Page 59 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Database Management Systems/Managing Database

Notes Query (b): List the number of employee in the company
Solution:
SELECT COUNT (*)
FROM Employee E.
Query (c): List the number of employees who are working on project number 44
Solution:
SELECT COUNT(*)
FROM Employee E, Department D

WHERE E.DNo = D.DNo AND
D.PNo = 44.
Query (d): Find the name and age of the eldest employee

Solution:
SELECT E.ename, E.age
FROM Employee E

WHERE E.age = (SELECT MAX(E2.age)
FROM employees E2)
(OR)

The above query can also be written as
SELECT E.ename, E.age
FROM Employee E

WHERE (SELECT MAX (E2.age)
FROM Employees E2 = E.age). Values

Task Discuss UNION operations.

3.11 Null Values

Null means nothing. Null values are those values which are assigned to an attribute if its value
is unknown or is not applicable. Null values are used by DBMS when the user does not know the
type information to be entered for a particular field, so the user doesn’t enter any data and DBMS
assigns that field a NULL value. A NULL value does not represent a zero or spaces it just means
the absence of value for that field and this value can be inserted later. For example in the
employee table it is not necessary that all the employees should have a phone number, so for the
employer who does not have one yet, a NULL value can be assigned for that. When we insert a
tuple (124, ‘Kelly’, 6, NULL, 26, NULL) in employer table then for the attribute e.sal and phone
NULL values are assigned. Assignment of NULL value simply indicates that the value is not
known or is inapplicable for that attribute. A field which is not declared as NOT NULL can have
NULL values. But this inclusion of special type of value may result in the complications of other
operations. NULL values are used to deal with the exceptional data or the data that is not

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