Directory listing of http: uap.unnes.ac.id ebook biblebook Visual Basic 6 com & Library Books
6
C H A P T E R
Accessing
Databases from
Visual Basic
✦
✦
✦
✦
In This Cha pter
O DBC and O LE DB
DAO o bject mo del
I
n this c hapter, I’m go ing to disc uss ho w Visual Basic talks
to a database server. ODBC and OLE DB are the lo w-level
tec hno lo gies that ac tually perfo rm the wo rk, while the DAO,
RDO, and ADO o bjec t mo dels enc apsulate these tec hno lo gies
to make them easy fo r the Visual Basic pro grammer to use.
M icrosoft Database
Programming APIs
Just bec ause yo ur database server is based o n SQL do esn’t
mean that yo u c an easily ac c ess it fro m yo ur favo rite pro gramming language. Many database vendo rs pro vide a spec ial prec o mpiler that translates embedded database statements into
database subro utine c alls that in turn c o mmunic ate with the
database server. The pro blem with this appro ac h is that yo u
need a different pre-c o mpiler fo r eac h pro gramming language
the database vendo r suppo rts. Of c o urse, sinc e the pre-c o mpilers are spec ific fo r eac h database server, yo u’ll need a pre-c o mpiler fo r eac h database server fo r whic h yo u develo p pro grams.
Rather than develo ping a large number o f pre-c o mpilers fo r
eac h c o mbinatio n o f database server and pro gramming language, Mic ro so ft develo ped a standard c alled Ope n Database
Co nne ctivity (ODBC), whic h later evo lved into OLE DB.
RDO o bject mo del
ADO o bject mo del
Visual Basic
database to o ls
✦
✦
✦
✦
90
Part I ✦ Database Programming Fundamentals
ODBC
The ODBC standard defines an Applicatio n Pro gramming Inte rface (API) fo r database
pro gramming. This allo ws yo u to write a pro gram using standard subro utine c alls
fo r any database server that suppo rts ODBC, making it po ssible fo r the same o bjec t
c o de to ac c ess any ODBC-c o mpatible database server.
ODBC architecture
ODBC is based o n the idea that the c alls to the API ro utines made by the applic atio n pro gram are translated to lo wer c alls that are passed o nto a database driver
(see Figure 6-1). The database driver in turn perfo rms the nec essary wo rk to talk to
the database server. Thus, the database vendo r need o nly pro vide an ODBC-c o mpatible driver fo r eac h c lient c o mputer system and no t fo r eac h c o mpiler o n eac h
c lient o perating system.
Client Computer
Application
Program
ODBC API
ODBC Database
Driver
Database Server
Database Engine
ODBC Interface
Figure 6-1: The client side of the ODBC architecture is based on the driver m odel.
On the database server side, all the database vendo r needs to do is pro vide a single
ODBC interfac e, whic h will be shared by all ODBC c lients. This means that database
vendo rs c an preserve their native interfac e, plus any o ther spec ialized interfac es
fo r o ther applic atio ns.
Chapter 6 ✦ Accessing Databases from Visual Basic
A utility pro gram c alled the ODBC Administrato r is inc luded as part o f the o perating system to manage the set o f ODBC drivers and database servers that are available to the ODBC API. This pro gram allo ws yo u to add and remo ve ODBC drivers,
spec ify ho w to c o nnec t to the database server, and inc lude sec urity info rmatio n
that will be used when the c o nnec tio n between the c lient c o mputer and the
database server is o pened.
Drawbacks to ODBC
While the arc hitec ture o f ODBC allo ws a great deal o f flexibility o n the part o f the
database vendo r, there are several drawbac ks to writing ODBC applic atio ns. First,
the ODBC APIs are diffic ult to use, espec ially if yo u aren’t pro gramming in C. Sec o nd,
the ODBC APIs are o ften slo wer than the native database interfac e. Third, the ODBC
API o ften impo ses restric tio ns o n the SQL statements that c an be used.
While the first drawbac k applies to all database servers, the sec o nd two drawbac ks
apply mainly to no n-Mic ro so ft database servers. Mic ro so ft uses ODBC as the native
interfac e to bo th the Jet database and SQL Server. They spent a lo t o f time tuning
the interfac e fo r o ptimal results. While o ther database vendo rs suppo rt ODBC, their
native interfac es may o ffer impro ved perfo rmanc e and func tio nality, espec ially fo r
no n-Mic ro so ft c o mpilers.
Database Access Objects (DAO)
Bec ause the ODBC API is diffic ult to use, Mic ro so ft c ho se to build an o bjec t-o riented
interfac e to ODBC c alled Data Acce ss Obje cts (DAO). DAO was o riginally develo ped
fo r Ac c ess and Visual Basic pro grammers who needed to ac c ess the Jet database.
Mo st o f the func tio nality available in DAO mirro rs features fo und in the Jet database.
While DAO c an be used to ac c ess o ther databases, the pro c ess is o ften diffic ult to
use and c lumsy in its implementatio n. To ac c ess an SQL Server database, yo u need
to c reate a Jet database and then go thro ugh the Jet database to ac c ess the remo te
database. While DAO allo ws yo u to ac c ess the ODBC database direc tly, eac h time
yo u o pen the database, yo u must do wnlo ad a lo t o f info rmatio n abo ut the database
struc tures that yo u might ac c ess. By using the Jet database, this info rmatio n is
saved lo c ally and need no t be do wnlo aded eac h time yo u o pen the database.
