( TABLES JOIN COLUMNS ) WHERE COLCOUNT < 3 ) [TABNAME, COLNAME] Means:

There are two tables "TABLES" and "COLUMNS" we are joint this two tables by TABNAME and COLNAME but when the COLCOUNT is less than 3 there apply the condtion. TABNAME and  COLNAME are tow columns which present in respective tables.