Database Management Systems

1. Database Management Systems

2. NESTING OF QUERIES • Some queries require that existing values in the database be retrieved and then used in a comparison condition. • A complete SELECT query, called a nested query , can be specified within the WHERE-clause of another query, called the outer query • Many of the previous queries can be specified in an alternative form using nesting

3. Query 1 • Retrieve the name and address of all employees who work for the 'Research' department. • SELECT FNAME, LNAME, ADDRESS FROM EMPLOYEE, DEPARTMENT WHERE DNAME='Research' AND DNUMBER=DNO; • SELECT FNAME, LNAME, ADDRESS FROM EMPLOYEE WHERE DNO IN ( SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research' )

4. NESTING OF QUERIES • The nested query selects the number of the 'Research' department • The outer query select an EMPLOYEE tuple if its DNO value is in the result of either nested query • The comparison operator IN compares a value v with a set (or multi-set) of values V, and evaluates to TRUE if v is one of the elements in V • In general, we can have several levels of nested queries • A reference to an unqualified attribute refers to the relation declared in the innermost nested query • In this example, the nested query is not correlated with the outer query

5. Query 2 • Make a list of all project numbers for projects that involve an employee whose last name is 'Smith' as a worker or as a manager of the department that controls the project. • (SELECT PNUMBER FROM PROJECT, DEPARTMENT, EMPLOYEE WHERE DNUM=DNUMBER AND MGR_SSN=SSN AND LNAME='Smith')UNION (SELECT PNUMBER FROM PROJECT, WORKS_ON, EMPLOYEE WHERE PNUMBER=PNO AND ESSN=SSN AND LNAME='Smith')

6. Query 2 using nested queries SELECT distinct pnumber FROM project WHERE pnumber IN ( SELECT Pnumber FROM project , department , employee WHERE dnum=dnumber and mgr_ssn= ssn and lname='Smith') or pnumber IN ( SELECT pno FROM works_on , employee WHERE essn=ssn and lname='Smith')

7. Nested Queries • The first nested query selects the project numbers of projects that have a ‘Smith’ involved as manager • The second nested query selects the project numbers of projects that have a ‘Smith’ involved as a worker • In the outer query, we use the OR logical connective to retrieve a project tuple if the pnumber value of the tuple is in the result of either nested query

8. Nested Queries • SQL allows the use of tuples of values in comparisons by placing them within parentheses. To illustrate this, consider the following query: SELECT DISTINCT ESSN FROM WORKS_ON WHERE (PNO, HOURS) IN (SELECT PNO, HOURS FROM WORKS_ON WHERE ESSN='123456789') • This query will select the social security numbers of all employees who work in the same (project, hours) combination on same project that employee 'John Smith' (whose ssn = ‘123456789') works on.

9. Nested Queries • In addition to the IN operator, a number of other comparison operators can be used to compare single value v(typically an attribute name) to a set or multiset V (typically a nested query). The = ANY (or = SOME) operator returns TRUE if the value v is equal to some value in the set V and is hence equivalent to IN. The keywords ANY and SOME have the same meaning. Other operators that can be combined with ANY (or SOME) include >, >=,<,< = , and <>. The keyword ALL can also be combined with each of these operators

10. Query 3 • Retrieve the names of employees whose salary is greater than the salary of all the employees in department 5 SELECT LNAME, FNAME FROM EMPLOYEE WHERE SALARY > ALL (SELECT SALARY FROM EMPLOYEE WHERE DNO=5)

11. Nested Queries • In general, we can have several levels of nested queries. We can once again be faced with possible ambiguity among attribute names if attributes of the same name exist—one in a relation in the FROM clause of the outer query, and another in a relation in the FROM clause of the nested query. The rule is that a reference to an unqualified attribute refers to the relation declared in the innermost nested query.

12. CORRELATED NESTED QUERIES • If a condition in the WHERE-clause of a nested query references an attribute of a relation declared in the outer query , the two queries are said to be correlated • The result of a correlated nested query is different for each tuple (or combination of tuples) of the relation(s) the outer query

13. Query 4 • Retrieve the name of each employee who has a dependent with the same first name and same sex as the employee. SELECT E.FNAME, E.LNAME FROM EMPLOYEE E WHERE E.SSN IN (SELECT ESSN FROM DEPENDENT WHERE E.FNAME=DEPENDENT_NAME AND E.SEX=SEX)

14. CORRELATED NESTED QUERIES • Correlated subquery runs once for each row selected by the outer query. It contains a reference to a value from the row selected by the outer query. • Nested subquery runs only once for the entire nesting (outer) query. It does not contain any reference to the outer query row.

15. THE EXISTS FUNCTION • EXISTS is used to check whether the result of a correlated nested query is empty (contains no tuples) or not. Also we can use NOT EXISTS • Query 5 : Retrieve the name of each employee who has a dependent with the same first name as the employee. SELECT FNAME, LNAME FROM EMPLOYEE WHEREEXISTS (SELECT * FROM DEPENDENT WHERE SSN=ESSN AND FNAME=DEPENDENT_NAME)

16. Query 5 • List the names of employees who have no dependents SELECT fname, lname FROM employee WHERE NOT EXISTS ( SELECT * FROM dependent WHERE ssn=ESSN)

17. Query 6 • Retrieve the name of each employee who works on all projects controlled by department number 5 SELECT lname , fname FROM employee where not exists (SELECT * FROM works_on B where (B.PNO in (select Pnumber from project where dnum=5)) AND NOT Exists ( select * from works_on c where c.essn=ssn and c.pno =B.pno))