Note
Visual Basic 6 and Access 2000: While Jet 3.51 is shipped w ith Visual Basic 6, Jet
4.0 is shipped w ith Access 2000. While it’s possible to use Visual Basic 6 and DAO
w ith Jet 4.0, m any of the new features found in Jet 4.0 can only be used if you’re
using ADO.
91
92
Part I ✦ Database Programming Fundamentals
Remote Database Objects (RDO)
The Re mo te Database Obje cts (RDO) o bjec t mo del was built to address the pro blems asso c iated with ac c essing SQL Server fro m Visual Basic . First available with
the Visual Basic 5 Enterprise Editio n o f Visual Basic , RDO o ffered a streamlined
o bjec t mo del whic h in turn o ffered muc h better perfo rmanc e than DAO when used
with a database suc h as SQL Server o r Orac le. Ho wever, RDO do esn’t wo rk with the
Jet database and o ther single-user database systems.
One feature lac king in RDO that is fo und in DAO is the ability to c reate a database
by defining o bjec ts. Given that RDO is really targeted at large installatio ns, this isn’t
nearly as limiting as it seems. These installatio ns generally have a gro up o f peo ple
kno wn as database administrato rs (DBAs), who se so le respo nsibility is the c reatio n
and maintenanc e o f database struc tures. DBAs o ften use spec ialized to o ls to help
them design and do c ument their databases.
OLE DB
After years o f wo rking with ODBC, Mic ro so ft rec o gnized that the database server to
database c lient mo del used by ODBC c o uld be generalized as a data pro vide r to data
co nsume r mo del (see Figure 6-2).
Data Request
Data Provider
Data Customer
Data Response
Figure 6-2: A data provider handles data requests from a data
consum er.
Data providers
A data pro vide r is a pro gram that supplies data to ano ther pro gram. In the ODBC
mo del, the data pro vider is the database server. Ho wever, in the OLE DB, nearly
anything that c an pro duc e data c o uld be c o nsidered a data pro vider. This allo ws
pro grammers to treat suc h things as an Exc el wo rkbo o k, a flat file, o r ano ther c usto m-built pro gram like a database server.
Chapter 6 ✦ Accessing Databases from Visual Basic
Data consumers
A data co nsume r is simply a c lient pro gram that requests data fro m a data pro vider.
Two types o f info rmatio n may be requested fro m the data pro duc er: data and me tadata. Data is the info rmatio n used by an applic atio n, while metadata is info rmatio n
abo ut the struc tures used to ho ld the data. Metadata inc ludes suc h info rmatio n as
the name o f eac h c o lumn returned, its size and data type, and o ther desc riptive
info rmatio n.
ActiveX Data Objects (ADO)
The Active X Data Obje cts (ADO) o bjec t mo del was c reated to replac e bo th DAO and
RDO o bjec t mo dels and was shipped fo r the first time in Visual Basic 6. It uses the
newer OLE DB API rather than the o lder ODBC API. In additio n, ADO has an even
simpler o bjec t mo del than fo und in the DAO and RDO, whic h makes it easier to use.
ADO is mo deled after RDO and inc ludes many o f the familiar o b jec ts fo und in
DAO and RDO, suc h as the Re cordSe t o b jec t. Ho wever, where RDO maintains a
relatively stric t o b jec t hierarc hy, ADO’s o b jec t struc ture is muc h lo o ser. This
means that yo u generally need fewer o b jec ts in yo ur pro gram, and the resulting
c o de is even simpler.
Like RDO, ADO do esn’t inc lude o bjec ts that yo u c an use to define yo ur database.
Ho wever, with versio n 2.5 o f ADO, Mic ro so ft inc luded a new set o f o bjec ts kno wn as
ADOX, whic h c an be used to define database struc tures and sec urity. These o bjec ts
wo uld be useful if yo u want to write a general-purpo se to o l that allo ws yo u to c reate and manipulate database struc tures fro m multiple database vendo rs. They c an
help smo o th o ver differenc es in ho w the vario us vendo rs implement their o wn SQL
extensio ns.
ADO MD is ano ther set o f extensio ns to the base ADO o bjec ts. These o bjec ts allo w
yo u to wo rk with a multidimensio nal database suc h as the OLAP Servic es bundled
with SQL Server. Yo u c an use these o bjec ts to build yo ur o wn pro grams fo r manipulating info rmatio n fro m yo ur o wn data wareho use.
OLE DB providers
Table 6-1 lists the OLE DB pro viders that are available with Visual Basic 6. Other
OLE DB pro viders may be available fro m yo ur spec ific database vendo r.
93
94
Part I ✦ Database Programming Fundamentals
Table 6-1
Common OLE DB Providers
Provider Name
Description
Microsoft Jet 3.51 OLE DB Provider
Supports Access 97/ Jet 3.51 databases
Microsoft Jet 4.0 OLE DB Provider
Supports Access 2000/ Jet 4.0 databases
Microsoft OLE DB Provider for SQL Server
Supports SQL Server 6.5 and 7.0
Microsoft OLE DB Provider for Oracle
Supports Oracle 7 and 8
Microsoft OLE DB Provider for Microsoft
Active Directory Service
Supports Microsoft Active Directory Service
found in Window s 2000 Server
Microsoft OLE DB Provider for Microsoft
Index Service
Supports Microsoft Index Server 2.0 and
new er
Microsoft OLE DB Provider for OLAP Services
Supports Microsoft OLAP Server
Microsoft OLE DB Provider for ODBC Drivers
Supports generic ODBC access
Even if yo ur database server do esn’t suppo rt OLE DB, that do esn’t mean yo u c an’t
ac c ess the database fro m ADO. There is a spec ial pro vider kno wn OLE DB Pro vider
fo r ODBC. This allo ws yo u to c o nnec t yo ur ADO-based applic atio n to any ODBC
database. Of c o urse, yo u sho uld try to use a native OLE DB pro vider whenever
po ssible.
