Managing Data Resources

1. Managing Data Resources 1

2. The Name of the Game • Information is a valuable resource. • It is expensive to collect, maintain, and use. • The goal of database management it to • maximize the benefits gained from information • maximize the accuracy of information • minimize the costs associated with information 2

3. Keeping Track of Things • Entity - person, place, thing or event on which we maintain information. • Attribute - A single piece of information describing a particular entity. 3

4. Data Hierarchy • Database - a collection of related files • File - a collection of uniform records • Record - a collection of related fields • Field - a collection of bytes • Byte (& words) • Bit 4

5. Terminology • Generic Database Spreadsheet • ----- Table Table/Sheet • Entity Record Row • Attribute Field Column 5

6. Key Field(Attribute) • A key field is an attribute that uniquely identifies a record in a file. • Examples: SSN, NAID • The values in the key field MUST be unique. • It is possible to use several fields to form a composite key. • Example: Lastname + firstname + middlename 6

7. Natural Keys • It is convenient and desirable to use attributes which “naturally occur” with an entity as a key. • Example - most students have a SSN by the time they enroll at NDSU, so the SSN would be natural key. 7

8. Accessing Information • Lookup items(records) by the value of their key. • Methods of access: • Sequential Access • Direct Access • Indexed Sequential Access 8

9. Ordered vs. Unordered • A database file (collection of records) may be: • ordered - physically arranged in the file so that the key field increases (or decreases) in a sequential fashion. • unordered - physically arranged in the file so the key field has no ordered relation with the preceding or succeeding key. 9

10. Costs & Benefits of Ordering • “In general” a record can be found faster in an ordered list than in an unordered list. • I’ll use the term file & list interchangeably. • “In general” you can turn an unordered list into an ordered list by sorting. • Sorting is a cost of keeping a list ordered. • In this course we will generally be dealing with ordered lists. 10

11. Sequential Access • Look at key of first record in file, • if not the target then look at next record, • if not the target then look at next record, … • If file has N records on average will have to look at N/2 records to find a random target. • Question - Why not just “skip over” some of the records? 11

12. Sequential Access • An employee database might use SSN as the key field. • If the target SSN is 540-12-3763, and • the first record SSN is 120-11-0007, then • how many records should you skip? • This is why sequential access has to look at every record. 12

13. Sequential Access • Historically data was stored on tapes. • Tapes store information sequentially and “only” allow for sequential access. • DASD (disks drives) can also store files sequentially. Files are written to the disk track-by-track, cylinder-by-cylinder in a “physically contiguous” fashion. 13

14. Direct Access • Direct access means that given a value for the key attribute the system can move “directly” to the corresponding record without having to look at an intervening records in the file. • Direct access requires that the system “know” the physical location of the target record on the disk. 14

15. Hashing Algorithms • To find the physical location on the disk a computation is performed on the key value which yields a “unique” physical address for the corresponding record. • Perfect hashing algorithms get you to a unique address. • Imperfect algorithms may hash several keys to the same address. 15

16. Hashing Example • Suppose that I were using SSN as the key and wanted to keep track of 100 entities. • Select 101 (a prime number closest to the number of records) and divide this into the SSN. • Remainder will always be a number between 0 and 100. 16

17. Hashing Example • The remainder represents the disk address. • A remainder of 52 could represent • cylinder 5, surface 2 • If two or more SSNs have the same remainder (hash to the same address) this is called a collision. Essentially these records are then searched sequentially. 17

18. Direct Access Note • The physical addresses in Direct Access have no relation to the sequential “order” of the keys. • For any two adjacent sequential keys there is no guarantee about the relationship between their physical locations on the disk, they may not be “physically contiguous”. 18

19. Sequential vs. Direct Access • Sequential Access • good when you want to process all records in key order, next record is always ready to be read/written. • Direct Access • good when you want to process records in a random order, next record can be found directly. 19

20. Indexed Sequential Access Method (ISAM) • Combines a sequential file with one or more levels of indexes. • Each index relates a physical location to the highest key value stored in that location. • You find physical location by looking in each level of the index and then sequentially searching the last physical location. 20

21. ISAM • In the library the books are laid out sequentially by call number (the key). • Look at floor index to determine the floor • Look at shelf index to determine the shelf • Sequentially search the shelf 21

22. ISAM • ISAM tries to give the best of both worlds. • When you want to process items sequentially you have an underlying sequential file. • When you want direct access you go through the indexes to get close, then a “small” sequential search at end. 22

23. Traditional File Systems • Also called: • flat file organization • data file approach • Typically an organization or a department within an organization would develop their applications and associated data files in an independent fashion. 23

24. Problems with Traditional Files • Data Redundancy • conflicting data • Program-Data Dependence • lack of flexibility • Lack of Data Sharing • no common names for attributes & entities • Poor Security 24

25. DBMS Approach • Database Management Systems approach places a common interface between the users of data (the application programs) and the data files. 25

