Directory listing of http: uap.unnes.ac.id ebook biblebook Visual Basic 6 com & Library Books
30
C H A P T E R
Creating
Database
Objects with
M icrosoft Jet
I
n this c hapter, I’m go ing to sho w yo u ho w to use the Visual
Data Manager add-in to Visual Basic to c reate a Mic ro so ft
Jet database and vario us o bjec ts, suc h as tables and indexes,
inside it. I’ll also c o ver ho w to c reate users and gro ups, and
ho w to grant them permissio n to ac c ess database o bjec ts.
Introducing the Visual
Data M anager
The Visual Data Manager ( also c alled VisData) is a Visual
Basic add-in that was written to make it easier to design a
Jet datab ase witho ut b uilding yo ur o wn applic atio n o r using
Mic ro so ft Ac c ess. To run the Visual Data Manager, c ho o se
Add-Ins ➪ Visual Data Manager fro m the main menu. This
will start the applic atio n.
Tip
You got it: The Visual Data Manager utility w as w ritten in
Visual Basic 5 and show s w hat you can do w ith a little bit
of w ork. The com plete source code to the Visual Data
Manager is included in the Samples\VisData directory
of Visual Basic.
Sinc e the Visual Data Manager is based o n DAO, yo u c an use it
to o pen many o ther types o f databases besides Jet databases.
Yo u c an ac c ess dBase, Fo xPro , and Parado x databases, as well
as Exc el wo rksheets and vario us types o f text files. Yo u c an
even o pen an ODBC c o nnec tio n to a remo te database, suc h as
✦
✦
✦
✦
In This Cha pter
Intro ducing Visual
Data Manag er
Co nstructing
databases, tables,
and indexes
Manag ing security
fo r users and g ro ups
✦
✦
✦
✦
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SQL Server o r Orac le, tho ugh depending o n yo ur c ho ic e o f database, so me o f the
c apabilities may no t be suppo rted.
Note
Why doesn’t it open the database I created with Access 2000?: Visual Data
Manager uses DAO 3.5 to access databases. To access a Jet 4.0 database created
w ith Access 2000, you m ust use DAO 3.6. While you could recom pile the Visual
Data Manager application using DAO 3.6, it’s probably just easier to use Access to
design your database.
Opening an existing database
To o pen an existing database, c ho o se File ➪ Open DataBase ➪ Mic ro so ft Ac c ess
fro m the Visual Data Manager main menu. This will display a File Open dialo g bo x.
Simply c ho o se the file c o ntaining yo ur database and press the Open butto n. After
yo u’ve o pened yo ur database, yo u’ll see a fo rm like the o ne sho wn in Figure 30-1.
By default, the Visual Data Manager o pens yo ur database and displays two multiple
do c ument interfac e (MDI) windo ws. The first windo w (Database Windo w) displays
yo ur database’s struc ture, while the sec o nd windo w (SQL Statement) allo ws yo u to
enter and run an SQL statement.
Figure 30-1: View ing an open database.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Viewing Database Information
The Visual Data Manager allo ws yo u to see the struc ture o f yo ur database (see
Figure 30-2). The tree view displays eac h table and its attributes, inc luding c o lumns
and indexes. As yo u expand eac h entry in the table, yo u will eventually reac h the
details that wo uld no rmally sho w up in the Properties c o llec tio n asso c iated with
the o bjec t.
Figure 30-2: View ing the structure of your database.
Running an SQL query
Ano ther useful to o l inc luded in the Visual Data Manager is the ability to exec ute an
SQL query against the c urrent database. Yo u c an enter the query direc tly in the SQL
Statement windo w (see Figure 30-3) o r use the Query Builder to c reate the query
(see Figure 30-4). Either way, when yo u run the query, yo ur results will be displayed
o ne ro w at a time in the Results windo w sho wn in Figure 30-5.
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Figure 30-3: Entering an SQL statem ent in the SQL statem ent w indow.
Using the Query Builder
Cho o sing Utility ➪ Query Builder fro m the Visual Data Manager main menu starts
the Query Builder (see Figure 30-4). This utility assists yo u with building a Select
statement. Yo u begin building the Select statement by c ho o sing the table yo u wish
to start with in the Tables sec tio n o f the fo rm, and the c o lumns yo u want to display
in the Fields to Sho w sec tio n o f the fo rm. If yo u do n’t selec t any c o lumns, all o f the
c o lumns will be returned.