Custom OLE DB providers
The arc hitec ture o f OLE DB allo ws yo u to write yo ur o wn OLE DB pro vider. This is
an advanc ed tec hnique that mo st peo ple wo n’t ever need to use. After all, ho w
many peo ple implement their o wn database management system? Ho wever, if yo u
have a c usto m data sto rage applic atio n, yo u c an c reate yo ur o wn OLE DB pro vider
fo r it. This will allo w pro grammers to ac c ess it using to o ls with whic h they are
already familiar.
Visual Basic Database Tools
To suppo rt database pro gramming, Visual Basic inc ludes a nic e asso rtment o f to o ls
that will assist yo u in c reating yo ur applic atio n, inc luding the fo llo wing:
✦ Data Enviro nment Designer
✦ Data View Windo w
Chapter 6 ✦ Accessing Databases from Visual Basic
✦ Database Designer
✦ SQL Edito r
✦ T-SQL Debugger
✦ Query Designer
✦ Data Repo rt Designer
✦ UserCo nnec tio n Designer
With the exc eptio n o f the UserCo nnec tio n Designer, these to o ls will wo rk o nly in an
ADO pro gramming enviro nment. The UserCo nnec tio n Designer wo rks o nly with
RDO o bjec ts.
Data Environment Designer
The Data Enviro nment Designer (see Figure 6-3) is an interac tive design-time to o l
that helps yo u c reate database o bjec ts fo r use at runtime. It is based o n the ADO
o bjec t mo del and will no t wo rk with RDO and DAO o bjec ts.
Figure 6-3: Using the Data
Environm ent Designer
Yo u c an use the Data Enviro nment Designer to perfo rm the fo llo wing tasks:
✦ Define Connection o bjec ts to yo ur database using either OLE DB data so urc es
o r ODBC data so urc es.
✦ Define Command o bjec ts using tables, queries, and sto red pro c edures fro m
yo ur database.
✦ Create hierarc hies o f Command o bjec ts that c an be used with hierarc hic al
to o ls like the HFlexGrid c o ntro l and the Data Reporter Designer.
95
96
Part I ✦ Database Programming Fundamentals
✦ Drag and dro p fields and tables fro m a Command o bjec t in the designer o nto
a Form o bjec t o r the Data Reporter designer that are auto matic ally bo und to
the Command o bjec t.
✦ Specify default the type o f co ntro l to be used as part o f drag and dro p o peratio ns.
✦ Bind data-aware c o ntro ls to Field o bjec ts within a Command o bjec t.
✦ Attac h c o de fo r Connection and Recordset o bjec ts in the Data Enviro nment
Designer.
✦ Trap all ADO events fo r the Connection and Command o bjec ts.
Data View Window
The Data View Windo w allo ws yo u to ac c ess yo ur database system thro ugh Visual
Basic instead o f a partic ular database vendo r’s utility (see Figure 6-4). Thro ugh the
Data View Windo w, yo u c an ac c ess the Mic ro so ft Visual Database To o ls.
Note
The Visual Database Tools are present only in the Enterprise Edition of Visual Basic.
This not only includes the Data View Window, but the Database Designer, the
Query Designer, the SQL Editor, and the T-SQL Debugger.
Figure 6-4: Using the Data View Window
to brow se and edit the database structures
in your database
Yo u c an use the Data View Windo w to perfo rm the fo llo wing tasks:
✦ Create a Connection o bjec t that c an be used to ac c ess yo ur database while in
design mo de o r at runtime.
✦ Design yo ur database graphic ally using the Database Designer.
Chapter 6 ✦ Accessing Databases from Visual Basic
✦ View the c o ntents o f a table o r view using a wo rksheet-like display.
✦ Create and edit sto red pro c edures using the SQL Edito r.
✦ Debug sto red pro c edures using the T-SQL Debugger.
✦ Create views using the Query Designer.
✦ Generate repo rts using the Data Repo rter Designer.
Database Designer
The Database Designer is a graphic al to o l that presents a database using a graphic al diagram (see Figure 6-5). The Database Designer wo rks with bo th Mic ro so ft SQL
Server and Orac le database systems.
Figure 6-5: Designing a database graphically using the Database Designer
Yo u c an use the Database Designer to perfo rm the fo llo wing tasks:
✦ Create and mo dify tables.
✦ Add and delete indexes.
✦ Define relatio nships between tables.
✦ Save SQL sc ripts to c reate o r update yo ur database struc tures.
97
98
Part I ✦ Database Programming Fundamentals
SQL Editor
The SQL Edito r pro vides a simple edito r to help yo u write sto red pro c edures (see
Figure 6-6).
Figure 6-6: Editing a sim ple stored procedure using the SQL Editor
Yo u c an use the SQL Edito r to perfo rm the fo llo wing tasks:
✦ Create and edit SQL sto red pro c edures.
✦ Exec ute sto red pro c edures.
✦ Use the T-SQL Debugger to debug yo ur sto red pro c edures.
T-SQL Debugger
The T-SQL Debugger helps yo u test and debug yo ur sto red pro c edures (see
Figure 6-7).
Yo u c an use the T-SQL Debugger to perfo rm the fo llo wing tasks:
✦ Display the c o ntents o f lo c al variables and parameters.
