1. Database Management Systems
Chapter 7
Calculations and Data
Manipulation

2. Database Programming Variables Computations Standard Functions Debug
Output
Input
Conditions
Loops
Arrays
Data on Forms
Properties
Events
DoCmd: Internal Commands
Data Transfer Across Forms
Data Lookup
Programming SQL
Database access programming
Functions & statements grouped by task

3. Computing Totals
Should you save totals in the database or just use queries and reports to obtain the values as needed?
Queries and reports are the preferred method.
Easier to create.
Always have current values.
But sometimes totals are stored to improve performance.
If data rarely changes, faster to compute it once (e.g., sales tax).
Calculations can lead to difficult queries.
Also, totals are sometimes stored to prevent them from being changed automatically.

4. Programming Environment
DBMS
Tables
Create code
(1) In forms and reports
(2) Within the query system
(3) Hosted in external programs
Queries
(2)
If ( . . ) Then
SELECT . . .
Else . . .
UPDATE . . .
End If
External
Program
Forms &
Reports
(3)
C++
if (. . .) {
// embed SQL
SELECT … }
(1)
If (Click) Then
MsgBox . . .
End If

5. Windows Environment Before Windows With Windows Monolithic Code
Programming
Appendix
Windows Environment
Before Windows
Your code did all of the work.
Programmer In complete control.
Code ran from top to bottom.
With Windows
Structure and interface are defined by Windows standards.
Event-driven. Code is in small pieces that are called as needed, when some event occurs.
Monolithic Code
Windows Form
On_Click
On_Change
MsgBox . . .
UPDATE . . .
Start here
Do this routine
If (. . .) Then
print . . .
End If
Wait for input
More routines
End here
Event/Trigger

6. Data on Forms Simple assignment Naming conventions Full names
[control] = value
value = [control]
Naming conventions
Is it a variable or a control?
Full names
Forms![myform]![ctlTax]
Null values:
IsNull([control])
Form Code
32.50
ctlTax
Sub ComputeTax
Total = …
taxRate = ...
[ctlTax] = taxRate*Total
End Sub
After Update

7. Control Properties and Methods
Set properties with code
Examples
Visible/Invisible
Locked/Unlocked
Combo box RowSource
Methods
SetFocus
Undo
Requery
Code examples
[control].Visible = False
[control].Locked = False
[control].SetFocus
[combo].RowSource= “SELECT ... FROM . . .”
Employee
Sub Employee_AfterUpdate()
If (Employee = “manager”) Then
[cmdSalary].Visible = True
[cmdSchedule].Visible = True
Else
[cmdSalary].Visible = False
[cmdSchedule].Visible = False
End If
End Sub

8. Transfer Data Across Forms
Full names:
Forms!
[formname1]!
[control1]
Forms must be open
Form1.Visible = True
Subforms:
Forms!
[formname1]!
[subform].Form!
[control1]

9. Common Properties Forms Controls Controls Record Source Caption
Default View
Menu/Scroll Bars
Navigation/Selection
Size/Center
Pop Up/Modal/Border
Controls
Name
Control Source
Format/Decimal/Default
Controls
Input Mask
Validation Rule/Text
Status Bar
Auto Tab/Enter
Visible
Enabled/Locked
Tab Stop/Index
Size, Position
Back/Fore Color/Effects
Border
Font

10. Common Events Forms Controls Current Before/After Insert
Before/After Update
Delete
Before/After Del Confirm
Open/Close
Load/Unload
Activate/Deactivate
Got/Lost Focus
Click/Double Click
Error/Timer
Controls
Before/After Update
Change
Enter/Exit
Got/Lost Focus
Click
Double Click
Mouse
Key Down/Up/Press

11. DoCmd: Internal Commands
See help system
Common uses
FindNext
FindRecord
GoToControl
GoToPage
GoToRecord
Hourglass
Maximize/Minimize
OpenForm
OpenQuery
OpenReport
Print
Quit
RunApp (Shell)
RunSQL
SetWarnings
TransferDatabase
TransferSpreadsheet

12. Programming: SQL Select Into Insert Into Update Delete
New table
Insert Into
Append rows
Update
Change data
Delete
Delete rows
Operate on sets of data
Use String for Where
Use DoCmd RunSQL

