1. “ Possible representation (1) “
Welcome: To the forth learning sequence
“ Possible representation (1) “
Recap : In the previous learning sequence, we discussed the basic definitions of database subject.
Present learning: We shall explore the following topics:
- Regular organization.
- Factoring out the CITY value.
- Indexing on city.
- Combination the previous representations.

2. Possible representation
- In this section we make a simple collection of sample data and consider some of many ways it could be represented in storage at the level of stored record interface.

3. Possible representation
The sample data is consists of information about five suppliers; for each supplier we wish to record a:
Supplier number (S#), a supplier name (SNAME), and location (CITY).

4. Possible representation
1- Note that the (Figure1) actually a very simple conceptual model of the data. S#
SNAME
CITY S1
Smith
London S2
JOHNS
Paris S3
BLACKE S4
CLARK S5
ADAMS
Athens
Fig 1: sample data regular organization

5. Possible representation
The representation in fig 1 (file containing five stored record occurrence) has the advantage of simplicity but would probably be inadequate for various reasons in a realistic situation.
Suppose, for example we have (10.000) suppliers instead of just of five, but that they were located in only (10) different cities.

6. Possible representation
2- If we assume that the amount of storage required for a pointer is less than that required a city name, the representation illustrated in fig2 will clearly save some storage space in such a situation.

7. Possible representation
SNAME
CITY POINTER S1
Smith S2
JOHNS S3
BLACKE S4
CLARK S5
ADAMS
CITY
Athens
London
Paris
City file
Supplier file
Fig2: Factoring out the CITY value

8. Possible representation
SNAME
CITY POINTER S1
Smith S2
JOHNS S3
BLACKE S4
CLARK S5
ADAMS
CITY
Athens
London
Paris
City file
Supplier file
Fig2: Factoring out the CITY value

9. Possible representation
Here we have two stored files, a supplier file and a city file, with pointers out of the former into the later.
These pointers are Stored Record Addresses (SRAs). The only advantage of this representation (compared with the previous one) is the saving in space.
A request to find all properties (attributes) of a given supplier (entity) will require at least one more access than before.

10. Possible representation
3- A request to find all suppliers (entities) in a given CITY will involve several more access.
If such a case requested and it is an important one.
The DBA may choose the alternative representation shown in fig3.

11. Possible representation
SNAME S1
Smith S2
JOHNS S3
BLACKE S4
CLARK S5
ADAMS
CITY
SUPPLEIRPOINTER
Athens
London
Paris
Supplier file
City file
Fig3: Indexing on city

12. Possible representation
SNAME S1
Smith S2
JOHNS S3
BLACKE S4
CLARK S5
ADAMS
CITY
SUPPLEIRPOINTER
Athens
London
Paris
Supplier file
City file
Fig3: Indexing on city

13. Possible representation
Here again we have two stored file. A supplier file and a city file but this time there are pointers out of the later city into the firmer supplier.
Each city stored record occurrence contains pointers to all corresponding supplier stored record occurrence.

14. Possible representation
It should be obvious that the last representation of data (factoring out the city) is better than the previous figure (Indexing on city) for queries asking for all supplier in a given city but worse for queries asking for all attributer of given supplier.
Here we conclude that representation of data is strongly dependent on the queries (access) asked by the system user.

15. Possible representation
Also it is worth to mention, that it is the database Management System (DBMS), not the Access Method that maintain the pointers; the access method is only responsible for assigning a unique stored record address (SRA) as it was mentioned before.
The interesting thing about the previous representation is that the CITY file is an index to the SUPPLIER file (an index controlled by the DBMS, not by the access method).

16. Possible representation
4- To obtain the advantages of each of the previous two representation (at the cost of repairing slightly more storage space and probably more significant having more pointer maintenance to do when changes occur); we can have the following representation for data (fig 4).

17. Possible representation
Athens .
London .
Paris . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams
Fig 4: Combination the previous

18. Possible representation
Athens .
London .
Paris . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 4: Combination the previous

19. Possible representation
Athens .
London .
Paris . . . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 4: Combination the previous

20. Possible representation
Athens .
London .
Paris . . . . S1
Smith . S2
Jones S3
Black S4
Clark S5
Adams .
Fig 4: Combination the previous

21. Summary: In this learning sequence, we discussed the following topic:
- Four possible representations for some sample data.

22. END