✦ Mo dify lo c al variables and parameters while exec uting the sto red pro c edure.
✦ Co ntro l exec utio n by using breakpo ints.
✦ Step thro ugh the sto red pro c edure.
✦ View glo bal variables.
✦ View the c all stac k.
Chapter 6 ✦ Accessing Databases from Visual Basic
Figure 6-7: Testing a sim ple stored procedure using the T-SQL Debugger
Query Designer
The Query Designer is a to o l that wo rks with the Data View Windo w to drag and
dro p tables and c o lumns to c reate a view o r query (see Figure 6-8).
Figure 6-8: Creating a view using the Query Designer
Yo u c an use the Query Designer to perfo rm the fo llo wing tasks:
✦ Drag and dro p tables fro m the Data View Windo w o nto yo ur query.
✦ Use a c riteria grid to spec ify searc h c riteria and so rt o rder.
99
100
Part I ✦ Database Programming Fundamentals
✦ Selec t the c o lumns to be o utput as a result o f the query.
✦ Bro wse and edit live views o f yo ur data in yo ur view.
Data Reporter Designer
The Data Repo rter Designer helps yo u build repo rts interac tively (see Figure 6-9).
Figure 6-9: Building a report using the Data Reporter Designer
Yo u c an use the Data Repo rter Designer to perfo rm the fo llo wing tasks:
✦ Create repo rts by dragging and dro pping fields fro m the Data View Windo w.
✦ Add c o ntro ls to yo ur repo rt, just like yo u add c o ntro ls to a Fo rm in a Visual
Basic pro gram.
✦ Implement a Print Preview func tio n in yo ur pro gram to allo w users to preview
the repo rt befo re sending it to the printer.
✦ Allo w the user to resume wo rking while the repo rt is running (i.e., async hro no us exec utio n o f the repo rt).
UserConnection Designer
The UserCo nnec tio n Designer is a to o l that assists the RDO pro grammer in building
c o nnec tio n ( rdoConnection ) and query ( rdoQuery ) o bjec ts. These o bjec ts are made
available at the pro jec t level and c an be used anywhere in yo ur pro gram. A key part
o f these o bjec ts is that they have a simplified metho d fo r respo nding to database
events. These o bjec ts also make it easier to c all sto red pro c edures at runtime.
Caution
RDO not ADO: The UserConnection Designer supports only the RDO object
m odel. Do not try to use it w ith DAO or ADO object m odels. The Data Environm ent
Designer is a m uch im proved version of the UserConnection Designer and is available for use w ith ADO program s.
Chapter 6 ✦ Accessing Databases from Visual Basic
Thoughts on Visual Basic
Visual Basic program m ers can choose from three different object m odels to access their
databases: DAO, RDO, and ADO. Deciding w hich m odel is best for you m ay seem difficult,
but it really shouldn’t be. ADO is the m odel that Microsoft w ould prefer you to use. It is
w here they are placing the m ajority of their efforts and it is w here you w ill see the m ost
enhancem ents in the future.
Microsoft isn’t going to abandon the DAO object model anytime soon. There are too many
Visual Basic programs that still use it. Dropping support for DAO in a future release of Visual
Basic would simply mean that many Visual Basic programmers wouldn’t upgrade. (Consider
the number of people still using Visual Basic 3 and 4 because Microsoft stopped supporting 16bit versions with Visual Basic 5.) So if you’re currently using DAO, you can continue to use it in
the foreseeable future. However, if you’re not currently using DAO, you probably shouldn’t start.
Directly accessing SQL Server databases using DAO isn’t really practical for large applications. As m ore and m ore people tried to do this, Microsoft developed RDO as a solution. Its
low overhead offered m uch better perform ance in large-scale applications. How ever, RDO’s
tim e is also past, and if you’re not using it now, don’t start.
ADO is the w ave of the future. You should be using it for new applications w herever practical. While you don’t have to drop everything you’re currently doing to convert your existing
applications to ADO now, you should have a plan in place that ensures that they w ill all be
converted over tim e. This is especially true in applications w ith high transaction rates or
applications that are offering services over the Internet.
In the form er case, m any of the things that Microsoft w ill do to im prove perform ance w ill be
tied to ADO. This includes facilities such as the COM+ Transaction Server and future versions of SQL Server. In the latter case, Microsoft is already adding features, such as XML support to ADO, w hich w ill not be added to DAO and RDO. Also, all of the tools designed to
m ake you m ore productive, such as the Data View Window and the Data Environm ent
Designer, also require an ADO-based program .
Summary
In this c hapter yo u learned that:
✦ ODBC was o riginally develo ped as an alternative to develo ping pre-c o mpilers
fo r eac h pro gramming language to translate SQL statements into exec utable
c o de.
✦ DAO was c reated to pro vide an o bjec t-o riented API to ac c ess Mic ro so ft Jet as
well as any ODBC database.
✦ RDO is a lo w-o verhead, general-purpo se ODBC interfac e.
✦ OLE DB is a general-purpo se interfac e that allo ws data c o nsumers to talk to
data pro duc ers.
101
102
Part I ✦ Database Programming Fundamentals
✦ ADO is the rec o mmended way fo r Visual Basic pro grammers to ac c ess OLE DB.
✦ Visual Basic inc ludes a number o f spec ialized to o ls fo r the database pro grammer, whic h inc lude the Data Enviro nment Designer, Data View Windo w, Database Designer, SQL Edito r, T-SQL Debugger, Query Designer, Data Repo rter
Designer, and the UserCo nnec tio n Designer.