13. ADO Update Changes set of values--based on Where Syntax Examples
Update table SET col1=val1, col2=val3 Where condition
Examples
Dim cnn as ADODB.Connection
Dim cmd as ADODB.Command
Set cnn = CurrentProject.Connection
Set cmd = CreateObject(“ADODB.Command”)
cmd.ActiveConnection = cnn
cmd.CommandType = adCmdText
strSQL = “Update Order Set EstShipDate=OrderDate+3”
cmd.CommandText = strSQL
cmd.Execute

14. SQL Example: Employee Titles
EID Name Phone Title
2298 Adams 2253 Manager
9983 Cuervo 9973 Supervisor
2736 Dubai 3385 Worker
Employee Table
Change titles
Create a form
Old title (combo)
New title
Command button
SQL
Manager Go
Team Leader
UPDATE Employee
SET Title = " Team Leader "
WHERE Title = " Manager ”;

15. SQL Example: New Titles
UPDATE Employee
SET Title = "Team Leader"
WHERE Title = "Manager";
Manager Go
Team Leader
Sub cmdGo_Click
Build Update command in a String,
using values from OldTitle and NewTitle.
Execute the command.
End Sub

16. Build the SQL Update Command: 97
Sub cmdGo_AfterUpdate
Dim strSQL As String
strSQL = "UPDATE Employee SET Title = ” & txtNewTitle _
& " WHERE Title = ” & cboOldTitle
End Sub
strSQL:
UPDATE Employee SET Title = Team Leader
WHERE Title = Manager
Problem: You need quotation marks around the titles.

17. SQL Update Example SQL Sample
UPDATE Employee SET Title = "“Team Leader "
WHERE Title = "Manager";
Sub cmdGo_AfterUpdate
Dim strSQL As String, qt as String
Dim cnn As ADODB.Connection, cmd As ADODB.Command
q = “’” ‘ single quotation mark
strSQL = "UPDATE Employee SET Title = " & q & [txtNewTitle] & q _
& " WHERE Title = " & q & [cboOldTitle] & q & " ; "
Set cnn = CurrentProject.Connection
Set cmd = CreateObject(“ADODB.Command”)
cmd.ActiveConnection = cnn
cmd.CommandType = adCmdText
cmd.CommandText = strSQL
cmd.Execute
End Sub

18. SQL UPDATE (Oracle and SQL Server)
UPDATE Employee SET Title = "Team Leader "
WHERE Title = "Manager";
Oracle
PROCEDURE ChangeTitle(oldTitle IN VARCHAR2(50), newTitle IN VARCHAR2(50)) IS
UPDATE Employee SET Title = newTitle
WHERE Title = oldTitle;
END ChangeTitle;
SQL Server
CREATE PROCEDURE VARCHAR(50)) AS
UPDATE Employee SET Title
WHERE Title GO
END

19. Debug your Query

20. Insert Into (1) Adds rows to a table Syntax
Insert Into table (col1, col2, …)
Values (value1, value2, …
CID Last First Phone …
938 Sosa Javier 8474
Masi Jorge
LastName FirstName
Masi
Jorge
strSQL = "Insert Into [Customer] (Last, First) "
strSQL = strSQL & " Values ( " & q & [ctlLast] & q
strSQL = strSQL & " , " & q & [ctlFirst] & q & “ )"
cmd.CommandText = strSQL
cmd.Execute

21. Insert Into (2) Copies data rows to a second table Syntax
INSERT INTO {new table, columns} SELECT {any SQL}
Example, move all customers who have not placed orders recently.
Let users define “recently” by picking the number of days.
SQL
INSERT INTO OldCustomer
SELECT * FROM Customer
WHERE CustomerID NOT IN
(SELECT CustomerID
FROM Order
WHERE (Odate > Date() - x);

22. Code Example for Insert
CustomerID Name Phone
7763 Juarez 9987
3635 Kramer 2285
4456 Ciaro 8474
CustomerID Name Phone
7763 Juarez 9987 …
Customers who have not placed an order within some time frame.
O# C# Odate
Dim strSQL
strSQL = “INSERT INTO OldCustomer”
strSQL = strSQL & “ SELECT * FROM Customer WHERE”
strSQL = strSQL & “ CustomerID NOT IN”
strSQL = strSQL & “ (SELECT CustomerID FROM Order”
strSQL = strSQL & “ WHERE (Odate > Date() - “
strSQL = strSQL & [txtDays]
cmd.CommandText = strSQL
cmd.Execute
Time frame (txtDays)
is given by user.

