1 Access: Create a Table from a Query, Eg 31 SELECT * INTO SmallCust3 FROM Customer WHERE CreditLimit <= 7500; INTO clause used to save the results of a query into a new table –Specified before FROM and WHERE clauses SmallCust3 table did not exist before INTO Data added to the new table is separate – if change in SmallCust3, no change in Customer

2 Figures : SQL Query to Create New Table

3 Access: Create a Table from a Query Can query newly created table like any other table. SELECT SmallCust3.CustomerNum FROM SmallCust3; Useful if have complex query: –Break up into two parts –Do 1 st part and store into table –Then do 2 nd part as query on stored table. Can do same with views or saved query in Access

4 Make-Table : QBE version of INTO. Eg: 2_18 Create new table with customer# and cname, and num, fname, lname of of customer’s sales rep. Do as normal SELECT query and then choose Make-Table from Query. SQL: SELECT Customer.CustomerNum, Customer.CustomerName, Customer.RepNum, Rep.FirstName, Rep.LastName INTO CustomerRep2 FROM Rep INNER JOIN Customer ON Rep.RepNum = Customer.RepNum;

5 Figure 2.37: Make-Table Query

6 Figure 2.39: Make-Table Query (con’t.)

7 Views CREATE VIEW REDPARTS AS SELECT * FROM P WHERE COLOR = ‘RED’; View are virtual tables –Not physically stored in database –Created on demand Created via view definition Looks just like a table

8 Query on a View Views can be queried just like relations –How is this done ? Query modification: DBMS changes a query on a view to a query on underlying table. SELECT * FROM REDPARTS WHERE WEIGHT > 15; is converted to SELECT * FROM P WHERE COLOR = ‘RED’ AND WEIGHT > 15;

9 Creating Views/Saving Queries in Access Microsoft Access does not support views, but can get similar functionality by saving the query – can write another query on saved query Create a query and save it. Eg: with the name: TempQuery –TempQuery can be queried as if it was a table. –SELECT * FROM TempQuery; Query402 : SELECT RepNum FROM eg10; Like INTO, a way of breaking up complex queries. Can use queries like tables when building new queries in QBE also

10 Figures : Access Query Design of View

11 Eg:INTO: alternative to Nested From: warehouse with largest #parts Save “WPCount” in a query/temporary table: SELECT Warehouse AS W, COUNT (PartNum) AS NumberofParts INTO WPcount FROM Part GROUP BY Warehouse; Now get warehouse with largest count: SELECT W FROM WPcount WHERE WPcount.NumberofParts = (SELECT MAX (NumberofParts) FROM WPcount);

12 Eg: Another way to do same thing Save “WPCount” query as query402 SELECT Warehouse AS W, COUNT (PartNum) AS NumberofParts FROM Part GROUP BY Warehouse; Now get warehouse with largest count: SELECT W FROM query402 WHERE query402.NumberofParts = (SELECT MAX (NumberofParts) FROM query402 );

13 Count Distinct Eg How many customers have placed an order SELECT COUNT(DISTINCT CustomerNum ) FROM Orders; Access does not support DISTINCT with aggregate ops. Eg: above will not work. How to do ? First do DISTINCT and store in temp table (query) Then run aggregate op query on temp table SELECT DISTINCT CustomerNum FROM Orders; Save query as query55, then do aggregate operation: SELECT COUNT (CustomerNum) FROM query55; This will work in Access

14 Views are dynamic Any change in underlying base table gets reflected in the view. CREATE VIEW REDPARTS AS SELECT *FROM P WHERE COLOR = ‘RED’; Suppose initially 2 red parts in P SELECT * FROM REDPARTS How many rows? 2 rows. Now insert one new red part into P. SELECT * FROM REDPARTS How many rows? 3 rows. Why ? Query modification. This query becomes: SELECT * FROM P WHERE COLOR = ‘RED’ ;

