Database vs. DBMS

© 1999, by Que Education and Training, Chapter 10, pages 511-545 of Introduction to Computer Programming with Visual Basic 6: A Problem-Solving Approach

Database vs. DBMS Database

A collection of organized, related tables (data stored in rows and columns)

Database Management System Also known as DBMS Software used to create and maintain a database


Entity-Relationship Diagram Also known as ERD

Graphical representation showing the relationships of entities in a database

Entities things of interest to business (products, employees,

suppliers, customers, purchases, sales,…)

Relationships real-world associations between entities

– products purchased by customers

– employees who designed a particular part

– suppliers who provided a particular raw material


ERD CardinalityRelated to one

Related to one or zero (optional)

Related to zero or more

Related to one or more

Student Course Teacher

Each course belongs to a specific teacher (manager)A teacher may teach one or more coursesA student can sign up for one or more coursesA course may have no students (not offered)


Normalized Databases Normalization

A process of organizing database tables to increase stability through reduced (controlled) data redundancy.

Higher normal forms further restrict how attributes are grouped within records

3NF (Third Normal Form) is the generally accepted, minimum standard for designing a database

Every nonkey field is directly related to only to the entire key field and nothing else

– {Emp # (key), Emp Name, Dept Code, Dept Name} is not in 3NF since Dept Name can be derived entirely from dept code (both non-key fields)

– {Prod # (key), Prod Name, Qty, Cost, ValueinInventory} is not in 3NF since ValueInInventory is a calculated value (from Qty & Cost)

– {RawMat # (key), Prod #, AmtReqrd} is not in 3NF since AmtReqrd is derived from both RawMat# and Prod #


Database TerminologyNorthwind DB tables

Records 3, 6 8 are condiments

Second record or row of Categories table describes condiments

Key fieldsRelationship based on shared data in two tables


Databases Accessible by VB VB Native format

Microsoft Jet database engine Microsoft Access

Jet engine can also access Excel, dBASE, FoxPro, Lotus, Paradox, text files

VB Professional with Open DataBase Connectivity (ODBC) drivers Can also access SQL Server, Oracle, and DB2


Data Control

data control

Provides access to data stored in databases using any one of three types of Recordset objects: table snapshot synaset

Enables data navigation viewed through bound controls.

Without a Data control, data-bound controls on a form can't automatically access data.

Validate event occurs just before new record becomes current.


Data Control Properties Caption

Text that appears inside the control on the form Connect

Identifies DBMS source of database (Access) DatabaseName

Assigns the database filename with path (use dialog) ReadOnly

Defaults to False & must be set to true if user is not allowed to change contents

RecordSource Assigns the table or query (view) of the database to be used by the

VB program BOF and EOF

Flags set to true when current record pointer positioned before first record or after last record


Data-Bound Controls Same controls we’ve used thus far with additional

properties set Labels (user cannot change), Textboxes (user can change),

Checkboxes (T/F or Y/N), etc.

DataSource point to database (specify name of data control)

DataField field name in database table that relates to this

It is possible to navigate data in a database simply by designing a form with the data control and binding data-aware controls to the data control -- no coding is needed!


Programming the Data Control Recordset Object

Defined by setting RecordSource property of data control

Accessing Recordset properties in code datName.Recordset.Property Since this represents a value, it can only be part of

a statement (e.g., assign it to variable or assign something to it)

Applying methods to Recordset object datName.Recordset.Method Since this represents a process, it is written like a

statement


Recordset Properties Checking the database pointer

datName.Recordset. BOF Flag that is true when record pointer is before first database

record (select previous on first record) datName.Recordset. EOF

Flag that is true when record pointer is after last database record (select next on last record)

