Lesson 2 network database system
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The document describes the network database model and CODASYL DBTG model. Some key points: - The network model uses a many-to-many relationship with owner and member records linked together. - The DBTG model simplified this to one-to-one and one-to-many relationships. It uses segments, sets, and links to represent records, relationships, and connections between records. - The DBTG model provides commands to retrieve, update, insert, and delete records as well as connect and disconnect them from sets. Programs access the database using templates, pointers, and status flags stored in a work area.
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Lesson 2 network database system
- 1. Network Database System Prepared by Sir Gio
- 2. Network Model Another widely used in data model Many-to-Many relationship Owner – parent record Member – child record Links – collection of records which are connected to one another
- 3. Concept of Owner-Member Relationship DEPARTMENT FACULTY MAJORS CLASSES STUDENTS OWNER-MEMBER RELATIONSHIP
- 4. OCCURRENCE DIAGRAMS Used to illustrate the distribution of owner and member records within a network database Helps in navigation of the database
- 5. An Occurrence Diagram Dr. Jao Faculty BSIT records Department Dr. records Ramiro BSCS DIT Faculty Dr. records Dr. Flavier Dr. J Belen Dr. Cruz Dr. Dr. Holmes Rizal Dr. No Faculty records
- 6. CODASYL-DBTG Model Conference On Data System Languages, Data Base Task Group Original network model presented in a report by CODASYL-DBTG Collection of set types, owner record types, and member record types
- 7. Characteristics of CODASYL Network Model 1. There must be one-to-many relationship between pairs of record types related with respect to an owner-with-member relationship 2. A given owner instance must posses a unique set of member instances 3. A given record type cannot be both the owner and member of the same set type Records and their fields are represented as record type type customer = record type account = record customer-name: string; account-number: integer; customer-street: string; balance: integer; customer-city: string;
- 8. DBTG Model Simplified by limiting the link options to one-to-one or one-to-many Entity relationship is referred to as DBTG set The arrowhead is used to Segment A denote: ownership Records or segments are represented as boxes Links are represented as Segment B lines
- 9. DBTG Model Just like in hierarchical databases, a particular set can have any number of set occurrences CUSTOMER A CUSTOMER B CUSTOMER C ACCOUNT 1 ACCOUNT 2 ACCOUNT 3 ACCOUNT 4 ACCOUNT 5 ACCOUNT 6 A descriptive CUSTACCT SET OCCURRENCES way of naming a DBTG database is by combining the names of the owner and member segments
- 10. DBTG Model Another way of illustrating a set occurrence is similar to the one used in mathematical sets CUSTOMER A CUSTOMER B CUSTOMER C CUSTOMER D ACCOUNT 1 ACCOUNT 4 ACCOUNT 6 ACCOUNT 1 ACCOUNT 5 ACCOUNT 1 CUSTACCT SET OCCURRENCES
- 11. DBTG Model A set of occurrence can be represented as a link list CUSTOMER A ACCOUNT 1 ACCOUNT 2 ACCOUNT 3 CUSTOMER B ACCOUNT 4 ACCOUNT 5 CUSTOMER C ACCOUNT 6 Illustrates that the member LINKED REPRESENTATION OF A SET OCCURRENCES segments are ordered
- 12. DBTG Model CUSTOMER BRANCH Let us add another owner segment and ACCOUNT call it BRANCH This segment is also an owner of the ACCOUNT segment
- 13. DBTG Model The Data Structure Diagram below shows the inclusion of segment BRANCH and fields of each segment CUSTOMER BRANCH name street city name assets city ACCOUNT number balance
- 14. DBTG Model A populated representation will help further understanding the existing owner-member relationship in set CUSTACCT and BRNACCT ACCOUNT 1 CUSTOMER ACCOUNT 2 BRANCH 1 A CUSTOMER ACCOUNT 3 BRANCH 2 B CUSTOMER ACCOUNT 4 C ACCOUNT 5 ACCOUNT 6 CUSTACCT AND BRNACCT OWNER-MEMBER RELATIONSHIP
- 15. DBTG Model OWNERS MEMBERS CUSTOMER A ACCOUNT 1, 2 AND 3 CUSTOMER B ACCOUNT 4 AND 5 CUSTOMER C ACCOUNT 6 BRANCH 1 ACCOUNT 1, 2 AND 4 BRANCH 2 ACCOUNT 3, 5 AND 6 SET COMPOSITION SUMMARY •Segments are owned by more than one owner segment •This illustrates the fact that entity relationships within a DBTG set is limited to one –to-one or one-to-many •This avoids the need to deal with complicated many-to-many relationships
- 16. DBTG Model CUSTOMER SEGMENT BRANCH SEGMENT NAME ADDRESS CITY NAME ASSETS CITY Hagar Kalayaan Quezon Grace Park 1000000000 Caloocan 1 Ana Avenida Manila Cervantes 1233333455 Manila Carlo Forbes Manila 6 ACCOUNTS SEGMENT NUMBER BALANCE 12340 1000 12341 100020 12342 10006 12343 123222 12344 21233 12345 123433 SAMPLE RECORDS OF THE THREE SAMPLE SEGMENT
- 17. Sample Schema
- 18. DBTG Data-Retrieval Facility The DBTG data manipulation language consists of a number of commands that are embedded in a host language Run unit System application program consisting of a sequence of host language and DBTG command statements Statements access and manipulate database items as well as locally declared variables Program work-area (or user work area) A buffer storage area the system maintains for each application program
- 19. DBTG Variables Record Templates Currency pointers Current of record type Current of set type Current of run unit Status flags DB-status is most frequently used Additional variables: DB-set-name, DB-record-name, and DB-data-name
- 20. Example Program Work Area Templates for three record types: customer Account branch Six currency pointers Three pointers for record types: one each tot he most recently accessed customer, account, and branch record Two pointers for set types: one to the most recently accessed record in an occurrence of the set depositor, one to the most recently accessed record in an occurrence of the set account- branch One run-unit pointer Status flags: four variables defined previously Following diagram shows an example program work area state
- 21. Example Program Work Area