15 What kinds of view can be built? Subset of rows Subset of columns Contain derived attributes Renaming of columns Can be from multiple tables

16 Views : subset of columns Column names can be given explicitly or be inherited from base table CREATE VIEW SOMERED (P#, PARTNAME, WT) AS SELECT P#, PNAME, WEIGHT FROM P WHERE COLOR = ‘RED’;

17 More on views Column names must be explicitly stated if derived Derived columns CREATE VIEW PARTQUANT (PNO, TOTALQT) AS SELECT P#, SUM (QTY) FROM SP GROUP BY P# ; Can be used to create other views CREATE VIEW HEAVYREDPARTS(P#, PARTNAME) AS SELECT P#, PARTNAME FROM SOMERED WHERE WT > 13;

18 E.R. (entity relationship model) High level data model: conceptual design stage Not an implementation model –Independent of lower level implementation details –What the tables look like won’t impact ER schema What is it good for : can communicate what world looks like by capturing –What are the different types of data –The relationships between them. –In an easy to understand language Helps in relational design

19 Elmasri Company Database The company is organized into DEPARTMENTS. Each department has a name, number and an employee who manages the department –We keep track of the start date of the department manager –Departments can have multiple locations Each department controls a number of PROJECTs. Each project has a name, number and is located at a single location.

20 Elmasri Company Database For each EMPLOYEE, we store the social security number, address, salary, sex, and birthdate. Employees may have a supervisor Each employee works for one department but may work on several projects. We keep track of the number of hours per week that an employee currently works on each project. Each employee may have a number of DEPENDENTs. –For each dependent, we keep track of their name, sex, birthdate, and relationship to employee.

21 ER model basics Entities: things (“real world objects”) distinguishable from other things. Attributes: properties of entities Relationships: between entities Have to figure out – What are the entities and relationships ? – What information about these entities and relationships should we store in the database? – What are the integrity constraints or business rules that hold? Can map an ER diagram into a relational schema.

22 Entities Entities: a thing with an independent existence. Eg : people, objects, events etc Eg: Each of the employees Alice, Bill, Mike is an entity Eg: Is Payroll an entity ? Eg: Is the social security number an entity ?

23 Entity Types Entity type: employee is entity type –Alice, Bill are entities. Similar to type and var in programming languages Entity type represented by We will often say entity instead of entity type What are the entities in the company database?

24 Attributes Attributes are properties used to describe an entity. –EMPLOYEE entity type will have a Name, SSN, Address, Sex etc. Notation: A specific entity will have a value for each of its attributes. Eg: specific employee –Name='John Smith’ –SSN=' ’ –Address ='731 Broad, Houston, TX ’ –Sex='M'

25 Key Key uniquely identifies which entity of that entity type we are talking about. –Eg: SSN for employees Notation: by underlining What are the keys for the other entities. Notation different from relational schemas –In ER diagrams we underline all keys, not only PK No notion of PK in ER diagrams –all keys equal Show composite key by underlining –Confusing since 2 diff. keys have same notation

26 Simple vs Composite Attributes Simple: Each entity has a single atomic value for the attribute: –Not broken into further details. Eg ? Eg : salary Notation: Composite: The attribute may be composed of several components –Is broken down into further details. Eg ? Eg : name Notation:

27 Multi valued/Single-valued attribute Multi-valued: A single entity may have more than one value for that attribute. Eg ? Dept locations : one Dept, diff. locations Notation: Single valued: Each entity can have a only a single value for the attribute. Eg ? BDate Notation: Is single valued same as a key ? No: Bdate is not key. Why ? Two employee can have same Bdate

28 Stored vs derived attributes Stored: Whose value has been entered i.e. they are not being derived from other attributes. Eg: BDate Notation: Derived: Whose value can be derived from the value of another attribute. Eg: Age. Derived from which attribute ? Birthdate Notation:

29 Relationships Relationship: connection between two entities –Alice works for Accounting –Bob works for Marketing –David works for Accounting Notation: Two entitites can participate in more than one relationship. Eg ? WORKS_FOR, MANAGES What are the relationships in Elmasri company database ?