By default, all o f the ro ws in the table will be returned. Ho wever, yo u c an use the
Field Name, Operato r, and Value fields to selec t a value o r range o f values fo r a spec ific c o lumn. List Po ssible Values will po pulate the Value dro p-do wn bo x with all o f
the values fo r that spec ific c o lumn in the table.
Onc e yo u’ve identified a field, o perato r, and value, yo u sho uld press the And into
Criteria butto n o r the Or into Criteria butto n to inc lude this expressio n in the
Criteria sec tio n at the bo tto m o f the fo rm. Then yo u c an selec t o ther c riteria by
selec ting a new c o lumn, o perato r, and value c o mbinatio n. Pressing the And into
Criteria butto n will insert an And o peratio n between the two c riteria, while pressing the Or into Criteria butto n will insert an Or o peratio n.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Figure 30-4: Building a query using the Query Builder.
Yo u c an c ho o se to inc lude Group By and Order By c lauses by using the Gro up By
and Order By dro p-do wn bo xes. If yo u have selec ted multiple tables, pressing the Set
Table Jo ins butto n will display a dialo g bo x that helps yo u selec t the jo in c o lumns.
The To p N Value field allo ws yo u to limit the number o f ro ws retrieved by spec ifying
an abso lute number o f ro ws o r a relative perc entage value.
When all o f the info rmatio n is entered into the fo rm, yo u c an press the Sho w butto n
to see the Select statement yo u just built. Pressing Save will save the Select statement as a QueryDef in the database fo r use with Ac c ess, while pressing Co py will
save a c o py o f the Select statement to the c lipbo ard so that yo u may paste it into a
different applic atio n. Pressing Clo se will c lo se the Query Builder windo w and plac e
the query yo u built in the SQL Statement windo w.
Getting your results
If yo u pressed the Enter key o r c lic ked o n the Exec ute butto n in the SQL Statement
windo w, o r c lic ked o n the Run butto n in the Query Builder, the Select statement will
be exec uted and the results will be returned in the Results windo w, as sho wn earlier
in Figure 30-5.
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Figure 30-5: Seeing the results from running your query.
One ro w at a time will be displayed in the fo rm. Clic king o n the sc ro ll bar at the
bo tto m o f the fo rm will mo ve yo u to ano ther ro w that was retrieved fro m the database. The butto ns at the to p o f the fo rm perfo rm the func tio ns yo u wo uld expec t.
Pressing the Add butto n will display an empty versio n o f the Edit fo rm in whic h yo u
may enter the info rmatio n yo u want to add to the database. Again, press Update to
save yo ur new ro w, o r Canc el to disc ard it. Pressing the Edit butto n will c hange the
c o nfiguratio n o f the fo rm slightly (see Figure 30-6). Yo u c an c hange any o f the values and press the Update butto n to save the c hanges to the database, o r press the
Canc el butto n to disc ard the c hanges. The Delete butto n is used to delete the c urrent ro w. Pressing the Clo se butto n will c lo se the Results windo w and return yo u to
the previo us windo w where yo u c reated the query. Clic king o n the So rt butto n will
display a message bo x pro mpting yo u to enter the so rt c o lumn, while c lic king o n
the Filter butto n will display ano ther message bo x pro mpting yo u to enter a filter
expressio n. The Mo ve butto n will allo w yo u to spec ify the number o f ro ws to be
mo ved. The Find butto n displays a dialo g bo x to help yo u find a partic ular ro w in
the query. Yo u must selec t a single field, a relatio nal o perato r, enter a value and
then c ho o se whether yo u want to find the first value, next value, previo us value o r
the last value in the table. Pressing the OK butto n will start the searc h and the
rec o rd will be displayed in the main fo rm.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Figure 30-6: Editing a row in your database.
Constructing Databases, Tables, and Indexes
Pro bably the mo st c o mmo n use fo r the Visual Data Manager utility is to c reate Jet
database o bjec ts witho ut using Ac c ess and witho ut building yo ur o wn applic atio n
pro gram.