23. Delete Delete a set of rows that match a condition
Syntax: Delete From table Where condition
Cascade on Delete!
Example: Move old customer data
strWhere = “CustomerID NOT IN (SELECT CustomerID FROM Order”
strWhere = strWhere & “ WHERE (ODate > Date() – “ & [txtDays] & “)”
strSQL = “INSERT INTO OldCustomer”
strSQL = strSQL & “ SELECT * FROM Customer WHERE “ & strWhere
cmd.CommandText = strSQL
cmd.Execute
strSQL = “DELETE FROM Customer WHERE “ & strWhere

24. Data Lookup Commands: D...
Syntax
D... (expr, domain, criteria)
“Column”
“Table”
“Where Clause”
Functions
DAvg, DCount, DFirst,
DLast, DMin, DMax,
DStDev, DStDevP,
DSum, DVar, DVarP
Dlookup
Usually better to use SQL.
Or to write DAO code.
Column
Table
V = DSum(“BalanceDue”, “Customer”,
“City=‘Chicago’”)
Where Clause

25. Row-Level Object Programming
Purpose
Track through table or query one row at a time.
Data cursor/pointer to active row.
Why?
Performance.
SQL cannot do everything.
Complex calculations.
Compare multiple rows.
Year Sales
,321
,998
,004
,736
,321
,998
,004
,736
MoveNext
MovePrevious
MoveFirst
MoveLast
Move
Test for Beginning and End of File

26. Application Data Object (ADO) Programming
ADODB.Connection
ADODB.Command
ADODB.Recordset
Dim cnn As ADODB.Connection
Dim rst As ADODB.Recordset
Dim strSQL As String
Set cnn = CurrentProject.Connection
Set rst = CreateObject(“ADODB.Recordset”)
strSQL = “my query”
rst.Open strSQL, cnn

27. ADODB Recordset Initialization
Dim cnn As ADODB.Connection
Dim rst As ADODB.Recordset
Dim strSQL As String
Set cnn = CurrentProject.Connection
Set rst = CreateObject("ADODB.Recordset")
strSQL = "SELECT LastName, FirstName FROM Customer"
rst.Open strSQL, cnn, adOpenForwardOnly, adLockReadOnly
Do Until rst.EOF
‘ Do something with the data
rst.MoveNext
Loop
rst.Close
Access 2000

28. Problems with Multiple Users
Original Data
Modified Data
Name Sales
Alice 444,321
Carl 254,998
Donna 652,004
Ed 411,736
Name Sales
Alice 444,321
Neal 333,229
Carl 254,998
Donna 652,004
Ed 411,736
New row is
added--while
code is running.
rst.Open “Data”, cnn, , , adCmdTableDirect
Value1 = rst(“Sales”) ‘ Alice (444,321)
rst.MoveNext ‘ Carl …
rst.MovePrevious ‘ ??? Which row

29. Table Locator Commands
Move Commands
.MoveNext
.MovePrevious
.MoveFirst
.MoveLast
.Move nRows
Location tests
BOF
EOF
Bookmarks
Dim MyMark As String
MyMark = rst.Bookmark
(Save position)
.Move . . .
(Move somewhere else)
rst.Bookmark = MyMark
(Return to mark)
Name Sales
Alice 444,321
Neal 333,229
Carl 254,998
Donna 652,004
Ed 411,736
Save
position
bmk = rst.Bookmark
rst.MoveNext
rst.Bookmark = bmk

30. DAO: Recordset Find Commands
.FindFirst “condition”
.FindLast “condition”
.FindNext “condition”
.FindPrevious “condition”
Inefficient: Sequential search
Use SQL instead
rs.NoMatch
rs.Seek
Indexed
One item
rst.Index = “PrimaryKey”
rst.Seek keyvalue, adSeekFirstEQ
If (Not rst.EOF) Then
‘ Make changes
End If