✦
✦
✦
C H A P T E R
Accessing
Databases from
Visual Basic
✦
✦
✦
✦
In This Cha pter
O DBC and O LE DB
DAO o bject mo del
I
n this c hapter, I’m go ing to disc uss ho w Visual Basic talks
to a database server. ODBC and OLE DB are the lo w-level
tec hno lo gies that ac tually perfo rm the wo rk, while the DAO,
RDO, and ADO o bjec t mo dels enc apsulate these tec hno lo gies
to make them easy fo r the Visual Basic pro grammer to use.
M icrosoft Database
Programming APIs
Just bec ause yo ur database server is based o n SQL do esn’t
mean that yo u c an easily ac c ess it fro m yo ur favo rite pro gramming language. Many database vendo rs pro vide a spec ial prec o mpiler that translates embedded database statements into
database subro utine c alls that in turn c o mmunic ate with the
database server. The pro blem with this appro ac h is that yo u
need a different pre-c o mpiler fo r eac h pro gramming language
the database vendo r suppo rts. Of c o urse, sinc e the pre-c o mpilers are spec ific fo r eac h database server, yo u’ll need a pre-c o mpiler fo r eac h database server fo r whic h yo u develo p pro grams.
Rather than develo ping a large number o f pre-c o mpilers fo r
eac h c o mbinatio n o f database server and pro gramming language, Mic ro so ft develo ped a standard c alled Ope n Database
Co nne ctivity (ODBC), whic h later evo lved into OLE DB.
RDO o bject mo del
ADO o bject mo del
Visual Basic
database to o ls
✦
✦
✦
✦
90
Part I ✦ Database Programming Fundamentals
ODBC
The ODBC standard defines an Applicatio n Pro gramming Inte rface (API) fo r database
pro gramming. This allo ws yo u to write a pro gram using standard subro utine c alls
fo r any database server that suppo rts ODBC, making it po ssible fo r the same o bjec t
c o de to ac c ess any ODBC-c o mpatible database server.
ODBC architecture
ODBC is based o n the idea that the c alls to the API ro utines made by the applic atio n pro gram are translated to lo wer c alls that are passed o nto a database driver
(see Figure 6-1). The database driver in turn perfo rms the nec essary wo rk to talk to
the database server. Thus, the database vendo r need o nly pro vide an ODBC-c o mpatible driver fo r eac h c lient c o mputer system and no t fo r eac h c o mpiler o n eac h
c lient o perating system.
Client Computer
Application
Program
ODBC API
ODBC Database
Driver
Database Server
Database Engine
ODBC Interface
Figure 6-1: The client side of the ODBC architecture is based on the driver m odel.
On the database server side, all the database vendo r needs to do is pro vide a single
ODBC interfac e, whic h will be shared by all ODBC c lients. This means that database
vendo rs c an preserve their native interfac e, plus any o ther spec ialized interfac es
fo r o ther applic atio ns.
Chapter 6 ✦ Accessing Databases from Visual Basic
A utility pro gram c alled the ODBC Administrato r is inc luded as part o f the o perating system to manage the set o f ODBC drivers and database servers that are available to the ODBC API. This pro gram allo ws yo u to add and remo ve ODBC drivers,
spec ify ho w to c o nnec t to the database server, and inc lude sec urity info rmatio n
that will be used when the c o nnec tio n between the c lient c o mputer and the
database server is o pened.
Drawbacks to ODBC
While the arc hitec ture o f ODBC allo ws a great deal o f flexibility o n the part o f the
database vendo r, there are several drawbac ks to writing ODBC applic atio ns. First,
the ODBC APIs are diffic ult to use, espec ially if yo u aren’t pro gramming in C. Sec o nd,
the ODBC APIs are o ften slo wer than the native database interfac e. Third, the ODBC
API o ften impo ses restric tio ns o n the SQL statements that c an be used.
While the first drawbac k applies to all database servers, the sec o nd two drawbac ks
apply mainly to no n-Mic ro so ft database servers. Mic ro so ft uses ODBC as the native
interfac e to bo th the Jet database and SQL Server. They spent a lo t o f time tuning
the interfac e fo r o ptimal results. While o ther database vendo rs suppo rt ODBC, their
native interfac es may o ffer impro ved perfo rmanc e and func tio nality, espec ially fo r
no n-Mic ro so ft c o mpilers.
Database Access Objects (DAO)
Bec ause the ODBC API is diffic ult to use, Mic ro so ft c ho se to build an o bjec t-o riented
interfac e to ODBC c alled Data Acce ss Obje cts (DAO). DAO was o riginally develo ped
fo r Ac c ess and Visual Basic pro grammers who needed to ac c ess the Jet database.
Mo st o f the func tio nality available in DAO mirro rs features fo und in the Jet database.
While DAO c an be used to ac c ess o ther databases, the pro c ess is o ften diffic ult to
use and c lumsy in its implementatio n. To ac c ess an SQL Server database, yo u need
to c reate a Jet database and then go thro ugh the Jet database to ac c ess the remo te
database. While DAO allo ws yo u to ac c ess the ODBC database direc tly, eac h time
yo u o pen the database, yo u must do wnlo ad a lo t o f info rmatio n abo ut the database
struc tures that yo u might ac c ess. By using the Jet database, this info rmatio n is
saved lo c ally and need no t be do wnlo aded eac h time yo u o pen the database.
