cp2030 visual basic for c++ programmers

Component 8, Slide 1CP2030 Visual basic for C++ Programmers, ‘The VB Team’ Copyright © University of Wolverhampton

CP2030Visual Basic For C++ Programmers

Week 8 - Databases continued:

Example using Further Features Find Method Reposition and Validate Event Multiple Data Controls Queries Other Topics to Consider Last Words on Databases


Example of Other Features

Below is an example (dental3.mak) which is used to demonstrate further data control functionality

– Modifying properties– Refresh method– Referencing fields– Unbound controls– EOF property– IsNull

The example shows the addition of a list box


Example - Code

Sub Form_Load () Data1.DatabaseName = "c:\files\uow\vb_sc_da\dental.mdb" Data1.RecordSource = "Patients"

Data1.Refresh 'Open database & recordset

Do While Not Data1.Recordset.EOF If Not IsNull(Data1.Recordset(“Surname”).Value) Then 'If

not empty, then List1.AddItem Data1.Recordset(“Surname”).Value 'add

to the list End If Data1.Recordset.MoveNext 'Move to next record Loop Data1.Refresh 'Rebuild the recordset End Sub


Property Setting

The DatabaseName, RecordSource and Datafield (the field within a database to which a data aware control links) properties can all be set at design and run time

The DataSource property (the data control to which a data aware control links e.g. Data1) can only be set at design time

e.g. in above code

Data1.DatabaseName = "c:\files\uow\vb_sc_da\dental.mdb"

Data1.RecordSource = "Patients"


Refresh Method

Used to open the database and build or reconstruct the dynaset in the controls recordset property at runtime

Has side affect of making the first record current in the recordset e.g.Data1.Refresh 'Open database & recordset

In the code example above it is used twice

i. To initially open the database and recordset

ii. To refresh the recordset, making the first record current, after the Do ... Loop has completed


Referencing Fields

Can move through records using MoveNext etc To select individual fields within a record,

need to reference it within the recordsets current record, can use any of the following e.g.

Data1.Recordset(“First Name”).ValueData1.Recordset.Fields(“First Name”)Data1.Recordset.Fields(0).ValueData1.Recordset.Fields(0)Data1.Recordset(“First Name”)

All equivalent since ‘Fields’ is a default collection the recordset and ‘Value’ is default property for a ‘Field’


Unbound Controls

To display data in controls other than the 5 data aware controls requires coding to manipulate the data in and out of the control

In the above code the required field is referenced and then added to a list boxes list e.g.List1.AddItem Data1.Recordset(“Surname”).Value

'add to the list

Here we’re adding the second field (Surname) of each record into the list box


EOF / BOF / IsNull

EOF is used to check if the end of the recordset has been reached, when EOF becomes true

– Do While Not Data1.Recordset.EOF In the example the code loops while the end of

the file has not been reached BOF is True when at beginning of recordset

IsNull is a Visual Basic command which checks if a variable is a null, here it is used to check the value of the referenced field of the current record

If Not IsNull(Data1.Recordset(1)) Then ‘If not empty, then


Example - Code Revisited

Sub Form_Load () Data1.DatabaseName = "c:\files\uow\vb_sc_da\dental.mdb" Data1.RecordSource = "Patients"

Data1.Refresh 'Open database & recordset

Do While Not Data1.Recordset.EOF If Not IsNull(Data1.Recordset(1)) Then 'If not empty, then List1.AddItem Data1.Recordset(1) 'add to the list End If Data1.Recordset.MoveNext 'Move to next record Loop Data1.Refresh 'Rebuild the recordset End Sub

Component 8, Slide 10

CP2030 Visual basic for C++ Programmers, ‘The VB Team’ Copyright © University of Wolverhampton

Alternative version (using For loop and RecordCount property)