31. ADO: Recordset Find & Seek Commands
rst.Find “condition”
Inefficient: Sequential search
Use SQL instead
rst.EOF ‘ True if not found
rst.Seek value, adSeekFirstEQ
rst.Open “Table”,cnn, adOpenDynamic ,adLockReadOnly, _ adCmdTableDirect
rst.Index = “PrimaryKey”
rst.Seek keyvalue, adSeekFirstEQ
If (not rst.EOF) Then
‘ Make changes
End If

32. Sample ADO Code Compute total of BalanceDue. Normally use SQL instead.
Dim dblSum as Double
Dim cnn as ADODB.Connection
Dim rst As ADODB.Recordset
Set cnn = CurrentProject.Connection
Set rst = CreateObject(“ADODB.Recordset”)
rst.Open “SELECT * FROM Customer”, cnn, _
adOpenForwardOnly, adLockReadOnly
dblSum = 0.0
Do Until (rst.EOF)
dblSum = dblSum + rst(“BalanceDue”)
rst.MoveNext
Loop
MsgBox “Total Due = “ & dblSum
rst.Close
cnn.Close
Compute total of
BalanceDue.
Normally use SQL instead.

33. Sample ADO Code to Change Data
Dim cnn as ADODB.Connection
Dim rst As ADODB.Recordset
Set cnn = CurrentProject.Connection
Set rst = CreateObject("ADODB.Recordset")
rst.Open strSQL, cnn, adOpenDynamic, adLockOptimistic
Do Until (rst.EOF)
rst(“BalanceDue”) = rst(“BalanceDue”)*(1 + [PctIncrease] )
rst.Update
rst.MoveNext
Loop
rst.Close
Add a 10% charge to the BalanceDue
for every customer. The 10% value is
entered on the form by the user.
What happens if the row is already locked?
Normally use SQL instead.

34. Error Handling Errors are events Simple code: Event Options
Display error and exit
MsgBox Err.Description, , “Title”
Resume Exit_Label
Event Options
On Error Goto [label]
On Error Goto 0
On Error Resume Next
Resume Options
Resume
Resume Next
Resume label
On Error Goto ErrSub1
Program code
ExitSub1:
Exit Sub
ErrSub1:
MsgBox Err.Description,,”Errors”
Resume ExitSub1
Error occurs

35. Alternate Error Handling
Sub mySubroutine
On Error Resume Next …
err.Clear ‘ clears error object
rst.Open strSQL, cnn
If (err.Number <> 0) Then
… Handle the error
End If
Exit Sub
Error occurs,
skip to next line.
Test for error,
handle it.

36. Concurrent Access Concurrent Access Force sequential Two processes
Multiple users or processes changing the same data at the same time.
Final data will be wrong!
Force sequential
Locking
Delayed, batch updates
Two processes
Receive payment ($200)
Place new order ($150)
Initial balance $800
Result should be $ = $750
Interference result is either $600 or $950
Customers
Receive Payment
Place New Order
ID Balance
Jones $800
$600
$950
1) Read balance 800
2) Subtract pmt -200
4) Save new bal. 600
3) Read balance 800
5) Add order 150
6) Write balance 950

37. Deadlock Deadlock Many solutions Data A Data B 1) Lock Data A
3) Wait for Data B
Deadlock
Two (or more) processes have placed locks on data and are waiting for the other’s data.
Many solutions
Random wait time
Global lock manager
Two-phase commit - messages
Data A
Data B
2) Lock Data B
4) Wait for Data A

38. Lock Manager

39. Concurrent Access Customer Balance Adams 152.35 Brown 315.81
Jones
Using &
locked
Running the program to add interest charges generates an error message.
Do until rst.EOF
rst(“Balance”) = rst(“Balance”)*(1.0+rate)
rst.Update
rst.MoveNext
Loop

40. Errors and Locks "Error (RS)” ) = vbRetry ) Then On Error Goto ErrSub1…
rst(“Balance”) = rst”(Balance”)*1.10
rst.Update
ExitSub1:
Exit Sub
ErrSub1:
If ( MsgBox (Err.Description, vbRetryCancel, _
"Error (RS)” ) = vbRetry ) Then
Resume
Else
Resume ExitSub1
End If
If the table is locked,
Edit will cause an error.
Let the user retry the edit or exit.
To do it automatically, wait
for a random number of seconds.