30 Recursive Relationships Recursive Relationship: Relationship between two entities of the same type. Eg ? SUPERVISION relationships between EMPLOYEE (in role of supervisor or boss) and (another) EMPLOYEE (in role of subordinate or worker). In ER diagram, need to display role names to distinguish participations.

31 Cardinality Capture information about max # relationships (of that type) that a particular entity can take part in? If 2 entity types in relationship, can be –one-to-many 1:N (or N:1) –one-to-one 1:1 –many-to-many M:N Derived from knowledge of mini-world constraints –Has to be based on the semantics of the data

32 Cardinality Eg of 1:N : WORKS_FOR –One employee can only work in one dept –One department can have many employees Notation: Put numbers on line for relationship –WORKS_FOR :Put 1 on Dept side, N on Emp side Eg of 1:1: MANAGES –One employee can manage at most one work dept –One department can have at most one manager Eg of M:N :WORKS_ON –One employee can work on many projects –One project can have many employees working on it

33 Partial/Total Participation What is min # of relationships (of that relationship type) that entity has to take part in. Partial participation: can be zero –OK for an entity to be not involved in this relationship Notation: single or thin line Total participation: one or more –Each entity has to take part in at least one such relationship Notation: double or fat line

34 Partial/Total Participation Could be: –Total on both sides. –Partial on both sides. –Partial on one side, total on the other. Employees manage Department ? Partial – total. Why ? Not every employee manages a Dept. –Partial from E Every Dept. has a manager –Total from D

35 Partial/Total Participation Employees work for Dept ? Total on both sides –Every Emp has to work for a Dept. –Every Dept has to have an Emp Not possible for Dept to exist without Emps Supervision? Partial – partial (what about cardinality?) –Not every employee is a supervisor –Not every employee has a supervisor Eg: WORKS_FOR, WORKS_ON, CONTROLS

36 Attributes of Relationship Eg: HoursPerWeek of WORKS_ON – # hours per week EMP works on a PROJECT. Notation Could this be moved to Emp or Project ? No : many-many relationship MGR_START_DATE: could this be moved to Emp or Dept ? Yes, one-one –But better to show as attribute of MANAGES relationship since it is really telling us about that

37 Weak Entities Want to keep track of dependents. What is the key ? –Can name be a key for dependents table? No : 2 employees could have a son called John Not regular entity: can’t identify since no key Has to be a weak entity : Notation How to distinguish one Dependent from another ? –What info do we need to figure out which Dependent we are talking about ? Name can be a partial key: –if we know Emp SSN and dependent name, can figure out which dependent we are talking about. Key will be {ESSN, Dependent Name}

38 Weak Entities A weak entity must participate in an identifying relationship with a regular (owner) entity. Notation –Weak entity DEPENDENT –Identifying relationship DEPENDENT_OF –Regular entity EMPLOYEE From the side of the weak entity : can this be many (N:N or N:1) ? No: has to be 1:N or 1:1. Can be partial? No: from weak entity side, has to be total

39 SUMMARY OF ER-DIAGRAM NOTATION FOR ER SCHEMAS Meaning ENTITY TYPE WEAK ENTITY TYPE RELATIONSHIP TYPE IDENTIFYING RELATIONSHIP TYPE ATTRIBUTE KEY ATTRIBUTE MULTIVALUED ATTRIBUTE COMPOSITE ATTRIBUTE DERIVED ATTRIBUTE TOTAL PARTICIPATION OF E 2 IN R CARDINALITY RATIO 1:N FOR E 1 :E 2 IN R Symbol E1E1 R E2E2 E1E1 R E2E2 N1

40 The ER conceptual schema diagram for the COMPANY database. On next slide we will see another alternative notion (min, max) from text book –Use if you like, but ONLY IF YOU UNDERSTAND IT !