Data1.Recordset.MoveLast ‘sets value of RecordCountData1.Recordset.MoveFirstFor I = 1 to Data1.Recordset.RecordCount If Not IsNull(Data1.Recordset(1)) Then 'If not empty, then List1.AddItem Data1.Recordset(1) 'add to the list End If Data1.Recordset.MoveNext 'Move to next

record Next I



Find Instructions

The FindFirst, FindLast, FindNext and FindPrevious instructions are used to locate records in the recordset e.g.Data1.Recordset.FindLast “[First Name] = ‘Jeremy’ ”

Location Field to Criteria

method search to find

A record which matches the criteria is made the current record,

Criteria can be variables, strings with wildcardsData1.Recordset.FindLast “[Surname] = ‘ “ & NameVar

& “ ’ ”

Data1.Recordset.FindLast “[First Name] = ‘J*’ ”



Reposition Event

A Reposition event occurs after a new record becomes current.

First happens when the Data control loads and makes the first record in the recordset current

Is triggered by ‘Move’ or ‘Find’ method

Data Control load

New

Code (Move / Find) RecordReposition

CurrentEvent



Validate Event

Allows validation of changes before database updates

A Validate event occurs before a new record becomes current.

It also occurs before an Update, Delete, Unload or Close operation Update/Delete

Data Control load Unload/Close

Code (Move / Find) Validate New

Event Record

Current



Data Control - Validate event

Private Sub Data1_Validate ( Action as Integer, Save as Integer )

Action parameter identifies current operation– 1 MoveFirst (includes navigation arrow)– 2 MovePrevious ( “ “ “ )– 3 MoveNext ( “ “ “ )– 4 MoveLast ( “ “ “ )– 5 AddNew– 6 Update– 7 Delete– etc…

Save parameter =True (-1) if any data attached to data control has

changed



Data Control - Validate event

Validate event can prevent the action from happening

E.g. Private Sub Data1_Validate ( Action as Integer, Save as

Integer ) If val(txtAge) < 18 then

MsgBox “Too young!” Action = 0 End If End Sub



Validate Event

Save - normally True(-1), set False(0) to stop new data being saved

Note. Don’t use methods under the Validate event e.g. MoveNext, as this creates an infinite loop

MoveNext Validate Event



Multiple Data Controls

Can use two data controls to bring in data from two separate tables

The tables can either be in the same database or in separate databases

Databases could even be different types But the data under each data control is

totally independent of the other data control

The sets of data aware controls update separately; they are dependent on the data control they are attached to



Queries

If want to relate the data from two tables together: Tables must be in same database,

and have a common field to relate Use a query to create the relationship Queries are written using SQL, structured

query language Complex language, but some databases

e.g. MSAccess allow you to build queries using blocks, it will then generate the SQL code which can be used by VB to link in a query



Query Example

If you had two tables having a common field of Patient category, one table gives patient demographics and the second costing rates and descriptors for each category.

The SQL to use as the RecordSource to relate the patient name to the cost rate would look like Data1.RecordSource = "SELECT Patients.*, [Cost

Rate] FROM Patients, Costing, Patients INNER JOIN Costing ON Patients.[Category] = Costing.[Category] Order by [Surname]"

It isn’t simple !!



Other Topics To Consider

Transaction Statements - Used to control a series of updates to a database, can be used to allow the ability to undo updates

Error Event - Happens when an error occurs such as an invalid database name being specified

UpdateRecord as for Update except doesn’t trigger a validate event

UpdateControl - reads back values of current record from the recordset into bound controls, means changes to their values can be undone



Other Topics To Consider

FieldSize - gives the size of a memo field GetChunk - gets a 64k chunk of data from

a memo field, it has an offset parameter for where to get the chunk

AppendChunk - appends data in 64k chunks to a memo field



Last Words on Database

Visual Basic Professional allows greater control over database development with

Ability to create dynaset in code without using a data control

Tools to create new databases and modify existing database structures

Visual Basic uses the MSAccess engine Visual Basic 3.0 won’t link to MSAccess 2.0

developed database, need to install a fix file COMLAYER.EXE. This gives full compatibility and link ability.

cp2030 visual basic for c++ programmers

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