Counting the number of DB records datName.Recordset.RecordCount

Setting or tracking the current DB record datName.Recordset.BookMark

Determining if desired record was found datName.Recordset.NoMatch


Recordset Move & Refresh Methods Navigating DB via Code

Move to next database record datName.Recordset.MoveNext

Move to last database record datName.Recordset.MoveLast

Move to previous database record datName.Recordset.MovePrevious

Move to first database record datName.Recordset.MoveFirst

Refresh method Explicitly opens the database

datName.Refresh


Adding a First buttonPrivate Sub cmdFirst_Click()

datEx.Recordset.MoveFirst

lblRecNum.Caption = 1

End Sub

cmdFirst_Click

RecNum = 1

End

Move to db record 1


Adding a Last buttonPrivate Sub cmdLast_Click()

datEx.Recordset.MoveLast

lblRecNum.Caption = _

datEx.Recordset.RecordCount

End Sub

cmdLast_Click

RecNum = RecordCount

End

Move to last db record


Adding a Next buttonPrivate Sub cmdNext_Click()

datEx.Recordset.MoveNext

If datEx.Recordset.EOF Then

Call MsgBox(“msg”,…)

Call cmdFirst_Click

Else


CInt(lblRecNum.Caption)+1

EndIf

End Sub

cmdNext_Click

cmdFirst_Click

End

Move to next db record

EOF?TrueFalse

RecNum=RecNum+1

Display already at end - go to start


Adding a Previous buttonPrivate Sub cmdPrevious_Click()

datEx.Recordset.MovePrevious

If datEx.Recordset.BOF Then

Call MsgBox(“msg”,…)

Call cmdLast_Click

Else


CInt(lblRecNum.Caption)-1

EndIf

End Sub

cmdPrevious_Click

cmdLast_Click

End

Move to previous db record

BOF?TrueFalse

RecNum=RecNum-1

Display already at start - go to end


Recordset Find Methods Search for a specific type of record and then move to

it in the specified direction. Specifying search criteria

<DBFieldName> <relational operator> <expression>

Find methods define starting point & search direction Start at first record and search forwards

datName.Recordset.FindFirst (search criteria) Start at last record and search backwards

datName.Recordset.FindLast (search criteria) Start at current record and search forwards

datName.Recordset.FindNext (search criteria) Start at current record and search backwards

datName.Recordset.FindPrevious (search criteria)


Searching Example1) Save the current record

CurRecord = datName.Recordset.BookMark

3) Assign search criteriaSearchStr = InputBox(“prompt”, “title”, “default”)

Target = “TableName.FieldName = `” & SearchStr & “`”

3) Use search criteriaCall datName.Recordset.FindFirst(Target)

If datName.RecordSet.NoMatch Then

Call MsgBox(“ERROR”)

Else

‘ whatever processing needs to be done with current record

End If


Example: Find similar namePrivate Sub cmdFindName_Click()

Dim CurRecord As Variant

Dim SearchString As String

' Save location of current record

CurRecord = datEmployee.Recordset.Bookmark

SearchString = "EmpName Like '" & _

txtSearchName.Text & "*'"

Call datEmployee.Recordset.FindFirst(SearchString)

If datEmployee.Recordset.NoMatch Then

Call MsgBox("No such employee", _

vbOkOnly + vbInformation)

' Move pointer to previously saved location

datEmployee.Recordset.Bookmark = CurRecord

Else

' fill display controls with found record’s data

Call ShowDBData

End If

End Sub


Methods to change DB content Delete current database record datName.Recordset.Delete

be sure to add step to go to a valid record AddNew database record datName.Recordset.AddNew

clears data bound controls so user can enter new data DB automatically updated by moving to another record or explicitly

saving record Edit database record datName.Recordset.Edit

moves current DB record to copy buffer to allow changes to occur Update database with addition datName.Recordset.Update

explicitly saves newly entered data Reset db to previous values

datName.Recordset.UpdateControls

resets data bound controls to previous values


Example: Add a new recordPrivate Sub cmdAddRec_Click()

If cmdAddRec.Caption = "&Add" Then ' 1) Clear input controls

Call DisableNavigation

Call datEmployee.Recordset.MoveLast

txtID.Text = Format(datEmployee.Recordset!empid + 1)

Call datEmployee.Recordset.AddNew

txtEmpName.Text = ""

txtBirthdate.Text = ""

txtPayRate.Text = ""

' Change caption to Save and disable other buttons

cmdAddRec.Caption = "&Save"

