CIS 338: Using DAO (Data Access Objects)

Dr. Ralph D. WestfallFebruary, 2003

Data Access Objects

VB objects that can be used in program code to work with databases can set properties in code can use methods " "

can do more than with Data control e.g., input validation based on business

rules handling data from automated sources

Starting a DAO Project

start a new VB standard EXE projectselect Project>Referencesclick Microsoft DAO 3.6 Object Library use 2.5/3.5 if will work with older

databases

Starting a DAO Project - 2

add code that will set up the database above Form_Load code (General):

Dim db[Name] As Database in Form_Load code:

Dim s[dbloc] As Strings[dbloc] = App.Path & "\[file].mdb" 'or use actual physical pathSet db[Name] = _ OpenDatabase(s[dbloc])

Recordset Type - Table

gives direct access to table/all records can change sort order can see changes by other users above Form_Load code:

Dim rs[name] As Recordset in Form_Load code:

Set rs[name] = _ db[name].OpenRecordset("[table]", _ dbOpenTable) 'sample database

Recordset Type - Dynaset

fields/records from one tableSet rs[name] = db[name].OpenRecordset_ ("[table]", dbOpenDynaset)

can use SQL to get data from multiple tables

Dim sSQL as StringsSQl = "SELECT … FROM … WHERE …"Set rs[name] = db[name].OpenRecordset_ (sSQL, dbOpenDynaset)

Recordset Type - Snapshot

gets "read only" copies from one or more tables faster processing uses more memory can't change sort order

Set rs[name] = db[name].OpenRecordset_ ("[table]", dbOpenSnapshot) 'table

Set rs[name] = db[name].OpenRecordset_ (sSQL, dbOpenSnapshot) 'SQL string

Recordset Syntax

Set rs[name] = [object].OpenRecordset _ (source, type, options, lockedits) object: usually a database source (required): table, query in

database, SQL string optional parameters:

type: must be dbOpenTable for table source, can be dbOpenDynaset, etc. if from query or SQL

options: dbReadOnly, dbForwardOnly, etc.

Putting DAO Data in Form

add controls (textboxes) to formwrite code to set property of control to field in database textboxes: set Text property

txt[name].Text = _ rs[name].Fields("[name]") 'space prob

txt[name].Text = rs[name]![name] could set label Caption similarly 'Notes

Using Record Pointer

record pointer identifies specific recordpositioning Move methods – moves pointer Find and Seek methods – goes to

specific record Bookmark property – saves location AbsolutePosition – go to record # PercentPosition – go % to of total items

Using Record Pointer - 2

Move MoveFirst, MoveNext, MovePrevious,

MoveLast Move n e.g. Move 2, Move –35

(negative)rs[name].MoveFirstrs[name].Move 14 'Notes

Using Record Pointer - 3

Find FindFirst, FindNext, FindPrevious, FindLast

rs[name].FindFirst [criteria] criteria = string like in a SQL WHERE clausesCriteria = "State='CA'"

rsCustomers.FindFirst sCriteria rs[name].NoMatch indicates if not foundIf rsCustomers.NoMatch = True 'not found

Using Record Pointer - 4

Bookmark identifies a location (like a record number, but stored as string)

sVariable = rs[name].Bookmark stores location of current record

rs[name].Bookmark = sVariable moves to bookmarked record

rs[name].PercentPosition = 10

Conditions in Strings

need to get single quotes around strings in an SQL query or Find criteriasCriteria = "[field] = ' " & [string] & " ' "

note ' " before string, " ' aftersCriteria = "Title = '" & sTitle & "'"

If sTitle = "Jaws" this evaluates to sCriteria = "Title like 'Jaws'"

'see p. 580

Conditions in Strings - 2

can store a single quote as a string variable to set up SQL strings as criteria

SQ = "'" 'single quote in double quotes

sCriteria = "Title = " & SQ & sTitle & SQ

'also mentioned on p. 580

Using Indexes

indexes control sort order different indexes for different sorts indexes must be created before

recordset can only use indexes with table

recordsets, not dynasets or snapshots rs[name].Index = "[name of index]"

'existing index

'creating a new index (MSDN site)

Filters

work with dynasets, snapshots, not tablesrs[name].Filter = [criterion] criterion = string like condition in a

SQL WHERE clause, but without the word WHERE

sCriterion = "Zip='90806'"

rsStudents.Filter = sCriterion

Sorting

works with dynasets and snapshots, but not with tablesrs[name].Sort = [field(s)] sorts according to "collating sequence"

in ASCII: spaces first, followed by some quotation marks, numbers, capital letters, more quotation marks, then lower case

rsCustomer.Sort = "City, Zip"

Updating Database: AddNew

adding a recordrs[name].AddNew

rs[name]![field1] = [value]

rs[name]![field2] = [value]

rs[name].Update

Updating Database: Edit

editing a record values replace data in current record

rs[name].Edit

rs[name]![field1] = [value]

rs[name]![field2] = [value]

rs[name].Update

'very similar to .Add in DAO

Updating Database: Delete

deleting a record current record is deleted can use record pointer positioning

commands (like Find) to identify record to delete

rs[name].Delete before other actions, need to be sure

record is not on EOF or will not be on BOF

Transaction Processing

used when transaction (group of actions) all need to happen to database at same time all or none basis

BeginTrans – starts transactionws[name].BeginTrans

'ws = workspace objectRollBack – removes all changes since BeginTransCommitTrans – permanently saves (can't be rolled back afterwards)