41 FIGURE 7.15 The ER diagram for COMPANY with (min, max) notation for participation and cardinality

42 Cardinality vs participation Cardinality : max # relationships for entity Participation : min # relationships for entity Independent of each other. Can be: Partial and 1. Eg: Manages from Emp Partial and N. Eg: Supervises from supervisor: –123 may not be supervisor –234 may be a supervisor for many supervisees Total and 1. Eg: Manages from Dept. Total and N. Eg: Employee works for Dept –Dept has to have at least one Emp, could have many

43 SQL REVIEW: Find age of youngest sailor with age > 18, for each rating with at least 2 sailors, of any age HAVING (SELECT COUNT (*) FROM Sailors S2 WHERE S.rating=S2.rating) > 1; Compare this with the query where we considered only ratings with 2 sailors over 18 SELECT S.rating, MIN (S.age) FROM Sailors S WHERE S.age > 18 GROUP BY S.rating

44 Ternary relationships Degree of relationship: # participating entitities. Binary Relationship: Relationship types of degree 2 –Eg: MANAGES Ternary Relationship: Relationship types of degree 3 –Eg: Elmasri SUPPLY relationship from next slide.

45 Elmasri FIGURE 7.17 Ternary relationship types: SUPPLY relationship.

46 Ternary relationships Can we replace a ternary relationship with two binary relationships? –Supp-Proj, Supp-Part No : will lose information. Eg: –S1 supplies to Proj1 –S1 supplies Part1 –Can we conclude S1 supplies Part1 to Proj1 ? No : possible that –S1 supplies Part2 to Proj1 –S1 supplies Part1 to Proj2

47 Ternary relationships Lossy decomposition: can’t recover original table. Can replace with three binary relationships? –supp-proj, supp-part, part-proj No. Counterexample ?

48 Ternary relationships Suppose we saw –S1 supplies to Proj1 –S1 supplies Part1 –Part1 supplied to Proj1 Can we conclude S1 supplies Part1 to Proj1 ? No –S1 supplies Part2 to Proj1 –S1 supplies Part1 to Proj2 –S2 supplies Part1 to Proj1

49 RG Eg: Ternary Relationships age pname Dependents Covers name Employees ssn lot Policies policyid cost Does this look OK ? Policy is for E or D ? Suppose one table (E,P,D): –(E1,P1,D1), (E1,P1,D2), (E1,P1,D3), … – P1 is for E1, repeated for each D How to fix ? Two binary relationships – how ?

50 RG Eg: Ternary Relationships: can sometimes be better with 2 binary 2 binary relationships, 2 tables – (E,P): –(P,D): No repetition RG: different notation for cardinality –Uses arrows age pname Dependents Covers name Employees ssn lot Policies policyid cost Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Bad design Better design

51 Points and Common Mistakes with ER diagrams May not be able to capture all constraints. –Eg: Emp in Acct dept have > 40k salary Have entity types and not actual entities –Eg: EMPLOYEE, not Mike Don’t represent the thing which the whole database is about as an entity –Eg: COMPANY as an entity

52 Entity vs. Attribute Should address be attribute of E or an entity – (connected to Employees by a relationship)? –Depends upon use we want to make of address information, and the semantics of the data: Eg: better as separate entity if: –several addresses per employee –If details of address (city, street, etc.) are important In a different database, could be a separate entity

53 Attribute vs relationship Eg: suppose we removed MANAGES relationship and had a MANAGER attribute for DEPT. Good idea ? No: managers are also employees –If we have as attribute, then we are not making the connection that managers have to be employees Eg: students and advisors ? If keeping track of professors, then relationship –Else possibly attribute

54 Redundancy Question: Any redundant info? –Eg: if we also had Manager as an attribute of DEPARTMENT. –Why is this redundant? Already have this info through the MANAGES relationship Eg: suppose have LOCATION entity connected via a relationship to PROJECT. Problem? Not OK to still have location information as attribute and via a relationship