41. Optimistic Locks Assume that collisions are rare
Improved performance, fewer resources
Allow all code to read any data (no locks)
When code tries to write a new value
Check to see if the existing value is different from the one you were given earlier
If it is different, someone changed the database before you finished, so it is a collision--raise an error
Reread the value and try again

42. Optimistic Locks for Simple Update
(1) Read the balance
(2) Add the new order value
(3) Write the new balance
(4) Check for errors
(5) If there are errors, go back to step (1).

43. Post’s Picky Programming
Use a naming convention.
Use proper indentation.
Comment your work.
Avoid spaces in variable names.
Use Option Explicit.
Recompile constantly.
Use as many parentheses as possible.
Split complex conditions.
Make it easy for the user.
Use the status bar and tool tips.
All code must be subject to error trapping.
Use Retry with rst.Edit sections.
Use subroutines and functions to simplify.
Keep backup copies.
Never use a raw number--use Const.
Remember that databases can be moved.
Test applications on different hardware.
Test all calculations by hand.

44. Comments Weak comments Useful comments
dblSum = 0# ' Initialize the accumulator
Do While Not rst.EOF ' Loop through the table
dblSum = dblSum + rst(“Balance”) ' Accumulate the balance
rst.MoveNext ' Move to the next row
Loop ' End the loop
' Need to compute total balance from sales
' Will use a loop instead of SQL
' Because some conditions will be added later
dblSum = 0#
Do While Not rst.EOF
' Add condition when user provides it, for example
' If this customer has more than three sales past due,
' only count the three most recent (write off the older ones)
dblSum = dblSum + rst(“Balance”)
rst.MoveNext
Loop
Useful comments

45. Sally’s Pet Store Main Switchboard Employee logs in.
Buttons are presented based on the management level of the employee.
Accounting
Marketing
Employees
Purchasing forms
are accessible by
this employee.
Not available
to this employee.

46. Sally’s Pet Store: Switchboard Logic
Event: EmployeeID AfterUpdate
On Error Goto ErrEIDAU
Declare variables.
Lookup assigned ManagementLevel of employee.
Get Management levels for each section.
Make two sections of buttons invisible.
If (MgtLevel > Level1) Then
Make first section of buttons visible.
If (MgtLevel > Level2) Then
make second section of buttons visible.
End If
ExitEIDAU:
Exit Sub
ErrEIDAU:
MsgBox
Resume ExitEIDAU

47. Sally’s Pet Store: Switchboard Code
Private Sub EmployeeID_AfterUpdate()
On Error GoTo ErrEIDAU
Dim varLevel, varMGTLEVEL1, varMGTLEVEL2
If Not IsNull(EmployeeID) Then
varLevel = DLookup("EmployeeLevel", "Employee", _
"EmployeeID=" & [EmployeeID])
If Not IsNull(varLevel) Then
varMGTLEVEL1 = DLookup("Value", "Preferences", _
"KeyID=" & qt & "MGTLEVEL1" & qt)
varMGTLEVEL2 = DLookup("Value", "Preferences", _
"KeyID=" & qt & "MGTLEVEL2" & qt)
End If
cmdAnimalPurchase.Visible = False
cmdMerchandisePurchase.Visible = False
cmdInventory.Visible = False
cmdAccounting.Visible = False
cmdMarketing.Visible = False
cmdEmployees.Visible = False

48. Sally’s Pet Store: Switchboard Code
If (varLevel > Val(varMGTLEVEL1)) Then
cmdAnimalPurchase.Visible = True
cmdMerchandisePurchase.Visible = True
cmdInventory.Visible = True
If (varLevel > Val(varMGTLEVEL2)) Then
cmdAccounting.Visible = True
cmdMarketing.Visible = True
cmdEmployees.Visible = True
End If
ExitEIDAU:
Exit Sub
ErrEIDAU:
MsgBox Err.Description, , "Unexpected Error (EIDAU)"
Resume ExitEIDAU
End Sub

49. Sally’s Pet Store: Employees
Enter a ZIP code and the form
tries to find a matching city.
Choose a city and the ZIP code
is entered automatically.
Spin buttons can be used
to set employee level.