Note
Visual Basic 6 and Access 2000: While Jet 3.51 is shipped w ith Visual Basic 6, Jet
4.0 is shipped w ith Access 2000. While it’s possible to use Visual Basic 6 and DAO
w ith Jet 4.0, m any of the new features found in Jet 4.0 can only be used if you’re
using ADO.
91
92
Part I ✦ Database Programming Fundamentals
Remote Database Objects (RDO)
The Re mo te Database Obje cts (RDO) o bjec t mo del was built to address the pro blems asso c iated with ac c essing SQL Server fro m Visual Basic . First available with
the Visual Basic 5 Enterprise Editio n o f Visual Basic , RDO o ffered a streamlined
o bjec t mo del whic h in turn o ffered muc h better perfo rmanc e than DAO when used
with a database suc h as SQL Server o r Orac le. Ho wever, RDO do esn’t wo rk with the
Jet database and o ther single-user database systems.
One feature lac king in RDO that is fo und in DAO is the ability to c reate a database
by defining o bjec ts. Given that RDO is really targeted at large installatio ns, this isn’t
nearly as limiting as it seems. These installatio ns generally have a gro up o f peo ple
kno wn as database administrato rs (DBAs), who se so le respo nsibility is the c reatio n
and maintenanc e o f database struc tures. DBAs o ften use spec ialized to o ls to help
them design and do c ument their databases.
OLE DB
After years o f wo rking with ODBC, Mic ro so ft rec o gnized that the database server to
database c lient mo del used by ODBC c o uld be generalized as a data pro vide r to data
co nsume r mo del (see Figure 6-2).
Data Request
Data Provider
Data Customer
Data Response
Figure 6-2: A data provider handles data requests from a data
consum er.
Data providers
A data pro vide r is a pro gram that supplies data to ano ther pro gram. In the ODBC
mo del, the data pro vider is the database server. Ho wever, in the OLE DB, nearly
anything that c an pro duc e data c o uld be c o nsidered a data pro vider. This allo ws
pro grammers to treat suc h things as an Exc el wo rkbo o k, a flat file, o r ano ther c usto m-built pro gram like a database server.
Chapter 6 ✦ Accessing Databases from Visual Basic
Data consumers
A data co nsume r is simply a c lient pro gram that requests data fro m a data pro vider.
Two types o f info rmatio n may be requested fro m the data pro duc er: data and me tadata. Data is the info rmatio n used by an applic atio n, while metadata is info rmatio n
abo ut the struc tures used to ho ld the data. Metadata inc ludes suc h info rmatio n as
the name o f eac h c o lumn returned, its size and data type, and o ther desc riptive
info rmatio n.
ActiveX Data Objects (ADO)
The Active X Data Obje cts (ADO) o bjec t mo del was c reated to replac e bo th DAO and
RDO o bjec t mo dels and was shipped fo r the first time in Visual Basic 6. It uses the
newer OLE DB API rather than the o lder ODBC API. In additio n, ADO has an even
simpler o bjec t mo del than fo und in the DAO and RDO, whic h makes it easier to use.
ADO is mo deled after RDO and inc ludes many o f the familiar o b jec ts fo und in
DAO and RDO, suc h as the Re cordSe t o b jec t. Ho wever, where RDO maintains a
relatively stric t o b jec t hierarc hy, ADO’s o b jec t struc ture is muc h lo o ser. This
means that yo u generally need fewer o b jec ts in yo ur pro gram, and the resulting
c o de is even simpler.
Like RDO, ADO do esn’t inc lude o bjec ts that yo u c an use to define yo ur database.
Ho wever, with versio n 2.5 o f ADO, Mic ro so ft inc luded a new set o f o bjec ts kno wn as
ADOX, whic h c an be used to define database struc tures and sec urity. These o bjec ts
wo uld be useful if yo u want to write a general-purpo se to o l that allo ws yo u to c reate and manipulate database struc tures fro m multiple database vendo rs. They c an
help smo o th o ver differenc es in ho w the vario us vendo rs implement their o wn SQL
extensio ns.
ADO MD is ano ther set o f extensio ns to the base ADO o bjec ts. These o bjec ts allo w
yo u to wo rk with a multidimensio nal database suc h as the OLAP Servic es bundled
with SQL Server. Yo u c an use these o bjec ts to build yo ur o wn pro grams fo r manipulating info rmatio n fro m yo ur o wn data wareho use.
OLE DB providers
Table 6-1 lists the OLE DB pro viders that are available with Visual Basic 6. Other
OLE DB pro viders may be available fro m yo ur spec ific database vendo r.
93
94
Part I ✦ Database Programming Fundamentals
Table 6-1
Common OLE DB Providers
Provider Name
Description
Microsoft Jet 3.51 OLE DB Provider
Supports Access 97/ Jet 3.51 databases
Microsoft Jet 4.0 OLE DB Provider
Supports Access 2000/ Jet 4.0 databases
Microsoft OLE DB Provider for SQL Server
Supports SQL Server 6.5 and 7.0
Microsoft OLE DB Provider for Oracle
Supports Oracle 7 and 8
Microsoft OLE DB Provider for Microsoft
Active Directory Service
Supports Microsoft Active Directory Service
found in Window s 2000 Server
Microsoft OLE DB Provider for Microsoft
Index Service
Supports Microsoft Index Server 2.0 and
new er
Microsoft OLE DB Provider for OLAP Services
Supports Microsoft OLAP Server
Microsoft OLE DB Provider for ODBC Drivers
Supports generic ODBC access
Even if yo ur database server do esn’t suppo rt OLE DB, that do esn’t mean yo u c an’t
ac c ess the database fro m ADO. There is a spec ial pro vider kno wn OLE DB Pro vider
fo r ODBC. This allo ws yo u to c o nnec t yo ur ADO-based applic atio n to any ODBC
database. Of c o urse, yo u sho uld try to use a native OLE DB pro vider whenever
po ssible.
