1. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 24- 1

2. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Chapter 24 Enhanced Data Models for Advanced Applications

3. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 24- 3 Outline Recap of SQL Advanced SQL Active database & Triggers Assertion Views

4. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 4 Recap of SQL Queries A query in SQL can consist of up to six clauses, but only the first two, SELECT and FROM, are mandatory. The clauses are specified in the following order: SELECT FROM [WHERE ] [GROUP BY ] [HAVING ] [ORDER BY ] There are three SQL commands to modify the database: INSERT, DELETE, and UPDATE

5. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 5 Aliases In SQL, we can use the same name for two (or more) attributes as long as the attributes are in different relations A query that refers to two or more attributes with the same name must qualify the attribute name with the relation name by prefixing the relation name to the attribute name Example: EMPLOYEE.LNAME, DEPARTMENT.DNAME

6. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 6 ALIASES Some queries need to refer to the same relation twice In this case, aliases are given to the relation name Query 8: For each employee, retrieve the employee's name, and the name of his or her immediate supervisor. Q8:SELECTE.FNAME, E.LNAME, S.FNAME, S.LNAME FROM EMPLOYEE E S WHEREE.SUPERSSN=S.SSN In Q8, the alternate relation names E and S are called aliases or tuple variables for the EMPLOYEE relation We can think of E and S as two different copies of EMPLOYEE; E represents employees in role of supervisees and S represents employees in role of supervisors

7. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 7 ALIASES (contd.) Aliasing can also be used in any SQL query for convenience Can also use the AS keyword to specify aliases Q8:SELECTE.FNAME, E.LNAME, S.FNAME, S.LNAME FROM EMPLOYEE AS E, EMPLOYEE AS S WHEREE.SUPERSSN=S.SSN

8. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 8 USE OF DISTINCT SQL does not treat a relation as a set; duplicate tuples can appear To eliminate duplicate tuples in a query result, the keyword DISTINCT is used For example, the result of Q11 may have duplicate SALARY values whereas Q11A does not have any duplicate values Q11:SELECT SALARY FROMEMPLOYEE Q11A: SELECT DISTINCT SALARY FROMEMPLOYEE

9. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 9 SET OPERATIONS SQL has directly incorporated some set operations There is a union operation (UNION), and in some versions of SQL there are set difference (MINUS) and intersection (INTERSECT) operations The resulting relations of these set operations are sets of tuples; duplicate tuples are eliminated from the result The set operations apply only to union compatible relations; the two relations must have the same attributes and the attributes must appear in the same order

10. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 10 SET OPERATIONS (contd.) Query 4: 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. Q4:(SELECT PNAME FROMPROJECT, DEPARTMENT, EMPLOYEE WHEREDNUM=DNUMBER AND MGRSSN=SSN AND LNAME='Smith') UNION (SELECT PNAME FROMPROJECT, WORKS_ON, EMPLOYEE WHEREPNUMBER=PNO AND ESSN=SSN AND NAME='Smith')

11. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 11 NESTING OF QUERIES 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 Query 1: Retrieve the name and address of all employees who work for the 'Research' department. Q1:SELECTFNAME, LNAME, ADDRESS FROM EMPLOYEE WHEREDNO IN (SELECT DNUMBER FROMDEPARTMENT WHEREDNAME='Research' )

12. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 12 NESTING OF QUERIES (contd.) 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

13. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 13 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 Query 12: Retrieve the name of each employee who has a dependent with the same first name as the employee. Q12: SELECT E.FNAME, E.LNAME FROMEMPLOYEE AS E WHEREE.SSN IN (SELECT ESSN FROMDEPENDENT WHEREESSN=E.SSN AND E.FNAME=DEPENDENT_NAME)

14. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 14 CORRELATED NESTED QUERIES (contd.) In Q12, the nested query has a different result in the outer query A query written with nested SELECT... FROM... WHERE... blocks and using the = or IN comparison operators can always be expressed as a single block query. For example, Q12 may be written as in Q12A Q12A:SELECT E.FNAME, E.LNAME FROMEMPLOYEE E, DEPENDENT D WHEREE.SSN=D.ESSN AND E.FNAME=D.DEPENDENT_NAME

15. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 15 CORRELATED NESTED QUERIES (contd.) The original SQL as specified for SYSTEM R also had a CONTAINS comparison operator, which is used in conjunction with nested correlated queries This operator was dropped from the language, possibly because of the difficulty in implementing it efficiently Most implementations of SQL do not have this operator The CONTAINS operator compares two sets of values, and returns TRUE if one set contains all values in the other set Reminiscent of the division operation of algebra

16. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 16 CORRELATED NESTED QUERIES (contd.) Query 3: Retrieve the name of each employee who works on all the projects controlled by department number 5. Q3:SELECT FNAME, LNAME FROMEMPLOYEE WHERE ( (SELECTPNO FROMWORKS_ON WHERESSN=ESSN) CONTAINS (SELECTPNUMBER FROMPROJECT WHEREDNUM=5) )

17. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 17 CORRELATED NESTED QUERIES (contd.) In Q3, the second nested query, which is not correlated with the outer query, retrieves the project numbers of all projects controlled by department 5 The first nested query, which is correlated, retrieves the project numbers on which the employee works, which is different for each employee tuple because of the correlation

18. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Three Types of Subquery 1. A scalar subquery returns a single column and single row 2. A row subquery returns multiple columns but a single row 3. A table subquery returns one or more columns and multiple rows. Slide 24- 18

19. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Illegal Subqueries? 1)WHERE branchNo = (SELECT branchNo FROM Branch WHERE street = ‘163 Main St’); 2) WHERE salary > AVG(salary) Slide 24- 19

20. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Subquery Slide 24- 20 List properties handled by staff at ‘163 Main St’. SELECT propertyNo, street, city, postcode, type, rooms, rent FROM PropertyForRent WHERE staffNo IN (SELECT staffNo FROM Staff WHERE branchNo = (SELECT branchNo FROM Branch WHERE street = ‘163 Main St’));

21. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Subquery Slide 24- 21 List all staff whose salary is greater than the average salary, and show by how much. SELECT staffNo, fName, lName, position, salary – (SELECT AVG(salary) FROM Staff) As SalDiff FROM Staff WHERE salary > (SELECT AVG(salary) FROM Staff);

22. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Subquery select firstname, lastname, salary from employee e1 where (salary) in(select max(e2.salary) from employee e2 where e1.workdept = e2.workdept) order by salary desc Slide 24- 22

23. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Subquery select firstname, lastname, salary from employee e1 where (salary, workdept) in (select max(e2.salary), e2.workdept from employee e2 group by e2.workdept) order by salary desc Slide 24- 23

24. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 24 ANY and ALL ANY and ALL may be used with subqueries that produce a single column of numbers. With ALL, condition will only be true if it is satisfied by all values produced by subquery. With ANY, condition will be true if it is satisfied by any values produced by subquery. If subquery is empty, ALL returns true, ANY returns false. SOME may be used in place of ANY.

25. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 25 Example 5.22 Use of ANY/SOME Find staff whose salary is larger than salary of at least one member of staff at branch B003. SELECT staffNo, fName, lName, position, salary FROM Staff WHERE salary > SOME (SELECT salary FROM Staff WHERE branchNo = ‘B003’);

26. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 26 Example 5.23 Use of ALL Find staff whose salary is larger than salary of every member of staff at branch B003. SELECT staffNo, fName, lName, position, salary FROM Staff WHERE salary > ALL (SELECT salary FROM Staff WHERE branchNo = ‘B003’);

27. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 27 THE EXISTS FUNCTION EXISTS is used to check whether the result of a correlated nested query is empty (contains no tuples) or not We can formulate Query 12 in an alternative form that uses EXISTS as Q12B

28. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 28 THE EXISTS FUNCTION (contd.) Query 12: Retrieve the name of each employee who has a dependent with the same first name as the employee. Q12B: SELECT FNAME, LNAME FROMEMPLOYEE WHEREEXISTS (SELECT* FROMDEPENDENT WHERESSN=ESSN AND FNAME=DEPENDENT_NAME)

29. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 29 THE EXISTS FUNCTION (contd.) Query 6: Retrieve the names of employees who have no dependents. Q6:SELECT FNAME, LNAME FROMEMPLOYEE WHERENOT EXISTS (SELECT* FROM DEPENDENT WHERE SSN=ESSN) In Q6, the correlated nested query retrieves all DEPENDENT tuples related to an EMPLOYEE tuple. If none exist, the EMPLOYEE tuple is selected EXISTS is necessary for the expressive power of SQL

30. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 30 EXPLICIT SETS It is also possible to use an explicit (enumerated) set of values in the WHERE- clause rather than a nested query Query 13: Retrieve the social security numbers of all employees who work on project number 1, 2, or 3. Q13:SELECT DISTINCT ESSN FROMWORKS_ON WHEREPNO IN (1, 2, 3)

31. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 31 NULLS IN SQL QUERIES SQL allows queries that check if a value is NULL (missing or undefined or not applicable) SQL uses IS or IS NOT to compare NULLs because it considers each NULL value distinct from other NULL values, so equality comparison is not appropriate. Query 14: Retrieve the names of all employees who do not have supervisors. Q14:SELECT FNAME, LNAME FROMEMPLOYEE WHERESUPERSSN IS NULL Note: If a join condition is specified, tuples with NULL values for the join attributes are not included in the result

32. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 32 Joined Relations Feature in SQL2 Can specify a "joined relation" in the FROM- clause Looks like any other relation but is the result of a join Allows the user to specify different types of joins (regular "theta" JOIN, NATURAL JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN, CROSS JOIN, etc)

33. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 33 Joined Relations Feature in SQL2 (contd.) Examples: Q8:SELECTE.FNAME, E.LNAME, S.FNAME, S.LNAME FROM EMPLOYEE E S WHEREE.SUPERSSN=S.SSN can be written as: Q8:SELECTE.FNAME, E.LNAME, S.FNAME, S.LNAME FROM (EMPLOYEE E LEFT OUTER JOIN EMPLOYEES ON E.SUPERSSN=S.SSN)

34. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 34 Joined Relations Feature in SQL2 (contd.) Examples: Q1:SELECTFNAME, LNAME, ADDRESS FROM EMPLOYEE, DEPARTMENT WHEREDNAME='Research' AND DNUMBER=DNO could be written as: Q1:SELECTFNAME, LNAME, ADDRESS FROM (EMPLOYEE JOIN DEPARTMENT ON DNUMBER=DNO) WHEREDNAME='Research’ or as: Q1:SELECTFNAME, LNAME, ADDRESS FROM (EMPLOYEE NATURAL JOIN DEPARTMENT AS DEPT(DNAME, DNO, MSSN, MSDATE) WHEREDNAME='Research’

35. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 35 Joined Relations Feature in SQL2 (contd.) Another Example: Q2 could be written as follows; this illustrates multiple joins in the joined tables Q2:SELECT PNUMBER, DNUM, LNAME, BDATE, ADDRESS FROM(PROJECT JOIN DEPARTMENT ON DNUM=DNUMBER) JOIN EMPLOYEE ON MGRSSN=SSN) ) WHERE PLOCATION='Stafford’

36. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Joining SQL provides alternative ways to specify joins: FROM Client c JOIN Viewing v ON c.clientNo = v.clientNo FROM Client JOIN Viewing USING clientNo FROM Client NATURAL JOIN Viewing In each case, FROM replaces original FROM and WHERE. However, first produces table with two identical clientNo columns. Slide 24- 36

37. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Join Three Tables Alternative formulation for FROM and WHERE: SELECT b.branchNo, b.city, s.staffNo, fName, lName, propertyNo FROM Branch b, Staff s, PropertyForRent p WHERE b.branchNo = s.branchNo AND s.staffNo = p.staffNo ORDER BY b.branchNo, s.staffNo, propertyNo; FROM (Branch b JOIN Staff s USING branchNo) AS bs JOIN PropertyForRent p USING staffNo Slide 24- 37

38. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 38 Outer Joins If one row of a joined table is unmatched, row is omitted from result table. Outer join operations retain rows that do not satisfy the join condition. Consider following tables:

39. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 39 Outer Joins The (inner) join of these two tables: SELECT b.*, p.* FROM Branch1 b, PropertyForRent1 p WHERE b.bCity = p.pCity;

40. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 40 Outer Joins Result table has two rows where cities are same. There are no rows corresponding to branches in Bristol and Aberdeen. To include unmatched rows in result table, use an Outer join.

41. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 41 Example 5.28 Left Outer Join List branches and properties that are in same city along with any unmatched branches. SELECT b.*, p.* FROM Branch1 b LEFT JOIN PropertyForRent1 p ON b.bCity = p.pCity;

42. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 42 Example 5.28 Left Outer Join Includes those rows of first (left) table unmatched with rows from second (right) table. Columns from second table are filled with NULLs.

43. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 43 Example 5.29 Right Outer Join List branches and properties in same city and any unmatched properties. SELECT b.*, p.* FROM Branch1 b RIGHT JOIN PropertyForRent1 p ON b.bCity = p.pCity;

44. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 44 Example 5.29 Right Outer Join Right Outer join includes those rows of second (right) table that are unmatched with rows from first (left) table. Columns from first table are filled with NULLs.

45. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 45 Example 5.30 Full Outer Join List branches and properties in same city and any unmatched branches or properties. SELECT b.*, p.* FROM Branch1 b FULL JOIN PropertyForRent1 p ON b.bCity = p.pCity;

46. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 46 Example 5.30 Full Outer Join Includes rows that are unmatched in both tables. Unmatched columns are filled with NULLs.

47. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 47 AGGREGATE FUNCTIONS Include COUNT, SUM, MAX, MIN, and AVG Query 15: Find the maximum salary, the minimum salary, and the average salary among all employees. Q15:SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY) FROMEMPLOYEE Some SQL implementations may not allow more than one function in the SELECT-clause

48. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 48 AGGREGATE FUNCTIONS (contd.) Query 16: Find the maximum salary, the minimum salary, and the average salary among employees who work for the 'Research' department. Q16: SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY) FROMEMPLOYEE, DEPARTMENT WHEREDNO=DNUMBER AND DNAME='Research'

49. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 49 AGGREGATE FUNCTIONS (contd.) Queries 17 and 18: Retrieve the total number of employees in the company (Q17), and the number of employees in the 'Research' department (Q18). Q17:SELECT COUNT (*) FROMEMPLOYEE Q18:SELECT COUNT (*) FROMEMPLOYEE, DEPARTMENT WHEREDNO=DNUMBER AND DNAME='Research’

50. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 50 GROUPING In many cases, we want to apply the aggregate functions to subgroups of tuples in a relation Each subgroup of tuples consists of the set of tuples that have the same value for the grouping attribute(s) The function is applied to each subgroup independently SQL has a GROUP BY-clause for specifying the grouping attributes, which must also appear in the SELECT-clause

51. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 51 GROUPING (contd.) Query 20: For each department, retrieve the department number, the number of employees in the department, and their average salary. Q20:SELECT DNO, COUNT (*), AVG (SALARY) FROMEMPLOYEE GROUP BYDNO In Q20, the EMPLOYEE tuples are divided into groups- Each group having the same value for the grouping attribute DNO The COUNT and AVG functions are applied to each such group of tuples separately The SELECT-clause includes only the grouping attribute and the functions to be applied on each group of tuples A join condition can be used in conjunction with grouping

52. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 52 GROUPING (contd.) Query 21: For each project, retrieve the project number, project name, and the number of employees who work on that project. Q21:SELECT PNUMBER, PNAME, COUNT (*) FROMPROJECT, WORKS_ON WHEREPNUMBER=PNO GROUP BYPNUMBER, PNAME In this case, the grouping and functions are applied after the joining of the two relations

53. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 53 THE HAVING-CLAUSE Sometimes we want to retrieve the values of these functions for only those groups that satisfy certain conditions The HAVING-clause is used for specifying a selection condition on groups (rather than on individual tuples)

54. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 8- 54 THE HAVING-CLAUSE (contd.) Query 22: For each project on which more than two employees work, retrieve the project number, project name, and the number of employees who work on that project. Q22: SELECT PNUMBER, PNAME, COUNT(*) FROMPROJECT, WORKS_ON WHEREPNUMBER=PNO GROUP BYPNUMBER, PNAME HAVINGCOUNT (*) > 2

55. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe CASE expression Simple CASE expression: CASE input_expression WHEN expression THEN result_expression... ELSE result_expression END Searched CASE expression CASE WHEN Boolean_expression THEN result_expression... ELSE result_expression END Slide 24- 55

56. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe CASE in SELECT Clause SELECT product, 'Status' = CASE WHEN quantity > 0 THEN 'in stock‘ ELSE 'out of stock' END FROM dbo.inventory; Slide 24- 56

57. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe CASE in WHERE Clause select firstnme, lastname, from employee where 50000 < case job when 'PRES' then salary when 'MANAGER' then salary*1.03 when 'DESIGNER' then salary*1.05 when 'CLERK' then salary*1.07 else salary*1.10 end Slide 24- 57

58. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe CASE in ORDER BY Clause SELECT BusinessEntityID, SalariedFlag FROM HumanResources.Employee ORDER BY CASE SalariedFlag WHEN 1 THEN BusinessEntityID END DESC, CASE WHEN SalariedFlag = 0 THEN BusinessEntityID END; Slide 24- 58

59. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe CASE in HAVING Clause SELECT JobTitle, MAX(ph1.Rate) AS MaximumRate FROM Employee AS e JOIN EmployeePayHistory AS ph1 ON e.BusinessEntityID = ph1.BusinessEntityID GROUP BY JobTitle HAVING (MAX(CASE WHEN Gender = 'M' THEN ph1.Rate ELSE NULL END) > 40.00 OR MAX(CASE WHEN Gender = 'F' THEN ph1.Rate ELSE NULL END) > 42.00) ORDER BY MaximumRate DESC; Slide 24- 59

60. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Use CASE in UPDATE statement UPDATE Employee SET VacationHours = ( CASE WHEN ((VacationHours - 10.00) < 0) THEN VacationHours + 40 ELSE (VacationHours + 20.00) END ) WHERE SalariedFlag = 0; Slide 24- 60

61. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Be Careful!!! UPDATE Employee SET VacationHours = (CASE WHEN ((VacationHours - 10.00) < 0) THEN VacationHours + 40 ELSE (VacationHours + 20.00) END ) WHERE SalariedFlag = 0; Slide 24- 61 UPDATE Employee SET VacationHours = VacationHours + 40 WHERE SalariedFlag = 0 and (VacationHours - 10.00) < 0 ; UPDATE Employee SET VacationHours = VacationHours + 20.00 WHERE SalariedFlag = 0 and (VacationHours - 10.00) >= 0 ; Is it the same with a following set of update statements?

62. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 62 Views View Dynamic result of one or more relational operations operating on base relations to produce another relation. Virtual relation that does not necessarily actually exist in the database but is produced upon request, at time of request.

63. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 63 Views Contents of a view are defined as a query on one or more base relations. With view resolution, any operations on view are automatically translated into operations on relations from which it is derived. With view materialization, the view is stored as a temporary table, which is maintained as the underlying base tables are updated.

64. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 64 SQL - CREATE VIEW CREATE VIEW ViewName [ (newColumnName [,...]) ] AS subselect [WITH [CASCADED | LOCAL] CHECK OPTION] Can assign a name to each column in view. If list of column names is specified, it must have same number of items as number of columns produced by subselect. If omitted, each column takes name of corresponding column in subselect.

65. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 65 SQL - CREATE VIEW List must be specified if there is any ambiguity in a column name. The subselect is known as the defining query. WITH CHECK OPTION ensures that if a row fails to satisfy WHERE clause of defining query, it is not added to underlying base table. Need SELECT privilege on all tables referenced in subselect and USAGE privilege on any domains used in referenced columns.

66. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 66 Example 6.3 - Create Horizontal View Create view so that manager at branch B003 can only see details for staff who work in his or her office. CREATE VIEW Manager3Staff ASSELECT * FROM Staff WHERE branchNo = ‘B003’;

67. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 67 Example 6.4 - Create Vertical View Create view of staff details at branch B003 excluding salaries. CREATE VIEW Staff3 ASSELECT staffNo, fName, lName, position, sex FROM Staff WHERE branchNo = ‘B003’;

68. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 68 Example 6.5 - Grouped and Joined Views Create view of staff who manage properties for rent, including branch number they work at, staff number, and number of properties they manage. CREATE VIEW StaffPropCnt (branchNo, staffNo, cnt) AS SELECT s.branchNo, s.staffNo, COUNT(*) FROM Staff s, PropertyForRent p WHERE s.staffNo = p.staffNo GROUP BY s.branchNo, s.staffNo;

69. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 69 Example 6.3 - Grouped and Joined Views

70. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 70 SQL - DROP VIEW DROP VIEW ViewName [RESTRICT | CASCADE] Causes definition of view to be deleted from database. For example: DROP VIEW Manager3Staff;

71. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 71 SQL - DROP VIEW With CASCADE, all related dependent objects are deleted; i.e. any views defined on view being dropped. With RESTRICT (default), if any other objects depend for their existence on continued existence of view being dropped, command is rejected.

72. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 72 View Resolution Count number of properties managed by each member at branch B003. SELECT staffNo, cnt FROM StaffPropCnt WHERE branchNo = ‘B003’ ORDER BY staffNo;

73. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 73 View Resolution (a) View column names in SELECT list are translated into their corresponding column names in the defining query: SELECT s.staffNo As staffNo, COUNT(*) As cnt (b) View names in FROM are replaced with corresponding FROM lists of defining query: FROM Staff s, PropertyForRent p

74. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 74 View Resolution (c) WHERE from user query is combined with WHERE of defining query using AND: WHERE s.staffNo = p.staffNo AND branchNo = ‘B003’ (d) GROUP BY and HAVING clauses copied from defining query: GROUP BY s.branchNo, s.staffNo (e) ORDER BY copied from query with view column name translated into defining query column name ORDER BY s.staffNo

75. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 75 View Resolution (f) Final merged query is now executed to produce the result: SELECT s.staffNo, COUNT(*) FROM staff s, PropertyForRent p WHERE s.staffNo = p.staffNo AND branchNo = ‘B003’ GROUP BY s.branchNo, s.staffNo ORDER BY s.staffNo;

76. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 76 Restrictions on Views SQL imposes several restrictions on creation and use of views. (a) If column in view is based on an aggregate function: Column may appear only in SELECT and ORDER BY clauses of queries that access view. Column may not be used in WHERE nor be an argument to an aggregate function in any query based on view.

77. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 77 Restrictions on Views For example, following query would fail: SELECT COUNT(cnt) FROM StaffPropCnt; Similarly, following query would also fail: SELECT * FROM StaffPropCnt WHERE cnt > 2;

78. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 78 Restrictions on Views (b) Grouped view may never be joined with a base table or a view. For example, StaffPropCnt view is a grouped view, so any attempt to join this view with another table or view fails.

79. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 79 View Updatability All updates to base table reflected in all views that encompass base table. Similarly, may expect that if view is updated then base table(s) will reflect change.

80. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 80 View Updatability However, consider again view StaffPropCnt. If we tried to insert record showing that at branch B003, SG5 manages 2 properties: INSERT INTO StaffPropCnt VALUES (‘B003’, ‘SG5’, 2); Have to insert 2 records into PropertyForRent showing which properties SG5 manages. However, do not know which properties they are; i.e. do not know primary keys!

81. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 81 View Updatability If change definition of view and replace count with actual property numbers: CREATE VIEW StaffPropList (branchNo, staffNo, propertyNo) AS SELECT s.branchNo, s.staffNo, p.propertyNo FROM Staff s, PropertyForRent p WHERE s.staffNo = p.staffNo;

82. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 82 View Updatability Now try to insert the record: INSERT INTO StaffPropList VALUES (‘B003’, ‘SG5’, ‘PG19’); Still problem, because in PropertyForRent all columns except postcode/staffNo are not allowed nulls. However, have no way of giving remaining non-null columns values.

83. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 83 View Updatability ISO specifies the views that must be updatable in system that conforms to standard. A view is updatable if and only if: - DISTINCT is not specified. - Every element in SELECT list of defining query is a column name and no column appears more than once. - FROM clause specifies only one table, excluding any views based on a join, union, intersection or difference. - No nested SELECT referencing outer table. - No GROUP BY or HAVING clause. - Also, every row added through view must not violate integrity constraints of base table.

84. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 84 Updatable View For view to be updatable, DBMS must be able to trace any row or column back to its row or column in the source table.

85. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 85 WITH CHECK OPTION Rows exist in a view because they satisfy WHERE condition of defining query. If a row changes and no longer satisfies condition, it disappears from the view. New rows appear within view when insert/update on view cause them to satisfy WHERE condition. Rows that enter or leave a view are called migrating rows. WITH CHECK OPTION prohibits a row migrating out of the view.

86. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 86 WITH CHECK OPTION LOCAL/CASCADED apply to view hierarchies. With LOCAL, any row insert/update on view and any view directly or indirectly defined on this view must not cause row to disappear from view unless row also disappears from derived view/table. With CASCADED (default), any row insert/ update on this view and on any view directly or indirectly defined on this view must not cause row to disappear from the view.

87. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 87 Example 6.6 - WITH CHECK OPTION CREATE VIEW Manager3Staff ASSELECT * FROM Staff WHERE branchNo = ‘B003’ WITH CHECK OPTION; Cannot update branch number of row B003 to B002 as this would cause row to migrate from view. Also cannot insert a row into view with a branch number that does not equal B003.

88. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 88 Example 6.6 - WITH CHECK OPTION If Manager3Staff is defined not on Staff directly but on another view of Staff: CREATE VIEW LowSalary ASSELECT * FROM Staff WHERE salary > 9000; CREATE VIEW HighSalary ASSELECT * FROM LowSalary WHERE salary > 10000 WITH LOCAL CHECK OPTION; CREATE VIEW Manager3Staff ASSELECT * FROM HighSalary WHERE branchNo = ‘B003’;

89. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 89 Example 6.6 - WITH CHECK OPTION UPDATE Manager3Staff SET salary = 9500 WHERE staffNo = ‘SG37’; Update would fail: although update would cause row to disappear from HighSalary, row would not disappear from LowSalary. However, if update tried to set salary to 8000, update would succeed as row would no longer be part of LowSalary.

90. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 90 Example 6.6 - WITH CHECK OPTION If HighSalary had specified WITH CASCADED CHECK OPTION, setting salary to 9500 or 8000 would be rejected because row would disappear from HighSalary. To prevent anomalies like this, each view should be created using WITH CASCADED CHECK OPTION.

91. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 91 Advantages of Views Data independence Currency Improved security Reduced complexity Convenience Customization Data integrity

92. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 92 Disadvantages of Views Update restriction Structure restriction Performance

93. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 93 View Materialization View resolution mechanism may be slow, particularly if view is accessed frequently. View materialization stores view as temporary table when view is first queried. Thereafter, queries based on materialized view can be faster than recomputing view each time. Difficulty is maintaining the currency of view while base tables(s) are being updated.

94. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 94 View Maintenance View maintenance aims to apply only those changes necessary to keep view current. Consider following view: CREATE VIEW StaffPropRent(staffNo) ASSELECT DISTINCT staffNo FROM PropertyForRent WHERE branchNo = ‘B003’ AND rent > 400;

95. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe 95 View Materialization If insert row into PropertyForRent with rent  400 then view would be unchanged. If insert row for property PG24 at branch B003 with staffNo = SG19 and rent = 550, then row would appear in materialized view. If insert row for property PG54 at branch B003 with staffNo = SG37 and rent = 450, then no new row would need to be added to materialized view. If delete property PG24, row should be deleted from materialized view. If delete property PG54, then row for PG37 should not be deleted (because of existing property PG21).

96. Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe References http://www.tizag.com/sqlTutorial/sqlcase.php http://msdn.microsoft.com/en- us/library/ms181765.aspx Slide 24- 96