52.  What is a query?

            A query with respect to DBMS relates to user commands that are used to interact with a data base. The query language can be classified into data definition language and data manipulation language.

 

53.  What do you mean by Correlated subquery?

            Subqueries, or nested queries, are used to bring back a set of rows to be used by the parent query. Depending on how the subquery is written, it can be executed once for the parent query or it can be executed once for each row returned by the parent query. If the subquery is executed for each row of the parent, this is called a correlated subquery.

A correlated subquery can be easily identified if it contains any references to the parent subquery columns in its WHERE clause. Columns from the subquery cannot be referenced anywhere else in the parent query. The following example demonstrates a non-correlated subquery.

 E.g. Select * From CUST Where ’10/03/1990′ IN (Select ODATE From ORDER Where CUST.CNUM = ORDER.CNUM)

 

54.  What are the primitive operations common to all record management systems?

Addition, deletion and modification.

 

55.  Name the buffer in which all the commands that are typed in are stored

            ‘Edit’ Buffer

 

56.  What are the unary operations in Relational Algebra?

            PROJECTION and SELECTION.

 

57.  Are the resulting relations of PRODUCT and JOIN operation the same?

            No.

            PRODUCT: Concatenation of every row in one relation with every row in another.

            JOIN: Concatenation of rows from one relation and related rows from another.

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