50. Sally’s Pet Store: Employee Spin Button
Private Sub SpinLevel_SpinDown()
If IsNull(EmployeeLevel) Then
EmployeeLevel = 0
Else
If (EmployeeLevel > 0) Then EmployeeLevel = EmployeeLevel - 1
End If
End Sub
Private Sub SpinLevel_SpinUp()
EmployeeLevel = 1
If (EmployeeLevel < 255) Then EmployeeLevel = EmployeeLevel + 1

51. Sally’s Pet Store: City
Private Sub CityID_AfterUpdate()
On Error GoTo ErrCIDAU
If IsNull([ZipCode]) Then
[ZipCode] = DLookup("ZipCode", "City", "CityID=" & [CityID])
End If
ExitCIDAU:
Exit Sub
ErrCIDAU:
MsgBox Err.Description, , "Unexpected Error (CIDAU)"
Resume ExitCIDAU
End Sub
‘ Requires a large city table.
‘ Do not replace an existing ZipCode entry.

52. Sally’s Pet Store: ZipCode
Private Sub Zipcode_AfterUpdate()
On Error GoTo ErrZCAU
Dim strZipShort As Variant, newCityID As Variant
strZipShort = Get5DigitZipCode(ZipCode)
newCityID = DLookup("CityID", "City", _
"ZipCode=" & qt & strZipShort & qt)
If Not IsNull(newCityID) Then
[CityID] = newCityID
End If
ExitZCAU:
Exit Sub
ErrZCAU:
MsgBox Err.Description, , "Unexpected Error (ZCAU)"
Resume ExitZCAU
End Sub
‘ City table only uses 5 digit codes.
‘ But we need to store 9 digits in ZipCode.

53. Appendix: SQL Server: Data Types
Primary Data Types
DECIMAL(precision, scale)
precision: Number of digits
scale: Round-off point
DECIMAL(7,4):
INTEGER
BIT
Yes/No
CHAR
Fixed length string
VARCHAR
Variable length string
IMAGE
Binary data
DATETIME

54. Appendix: SQL Server Structure
CREATE PROCEDURE integer) AS
BEGIN
DECLARE myVar decimal
IF > 10)
END

55. Appendix: Variables
CREATE PROCEDURE integer) AS
BEGIN
DECLARE myVar decimal
IF > 10)
END

56. Appendix: Transact-SQL Operators
Operation
Power(x, n)
Exponentiation
*, /
Multiply, Divide
+, -, +
Add, Subtract, Concatenate
=, <>, <, >, <=, >=,
IS NULL, LIKE, BETWEEN, IN
Comparison
NOT, AND, OR
Logical operators

57. Appendix: Transact-SQL CASE
CASE Account
WHEN ‘P’ THEN -- do personal accounts
WHEN ‘C’ THEN -- do corporate accounts
WHEN ‘S’ THEN -- do small business
ELSE -- handle error
END
IF (condition)
BEGIN
-- statements
END
ELSE
-- other statements

58. Appendix: Transact-SQL Loops
integer
= 0
WHILE < 10)
BEGIN
IF (error condition)
BREAK
@I + 1
END

59. Appendix: Procedures or Subroutines
CREATE Procedure datetime) AS
-- local variables are defined here
BEGIN
-- first copy the data to a backup table
INSERT INTO OldAccounts
SELECT * FROM Account WHERE AccountID NOT IN
(SELECT AccountID FROM Order WHERE
ODate GO
-- Copy additional tables …
-- Delete from Account automatically cascades to others
DELETE FROM Account WHERE AccountID NOT IN
END

60. Appendix: SQL Cursors
datetime,
@varTotal money
DECLARE c1 CURSOR FORWARD_ONLY READ_ONLY
SELECT Name, Salary, DateHired FROM Employee
OPEN c1
FETCH NEXT FROM @Hired
= 0
WHILE
BEGIN
END
CLOSE c1
DEALLOCATE c1 GO
-- Now do something

61. Appendix: Error Handling
CREATE PROCEDURE myProc ( ) AS
DECLARE -- declare all local variables
BEGIN
-- SQL statement GO
IF <> 0)
-- Transact-SQL code to handle the error
END