Custom OLE DB providers
The arc hitec ture o f OLE DB allo ws yo u to write yo ur o wn OLE DB pro vider. This is
an advanc ed tec hnique that mo st peo ple wo n’t ever need to use. After all, ho w
many peo ple implement their o wn database management system? Ho wever, if yo u
have a c usto m data sto rage applic atio n, yo u c an c reate yo ur o wn OLE DB pro vider
fo r it. This will allo w pro grammers to ac c ess it using to o ls with whic h they are
already familiar.
Visual Basic Database Tools
To suppo rt database pro gramming, Visual Basic inc ludes a nic e asso rtment o f to o ls
that will assist yo u in c reating yo ur applic atio n, inc luding the fo llo wing:
✦ Data Enviro nment Designer
✦ Data View Windo w
Chapter 6 ✦ Accessing Databases from Visual Basic
✦ Database Designer
✦ SQL Edito r
✦ T-SQL Debugger
✦ Query Designer
✦ Data Repo rt Designer
✦ UserCo nnec tio n Designer
With the exc eptio n o f the UserCo nnec tio n Designer, these to o ls will wo rk o nly in an
ADO pro gramming enviro nment. The UserCo nnec tio n Designer wo rks o nly with
RDO o bjec ts.
Data Environment Designer
The Data Enviro nment Designer (see Figure 6-3) is an interac tive design-time to o l
that helps yo u c reate database o bjec ts fo r use at runtime. It is based o n the ADO
o bjec t mo del and will no t wo rk with RDO and DAO o bjec ts.
Figure 6-3: Using the Data
Environm ent Designer
Yo u c an use the Data Enviro nment Designer to perfo rm the fo llo wing tasks:
✦ Define Connection o bjec ts to yo ur database using either OLE DB data so urc es
o r ODBC data so urc es.
✦ Define Command o bjec ts using tables, queries, and sto red pro c edures fro m
yo ur database.
✦ Create hierarc hies o f Command o bjec ts that c an be used with hierarc hic al
to o ls like the HFlexGrid c o ntro l and the Data Reporter Designer.
95
96
Part I ✦ Database Programming Fundamentals
✦ Drag and dro p fields and tables fro m a Command o bjec t in the designer o nto
a Form o bjec t o r the Data Reporter designer that are auto matic ally bo und to
the Command o bjec t.
✦ Specify default the type o f co ntro l to be used as part o f drag and dro p o peratio ns.
✦ Bind data-aware c o ntro ls to Field o bjec ts within a Command o bjec t.
✦ Attac h c o de fo r Connection and Recordset o bjec ts in the Data Enviro nment
Designer.
✦ Trap all ADO events fo r the Connection and Command o bjec ts.
Data View Window
The Data View Windo w allo ws yo u to ac c ess yo ur database system thro ugh Visual
Basic instead o f a partic ular database vendo r’s utility (see Figure 6-4). Thro ugh the
Data View Windo w, yo u c an ac c ess the Mic ro so ft Visual Database To o ls.
Note
The Visual Database Tools are present only in the Enterprise Edition of Visual Basic.
This not only includes the Data View Window, but the Database Designer, the
Query Designer, the SQL Editor, and the T-SQL Debugger.
Figure 6-4: Using the Data View Window
to brow se and edit the database structures
in your database
Yo u c an use the Data View Windo w to perfo rm the fo llo wing tasks:
✦ Create a Connection o bjec t that c an be used to ac c ess yo ur database while in
design mo de o r at runtime.
✦ Design yo ur database graphic ally using the Database Designer.
Chapter 6 ✦ Accessing Databases from Visual Basic
✦ View the c o ntents o f a table o r view using a wo rksheet-like display.
✦ Create and edit sto red pro c edures using the SQL Edito r.
✦ Debug sto red pro c edures using the T-SQL Debugger.
✦ Create views using the Query Designer.
✦ Generate repo rts using the Data Repo rter Designer.
Database Designer
The Database Designer is a graphic al to o l that presents a database using a graphic al diagram (see Figure 6-5). The Database Designer wo rks with bo th Mic ro so ft SQL
Server and Orac le database systems.
Figure 6-5: Designing a database graphically using the Database Designer
Yo u c an use the Database Designer to perfo rm the fo llo wing tasks:
✦ Create and mo dify tables.
✦ Add and delete indexes.
✦ Define relatio nships between tables.
✦ Save SQL sc ripts to c reate o r update yo ur database struc tures.
97
98
Part I ✦ Database Programming Fundamentals
SQL Editor
The SQL Edito r pro vides a simple edito r to help yo u write sto red pro c edures (see
Figure 6-6).
Figure 6-6: Editing a sim ple stored procedure using the SQL Editor
Yo u c an use the SQL Edito r to perfo rm the fo llo wing tasks:
✦ Create and edit SQL sto red pro c edures.
✦ Exec ute sto red pro c edures.
✦ Use the T-SQL Debugger to debug yo ur sto red pro c edures.
T-SQL Debugger
The T-SQL Debugger helps yo u test and debug yo ur sto red pro c edures (see
Figure 6-7).
Yo u c an use the T-SQL Debugger to perfo rm the fo llo wing tasks:
✦ Display the c o ntents o f lo c al variables and parameters.
