《VB2008从入门到精通(PDF格式英文版)》第178章

ing　a　set　of
individual　references。　This　causes　a　mismatch；　and　trying　to　bind　to　the　two　technologies　is　difficult。　
So　when　you　are　writing　database　code；　you　are　trying　to　automate　the　fitting　of　a　square　peg　into　a　round
hole。　You　get　many　extremely　creative　ideas　and　results；　but　at　the　end　of　the　day；　you　are　still　fitting　a　square
peg　into　a　round　hole。
The　essence　of　the　problem　when　dealing　with　sets　of　data　in　a　programming　language　is　how　to
integrate　the　two。　There　is　light　at　the　end　of　the　tunnel；　in　the　form　of　programming　language　alterations
such　as　LINQ。　LINQ　will　be　discussed　in　the　next　chapter。
Accessing　Relational　Databases　
Regardless　of　the　database　implementation　that　you　use；　a　mon　architecture　is　employed；
as　illustrated　in　Figure　14…1。　
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Figure　14…1。　mon　database　architecture　
Most　relational　database　servers　are　separate　applications　that　run　on　their　own。　To　interact
with　a　running　relational　database；　the　database　vendor　provides　a　database　driver。　In　；　a
database　driver　is　a　piece　of　proprietary　code　that　talks　to　the　relational　database　server；　but
exposes　its　functionality　using　the　ADO　layer。　
The　ADO　layer　is　a　technology　that　abstracts　the　database　client　into　a　neutral　set　of
interfaces。　By　itself；　ADO　does　not　implement　any　technologies；　but　it　defines　the　inter
faces　that　a　database　needs　to　have　implemented。　ADO　is　similar　to　the　lighting　manager
application　introduced　in　Chapter　8；　where　specific　lighting　implementations　need　to　imple
ment　interfaces。　
The　ADO　code　can　be　accessed　directly　in　your　application　by　your　code。　However；
doing　that　means　accessing　the　individual　rows；　columns；　and　database　tables。　It　is　not　a　difficult
undertaking；　but　it　means　you　need　to　do　everything　manually。　With　respect　to　the　lottery
example；　it　means　manipulating　the　lottery　winners　and　the　lottery　draws　tables。　
Visual　Basic　Express　and　other　Visual　Studio　editions　have　a　number　of　tools　that　will
generate　code　that　automatically　binds　to　tables　and　rows；　reducing　the　amount　of　work　that　you
need　to　do。　In　Figure　14…1；　the　user　code　has　arrows　pointing　to　both　the　Visual　Studio–generated
code　and　the　ADO　code。　These　arrows　indicate　that　your　code　does　not　need　to　use　the
Visual　Studio–generated　code；　it　is　optional。　But　the　advantage　of　using　the　Visual　Studio　code
is　that　it　reduces　the　amount　of　grunt　work　that　your　code　needs　to　do。　The　Visual　Studio–　
generated　code　is　a　thin　functional　layer　on　top　of　the　ADO　code　that　maps　directly　to　the
tables　that　you　are　manipulating。　
When　you　are　developing　an　application　that　accesses　a　relational　database；　you　need　to
consider　the　following　issues：　
ADO　drivers　：　When　you　write　ADO　code；　you　will　need　an　ADO　driver　for
each　database。　Thus；　if　you　wish　to　use　MySQL；　you　will　need　a　MySQL　driver。　However；
for　the　most　part；　the　code　that　you　write　in　your　application　will　remain　identical；　regard
less　of　which　type　of　database　you　are　using。
Abstraction　：　There　will　always　be　differences　in　the　code　used　to　access　different　relational
databases。　Be　prepared　to　abstract　your　database　code　if　you　access　the　database　directly
using　ADO。
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Designing　a　Database　Using　Visual　Basic　Express　
Visual　Basic　Express　is　very　helpful　when　designing　and　building　database　applications。　It　provides
interface…based　tools　for　designing　the　database；　managing　connections；　and　managing　data
access。　
With　Visual　Basic　Express；　you　can　directly　integrate　the　ADO　drivers　for　only　Microsoft
SQL　Server。　（This　does　not　mean　you　cannot　use　a　different　ADO　driver　using　code。）　If　you
want　to　use　the　GUI　tools　for　a　database　driver　other　than　for　SQL　Server；　you　will　need　to　upgrade
your　Visual　Studio　edition。　For　this　chapter’s　examples；　we’ll　use　the　SQL　Server　Express　Edition
driver　（http：//msdn。microsoft。/vstudio/express/sql/）。　
You　can　add　the　GUI　tool…based　database　support　to　any　Visual　Basic　project　type。　For　this
chapter；　we’ll　use　a　console　project　called　DatabaseConsoleEx。　
After　you　have　created　the DatabaseConsoleEx　console　project　in　Visual　Basic　Express
（using　the　procedure　outlined　in　Chapter　1）；　you　can　set　up　the　database；　and　then　add　tables
to　your　new　database。　
Configuring　the　Data　Source　
Using　the　Visual　Basic　Express　Data　Source　Configuration　Wizard；　you　can　add　a　database　as
your　data　source；　set　up　the　database　connection；　and　select　database　objects　to　include　in　the
project。　Follow　these　steps　to　use　the　wizard：　
1。 Select　Data Add　New　Data　Source　to　start　the　Data　Source　Configuration　Wizard。　
2。 Choose　Database　as　the　data　source　and　click　Next。　
3。 You’re　asked　to　choose　a　data　connection。　When　choosing　the　data　connection；　you　are
defining　the　connection　settings　to　your　relational　database。　Since　we　are　creating　a
new　database　in　this　example；　click　the　New　Connection　button。　
4。 Choose　Microsoft　SQL　Server　Database　File　and　click　Continue。　
5。 In　the　Add　Connection　dialog　box；　fill　in　the　database　file　name。　For　this　example；　enter
the　name　lot