Lecture 4. Network Data Model
The first and the last member record in the set occurrence
Syntax:
FIND FIRST|LAST [recordname] RECORD IN [setname] SET
Example:
FIND FIRST Lecture RECORD IN Has SET;
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Current occurrence of the set “Has”
L1 - the first member record of the set “Has”
Error_Status != 0 – the record occurrence do not have member records
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Lecture 4. Network Data Model
Location of the owner record of the current set occurence
Syntax:
FIND OWNER OF [setname] SET
Example:
FIND OWNER OF Has SET
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Current occurrence of the set “Has”
L2 - current member record of the set “Has”
T1 - owner record of the set “Has”
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Lecture 4. Network Data Model
Example
X = ”T1”;
FIND Teacher RECORD BY DATABASE KEY X; FIND LAST Lecture RECORD IN Has SET;
FIND OWNER OF Is_taught_at SET; 
FIND LAST Lecture RECORD IN Is_taught_at SET; FIND NEXT Lecture RECORD IN Has SET;
FIND OWNER OF Is_taught_at SET;
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Lecture 4. Network Data Model
Put a record occurrence into the database
create a record occurrence in UWA;
execute the command STORE.
Syntax:
STORE [recordname] RECORD
Example:
STORE Teacher RECORD
Stored record automatically connected to the current set occurrence.
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Lecture 4. Network Data Model
Attaching record occurrence from DB to the set occurrence
Make required set occurrence the current of set type
Make inserted record occurrence the current one
Execute the command INSERT
Syntax:
INSERT [recordname] RECORD INTO [setname] SET
Example:
X = 31; FIND Teacher RECORD BY DATABASE KEY x; Y = 24; FIND Lecture RECORD BY DATABASE KEY y; INSERT Lecture RECORD INTO Has SET;
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Lecture 4. Network Data Model
Remove a record occurrence of a certain type from a certain set occurrence
Make designated record be the current one
Execute the command REMOVE
Syntax:
REMOVE [recordname] RECORD FROM [setname] SET
Example: Remove all lectures of the teacher with DBK 31.
x=31; FIND Teacher RECORD BY DATABASE KEY x; Loop: FIND NEXT Lecture RECORD IN Has SET;
IF Error_Status=0 THEN
BEGIN; REMOVE Lecture RECORD FROM Teacher SET; GO TO Loop;
END;
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Lecture 4. Network Data Model
Updating record occurrence
The MODIFY command copies values from a record in UWA into the current record occurrence of the DB.
Example: For teacher with DBK 31 change name to “Smith”.
x=31; FIND Teacher RECORD BY DATABASE KEY x; GET Teacher RECORD;
Teacher.Name = ”Smith”; MODIFY RECORD;
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Lecture 4. Network Data Model
Deleting record occurrence from DB
The DELETE command removes from DB the current record occurrence
The set owner may be deleted when this set do not contains member records
Syntax:
DELETE [recordname] RECORD
Example: Delete record with DBK 21.
x=21; FIND Lecture RECORD BY DATABASE KEY x; DELETE Lecture RECORD;
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Lecture 4. Network Data Model
Advantages and drawbacks
The network data model basically has the same advantages and drawbacks as hierarchical data model.
The only basic difference of the network data model in comparison with hierarchical data model is the opportunity to represent many-to-many relationships.
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