- 22. The Find and Get Commands find Locates a record in the database and sets the appropriate currency pointers get Copies of the record to which the current of run-unit points from the database to the appropriate program work area template Example: Executing a find command to locate the customer record belonging to Johnson causes the following changes to occur in the state of the program work area. The current of the record type customer now points to the record of Johnson The current of set type depositor now points to the set owned by Johnson The current of run unit now points to customer record Johnson
- 23. Access of Individual Records find any <record type> using <record-field> Locates a record of type <record type> whose <record- field> value is the same as the value of <record-field> in the <record type> template in the program work area. Once such a record is found, the following currency pointers are set to point to that record: The current of run-unit pointer The record-type currency pointer for <record type> For each set in which that record belongs, the appropriate set currency pointer find duplicate <record type> using <record-field> Locates (according to a system-dependent ordering) the next record that matches the <record-field>
- 24. Access of Records Within a Set Other find commands locate records in the DBTG set that is pointed to by the <set-type> currency pointer find first <record type> within <set-type> Locates the first database record of type <record type> belonging to the current <set-type> find next <record type> within <set-type> which finds the next element in the set <set-type> find owner within <set-type> Locates the owner of a particular DBTG set
- 25. Predicates For queries in which a field value must be matched with a specified range of values, rather than to only one, we need to: get the appropriate records into memory examine each one separately for a match determine whether each is the; target of our find statement
- 26. Example DBTG Query Print the total number of accounts in the Perryridge branch with a balance greater than $10,000 count := 0; branch.branch-name := “Perryridge”; find any branch using branch-name; find first account within account-branch; while DB-status = 0 do begin get account if account.balance > 10000 then count := count + 1; find next account within account-branch; end print (count);
- 27. DBTG Update Facility DBTG mechanisms are available to update information in the database. To create a new record of type <record type> insert the appropriate values in the corresponding <record type> template add this new record to the database by executing store <record type> Can create and add new records only one at a time
- 28. DBTG Update Facility To modify an existing record of type <record type> find that record in the database get that record into memory change the desired fields in the template of <record type> reflect the changes to the record to which the currency point of <record type> points by executing modify <record type>
- 29. DBTG Update Facility To delete an existing record of type <record type> make the currency pointer of that type point to the record in the database to be deleted delete that record by executing erase <record type> Delete an entire set occurrence by finding the owner of the set and executing erase all <record type> Deletes the owner of the set, as well as all the set’s members. If a member of the set is an owner of another set, the members of that second set also will be deleted erase all is recursive
- 30. DBTG Set-Processing Facility Mechanisms are provided for inserting records into and removing records from a particular set occurrence Insert a new record into a set by executing the connect statement connect <record type> to <set-type> Remove a record from a set by executing the disconnect statement disconnect <record type> from <set-type>
- 31. Example disconnect Query Close account A-201, that is, delete the relationship between account A-201 and its customer, but archive the record of account A- 201 The following program removes account A-201 from the set occurrence of type depositor. The account will still be accessible in the database for record- keeping purposes account.account-number := “A-201”; find for update any account using account-number. get account, find owner within depositor, disconnect account from depositor.
- 32. DBTG Set-Processing Facility (Cont.) To move a record of type <record type> from one set occurrence to another set occurrence of type <set- type> Find the appropriate record and the owner of the set occurrences to which that record is to be moved. Move the record by executing reconnect <record type> to <set-type> Example: Move all accounts of Hayes that are currently at the Perryridge branch to the Downtown branch.
- 33. Example reconnect Query customer.customer-name := “Hayes”; find any customer using customer-name; find first account within depositor; while DB-status = 0 do begin find owner within account-branch; get branch; if branch.branch-name = “Perryridge” then begin branch.branch-name := “Downtown”; find any branch using branch-name; reconnect account to account-branch; end find next account within depositor, end
- 34. DBTG Set-Processing Facility (Cont.) A newly created member record of type <record type> of a set type <set-type> can be added to a set occurrence either explicitly (manually) or implicitly (automatically). Specify the insert mode at set-definition time via insertion is <insert mode> manual: connect <record type> to <set-type> automatic: store <record type>
- 35. Set Insertion Example Create account A535 for customer Hayes at the Downtown branch. Set insertion is manual for set type depositor and is automatic for set type account-branch. branch.branch-name := “Downtown”; find any branch using branch-name; account.account-number := “A-535”; account.balance := 0; store account; customer.customer-name := “Hayes”; find any customer using customer-name; connect account to depositor;