Lab 21
- The previous two labs have introduced the concept of a database—a piece of software that stores (maintains) data on a disk drive, and releases this data to a client as requested. This description fits a file system as well except that for the simpler file system the user is expected to know the format of the data on the disk and its organization. All the file system does is bring disk sectors into memory and allow the user to access the sector contents by keeping a file pointer for sequential access and calculating the access point for random access. The file systems relieves the program of the buffer management so that the program can pretend that the whole file is in memory and any record can be accessed with simply a GET # or PUT #. In contrast, the DBMS allows the program to pretend that all the data is organized into tables, and that new tables can be automatically generated via projection or inner join operations. The DBMS accepts commands to generate such tables and returns to the program a set of records that satisfy the command. The program does not have to know about buffering these records, or how they were generated, or how the data actually resides on the disk drive. The transition of data from drive to program is taken care of by the DBMS and the program’s agent the ADO control. The program generates the specification of the recordset, sets this specification in the ADO, and triggers the ADO to interact with the DBMS and refresh the local store of data. Commands can then be issued to manipulate the local data. Once the local data is processed, the ADO is instructed to returned it to the DBMS. The ADO acts as the program’s buffer and other controls can be connected to this buffer to permit the program and the user to give instructions to the ADO concerning its records. These processing controls, like the DataGrid , react to any changes made to the recordset. Indeed, data aware controls can use data from multiple recordsets as illustrated in Figure 18.22 where a value is moved from one control to another and used to update a recordset. The ADO is not the only control that can serve as a pipe from the DBMS to the program, the Data Environment Designer, illustrated in sections 18.10 is an alternative and more powerful control for this task. It also makes a connection to the DBMS and retrieves a recordset, but it has more convenient commands for operating on the records. Both permit the program to send SQL commands through the pipe to be executed by the DBMS. Complete lab 20.
As a review of where we have come, fill in the following table which lists programs that we have studied and/or written, determining the purpose of the illustration and how well you feel you have absorbed the point and feel comfortable with the code:
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Program |
VB feature(s) illustrated for the first time |
% mastered |
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Ex 5.16 |
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Ex 5.21 |
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Fig 6.19 |
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Fig 6.24 |
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Ex 6.16 |
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Ex 6.39 |
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Fig 8.13 |
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EX 8.28 |
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Fig 10.22 |
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Fig 10.41 |
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Fig 11.10 |
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Fig 11.15 |
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Fig 11.16 |
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Fig 12.3 |
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Fig 12.5 |
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Fig 12.7 |
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Fig 12.13 |
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Fig 12.14 |
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Fig 13.1 |
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Fig 13.5 |
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Fig 13.7 |
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Fig 13.9 |
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Fig 13.10 |
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Fig 14.9 |
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Fig 14.17 |
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Fig 14.19 |
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Fig 14.20 |
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Fig 15.4 |
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Fig 15.5 |
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Fig 15.6 |
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Fig 15.7 |
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Fig 18.15 |
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Fig 18.20 |
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3. Point your web browser at http://cbt.cs.ualr.edu. Log on with the user name <two initials><last name> in caps (no password). Click on the green Training button and then run the unit Introduction to Data Access under Microsoft Visual Basic.
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Microsoft Visual Basic 6.0
Desktop: Creating COM Components 1 |
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Microsoft Visual Basic 6.0
Desktop: Introduction to Data Access |
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Summarize below how this material supplements the text presentation.