26. DBMS Components • Data Definition Language, DDL • Data Manipulation Language, DML • Structured Query Language, SQL • Data Dictionary, DD 26

27. Logical & Physical Views • Logical View • how the user sees the data • Physical View • how the data is physically saved on the storage media • The DBMS gives each user their own logical view while storing the data using a single physical view. 27

28. Advantages of DBMS • Complexity & Confusion reduced • all data stored in single centralized physical view • Data redundancy & inconsistency reduced • data dictionary shows what data elements are available, data element only present “once” • Program-data dependence reduced • each user can get desired logical view 28

29. Advantages of DBMS • Security • single point of access to data • Reduced cost • initial purchase cost of DBMS and related staff are high, but savings in future development and maintenance usually offset these costs • Access & Flexibility • DML usually provides easier access to data 29

30. Designing Databases • Hierarchical Data Model • Network Data Model • Relational Data Model 30

31. Hierarchical Data Model Author 1 Book 1 Book 2 Book 3 Publisher A Publisher B Publisher A 31

32. Hierarchical Data Model • Data records are broken into segments • Each segment contains some attributes • Segments are arranged into a hierarchical “tree-like” structure • Physical locations pointers join related segments into records • Child segments can only have one parent 32

33. Network Data Model Author 1 Book 1 Book 2 Book 3 Publisher A Publisher B 33

34. Network Data Model • Same organization as hierarchical data model • Except that a child segment can have multiple parents 34

35. Relational Data Model Author 1 Author 2 Author 3 Book 1 Book 2 Book 3 Book 4 Book 5 Publisher 1 Publisher 2 35

36. Relating Fields A1 Author 1 A2 Author 2 A3 Author 3 Book 1 A1 P1 Book 2 A3 P2 Book 3 A2 P2 Book 4 A1 P2 Book 5 A1 P1 P1 Publisher 1 P2 Publisher 2 36

37. Relating Fields A1 Author 1 A2 Author 2 A3 Author 3 Book 1 A1 P1 Book 2 A3 P2 Book 3 A2 P2 Book 4 A1 P2 Book 5 A1 P1 P1 Publisher 1 P2 Publisher 2 37

38. ID Author A1 Author 1 A2 Author 2 A3 Author 3 Book-table Title AID PID Book 1 A1 P1 Book 2 A3 P2 Book 3 A2 P2 Book 4 A1 P2 Book 5 A1 P1 ID Publisher P1 Publisher 1 P2 Publisher 2 Relational Data Model Author-table Publisher-table 38

39. Relational Data Model • Data Records are broken into segments • Each segment contains some attributes • Segments are arranged in tables • There are NO “physical” location pointers between tables • Relations between tables are “implied” by relating fields 39

40. Relations Generated When Asked • Relationships between segments are not predefined by pointers in the relational model. • Tables are JOINed together to display relationships. • JOINs occur at query time. • Tables must have a common data element to be joined. 40

41. Example JOIN Select Author, Title, Publisher FROM Author-table, Book-table, Publisher-table WHERE Author-table.ID = Book-table.AID, and Book-table.PID = Publisher-table.ID 41

42. Author Title Publisher Author 1 Book 1 Publisher 1 Author 1 Book 4 Publisher 2 Author 1 Book 5 Publisher 1 Author 2 Book 3 Publisher 2 Author 3 Book 4 Publisher 2 Results of Join Answer-table 42

43. Relational Model Operations • Selection • select which rows to display • Projection • select which columns to display • Join • combine two or more tables 43

44. Types of Relations • 1-1 • 1-to-1 • 1-n • 1-to-many • n-n • many-to-many 44

45. Name of the game • Using the relational model, • Represent each type of relationship • as simply as possible (using the fewest tables), • with a minimum of duplicated data, and • with a minimum of wasted space (empty fields) 45

46. Tables needed for 1-1 Book Author Title Author1 Book1 Author2 Book2 Author3 Book3 46

47. Tables needed for 1-n Book Author ID Title 1 Book1 1 Book2 2 Book3 3 Book4 2 Book5 ID Name 1 Author1 2 Author2 3 Author3 47

48. AID BID 1 1 1 2 2 1 2 2 3 1 3 5 3 4 1 5 2 5 ID Title 1 Book1 2 Book2 3 Book3 4 Book4 5 Book5 ID Name 1 Author1 2 Author2 3 Author3 Tables needed for n-n Writes Book Author 48

49. Advantages & Disadvantages • Hierarchical & Network Data Models • faster for “pre-defined” queries • slower for ad-hoc queries • inflexible, more expensive to maintain • Relational Data Models • flexible, less expensive to maintain • most queries require joins and are slower than “pre-defined” queries mentioned above 49

50. Entity-relationship diagram • A conceptual model useful in database design. • Illustrates the relationships between various entities in the database. • Entities are represented by rectangles. • Relationships represented by diamonds. • Attributes can be assigned to both entities and relationships. 50