Call txtEmpName.SetFocus

Else ' the caption must be &Save ' 2) Write user input to DB

Call WriteDBData

datEmployee.Recordset!empid = txtID.Text

Call datEmployee.Recordset.Update

' Change caption to Add and enable other buttons

cmdAddRec.Caption = "&Add"

Call EnableNavigation

End If

End Sub


Example: Delete a recordPrivate Sub cmdDeleteRec_Click()

Dim CurRec As Variant

CurRec = datEmployee.Recordset.Bookmark ' The record to delete

datEmployee.ReadOnly = False

Call datEmployee.Refresh

Call datEmployee.Recordset.MoveLast

datEmployee.Recordset.Bookmark = CurRec

Call datEmployee.Recordset.Delete

datEmployee.ReadOnly = True

Call datEmployee.Refresh

' Move to the last record

Call cmdLast_Click

If datEmployee.Recordset.EOF Then ' Last record was deleted

Call cmdFirst_Click

If datEmployee.Recordset.BOF Then ' No records left

Call MsgBox("There are no records left-the database" & _

" is empty", vbOkOnly + vbInformation)

End If

End If

End Sub


Example: Change a recordPrivate Sub cmdModifyRec_Click()

' The current record will be the one to be modified

If cmdModifyRec.Caption = "&Modify" Then ' 1) Allow edit of record

Call DisableNavigation

Call datEmployee.Recordset.Edit

' Change caption to Save and disable other buttons

Call txtEmpName.SetFocus

cmdModifyRec.Caption = "&Save"

cmdDeleteRec.Enabled = False

cmdAddRec.Enabled = False

Else ' the caption must be &Save ' 2) Copy updates to DB

Call WriteDBData

Call datEmployee.Recordset.Update

' Change caption to Modify and enable other buttons

cmdModifyRec.Caption = "&Modify"

cmdDeleteRec.Enabled = True

cmdAddRec.Enabled = True

Call EnableNavigation

End If

End Sub


Preventing Errors

Examples: The primary key must

be non-blank All data entered must

be of correct type and length

The database table has valid records (not an empty table)

Pseudocode/Algorithm

If a required field is empty and both BOF and EOF are False Then

Show error msg

Reset error condition so program can run

Don’t save changes

Endif

Add code to the Validate Event of data control Add error handlers in all procedures that access DB


Checking for Empty RecordsetsIf BOF and EOF Then

Display error message

End If By checking for both BOF & EOF, you are

checking for a recordset with no members


Queries: Joining DB tables1) Define query to get all unique field names from both tables

Query = “Select * From TableName1, TableName2 Where ” & _

“ TableName1.FieldX = TableName2.FieldY”

Query = “Select TableName1.FieldA, TableName1.FieldB, “ & _

“TableName2.FieldZ Where TableName1.FieldX = “ & _

“TableName2.FieldY”

2) Assign Query to RecordSource property & Open the DBdatName.RecordSource = Query

Call datName.Refresh

3) Start at the first recordCall datName.Recordset.MoveFirst

4) Assign value in current DB field to control on formlblName.Caption = datName.Recordset(“FieldName”)

lblName.Caption = datName.Recordset!FieldName


Filling list with DB field choices1) Get filename from user (common dialog)

2) Assign user’s file selection to DatabaseName property of data control

3) Define query to get all unique field namesQuery = “Select Distinct FieldName From TableName”

datName.RecordSource = Query

4) Use Refresh method to open the databaseCall datName.Refresh

5) Clear the list & add any general choices (“Show All”)

6) For each item in the database, add the item to the list and go to next item until doneCall cboName.AddItem(datName.Recordset(”FieldName"))

Call cboName.AddItem(datName.Recordset!FieldName)


Returning select DB records1) Define query using user’s selection

a) General “Show All” choice: Query = “Select * From TableName”

b) Otherwise: Query = “Select * From TableName Where “ & _

“FieldName = `“ & cboName.Text & “`”

2) Assign Query to data controldatName.RecordSource = Query

3) Use Refresh method to open the databaseCall datName.Refresh

4) If flexgrid is used on the form, it will automatically contain all fields of the desired records based on the search criteria

Documents

Database vs. DBMS