✦ Mo dify lo c al variables and parameters while exec uting the sto red pro c edure.
✦ Co ntro l exec utio n by using breakpo ints.
✦ Step thro ugh the sto red pro c edure.
✦ View glo bal variables.
✦ View the c all stac k.
Chapter 6 ✦ Accessing Databases from Visual Basic
Figure 6-7: Testing a sim ple stored procedure using the T-SQL Debugger
Query Designer
The Query Designer is a to o l that wo rks with the Data View Windo w to drag and
dro p tables and c o lumns to c reate a view o r query (see Figure 6-8).
Figure 6-8: Creating a view using the Query Designer
Yo u c an use the Query Designer to perfo rm the fo llo wing tasks:
✦ Drag and dro p tables fro m the Data View Windo w o nto yo ur query.
✦ Use a c riteria grid to spec ify searc h c riteria and so rt o rder.
99
100
Part I ✦ Database Programming Fundamentals
✦ Selec t the c o lumns to be o utput as a result o f the query.
✦ Bro wse and edit live views o f yo ur data in yo ur view.
Data Reporter Designer
The Data Repo rter Designer helps yo u build repo rts interac tively (see Figure 6-9).
Figure 6-9: Building a report using the Data Reporter Designer
Yo u c an use the Data Repo rter Designer to perfo rm the fo llo wing tasks:
✦ Create repo rts by dragging and dro pping fields fro m the Data View Windo w.
✦ Add c o ntro ls to yo ur repo rt, just like yo u add c o ntro ls to a Fo rm in a Visual
Basic pro gram.
✦ Implement a Print Preview func tio n in yo ur pro gram to allo w users to preview
the repo rt befo re sending it to the printer.
✦ Allo w the user to resume wo rking while the repo rt is running (i.e., async hro no us exec utio n o f the repo rt).
UserConnection Designer
The UserCo nnec tio n Designer is a to o l that assists the RDO pro grammer in building
c o nnec tio n ( rdoConnection ) and query ( rdoQuery ) o bjec ts. These o bjec ts are made
available at the pro jec t level and c an be used anywhere in yo ur pro gram. A key part
o f these o bjec ts is that they have a simplified metho d fo r respo nding to database
events. These o bjec ts also make it easier to c all sto red pro c edures at runtime.
Caution
RDO not ADO: The UserConnection Designer supports only the RDO object
m odel. Do not try to use it w ith DAO or ADO object m odels. The Data Environm ent
Designer is a m uch im proved version of the UserConnection Designer and is available for use w ith ADO program s.
Chapter 6 ✦ Accessing Databases from Visual Basic
Thoughts on Visual Basic
Visual Basic program m ers can choose from three different object m odels to access their
databases: DAO, RDO, and ADO. Deciding w hich m odel is best for you m ay seem difficult,
but it really shouldn’t be. ADO is the m odel that Microsoft w ould prefer you to use. It is
w here they are placing the m ajority of their efforts and it is w here you w ill see the m ost
enhancem ents in the future.
Microsoft isn’t going to abandon the DAO object model anytime soon. There are too many
Visual Basic programs that still use it. Dropping support for DAO in a future release of Visual
Basic would simply mean that many Visual Basic programmers wouldn’t upgrade. (Consider
the number of people still using Visual Basic 3 and 4 because Microsoft stopped supporting 16bit versions with Visual Basic 5.) So if you’re currently using DAO, you can continue to use it in
the foreseeable future. However, if you’re not currently using DAO, you probably shouldn’t start.
Directly accessing SQL Server databases using DAO isn’t really practical for large applications. As m ore and m ore people tried to do this, Microsoft developed RDO as a solution. Its
low overhead offered m uch better perform ance in large-scale applications. How ever, RDO’s
tim e is also past, and if you’re not using it now, don’t start.
ADO is the w ave of the future. You should be using it for new applications w herever practical. While you don’t have to drop everything you’re currently doing to convert your existing
applications to ADO now, you should have a plan in place that ensures that they w ill all be
converted over tim e. This is especially true in applications w ith high transaction rates or
applications that are offering services over the Internet.
In the form er case, m any of the things that Microsoft w ill do to im prove perform ance w ill be
tied to ADO. This includes facilities such as the COM+ Transaction Server and future versions of SQL Server. In the latter case, Microsoft is already adding features, such as XML support to ADO, w hich w ill not be added to DAO and RDO. Also, all of the tools designed to
m ake you m ore productive, such as the Data View Window and the Data Environm ent
Designer, also require an ADO-based program .
Summary
In this c hapter yo u learned that:
✦ ODBC was o riginally develo ped as an alternative to develo ping pre-c o mpilers
fo r eac h pro gramming language to translate SQL statements into exec utable
c o de.
✦ DAO was c reated to pro vide an o bjec t-o riented API to ac c ess Mic ro so ft Jet as
well as any ODBC database.
✦ RDO is a lo w-o verhead, general-purpo se ODBC interfac e.
✦ OLE DB is a general-purpo se interfac e that allo ws data c o nsumers to talk to
data pro duc ers.
101
102
Part I ✦ Database Programming Fundamentals
✦ ADO is the rec o mmended way fo r Visual Basic pro grammers to ac c ess OLE DB.
✦ Visual Basic inc ludes a number o f spec ialized to o ls fo r the database pro grammer, whic h inc lude the Data Enviro nment Designer, Data View Windo w, Database Designer, SQL Edito r, T-SQL Debugger, Query Designer, Data Repo rter
Designer, and the UserCo nnec tio n Designer.
✦
✦
✦