Creating a new database
The fo llo wing steps will sho w yo u ho w to use the Visual Data Manager to c reate a
new database:
1. Start the Visual Data Manager and c ho o se File ➪ New ➪ Mic ro so ft Ac c ess ➪
Versio n 7.0 MDB fro m the main menu. This will display the Selec t Mic ro so ft
Ac c ess Database to Create dialo g bo x ( see Figure 30-7) . Unless yo u have a
spec ific need fo r an o bso lete database, do n’t c reate a Versio n 2.0 MDB.
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Figure 30-7: Choosing the nam e of your new
database.
2. Using the Selec t Mic ro so ft Ac c ess Database to Create dialo g bo x, c ho o se a
name fo r yo ur database and press Save to c reate it.
Tip
An empty database is a good thing: Because your database file is really just
another disk file, I find it useful to keep an em pty copy of the database file around
w hen I’m developing an application. I can just copy the .MDB file to another file
and use that to test m y application, w ithout having to go through the process of
re-creating the database. Also, you m ay w ant to keep a populated database
around w ith a set of know n values. You can create a new copy of this file to test
your application, w hile leaving the original populated file untouched for the next
tim e you w ant to do som e testing.
Creating a table
After c reating a new database o r o pening an existing database, yo u c an easily c reate a new table by fo llo wing these steps:
1. While in the Visual Data Manager, selec t the Database windo w. Right c lic k
anywhere in the windo w and c ho o se New Table. This will display the Table
Struc ture windo w, as sho wn in Figure 30-8. No te that yo u c an use the same
windo w to display info rmatio n abo ut an existing table by right c lic king o n the
table’s name in the Database windo w and c ho o sing Design fro m the po p-up
menu.
2. Enter the name o f the table in the Table Name field. No te that yo ur table name
sho uld no t duplic ate any tables already defined in yo ur database.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Figure 30-8: Creating a new table w ith the Table
Structure w indow.
3. To add fields to the table, press the Add Field butto n. This will display the Add
Field dialo g bo x, as sho wn in Figure 30-9. Enter the name o f the c o lumn in the
Name field and selec t its data type in the Type dro p-do wn bo x. Yo u c an igno re
the OrdinalPo sitio n bo x, sinc e this value will be c o mputed when the field is
added. Yo u may also spec ify a validatio n rule in the Validatio nRule bo x. If yo u
do so , yo u may also indic ate the text that will be displayed when the validatio n
rule fails in the Validatio nText bo x. The value yo u enter in the DefaultValue text
bo x will be used, instead o f a Null value, whenever a new rec o rd is c reated.
When yo u have entered the info rmatio n fo r a field, press the OK butto n. The
fo rm will be c leared and yo u c an enter the next field. When yo u have finished
entering all o f the fields, press the OK butto n to save the last field and then
press the Clo se butto n to return to the Table Struc ture windo w.
Figure 30-9: Adding a field to your table.
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Part IX ✦ M icrosoft Jet
4. Yo u c an view the info rmatio n asso c iated with a field by c lic king o n the field’s
name in the Field List bo x. The fields to the right o f the Field List will be po pulated with the info rmatio n. Yo u c an c hange any o f the info rmatio n asso c iated
with a field, exc ept fo r its data type and related info rmatio n. If yo u wish to
c hange this info rmatio n, yo u must delete the field and add it bac k again.
5. When yo u have entered all o f the fields, yo u may press the Clo se butto n, o r
c o ntinue this pro c ess by adding an index.
Creating an index
In o rder to c reate an index, yo u must have an existing table. Yo u c an c reate an
index at the same time yo u c reate the table, o r yo u c an add an index to an existing
table by right c lic king o n the table name in the Database windo w and selec ting
design fro m the po p-up menu to display the Table Struc ture windo w sho wn in
Figure 30-9. Then fo llo w these steps:
1. Press the Add Index butto n. This will display the Add Index dialo g bo x, as
sho wn in Figure 30-10.
Figure 30-10: Adding an index to a table.
2. Enter the name o f the index in the Name field. Cho o se Primary, Unique, and
Igno reNulls as appro priate. Just remember that yo u may have o nly o ne
Primary index, and it sho uld always be unique.
3. Add fields to the index by do uble-c lic king o n the field name in Available
Fields. The field name will be added to the list o f field names in the Indexed
Fields field. If yo u make a mistake, yo u c an delete the field by simply selec ting
the text in the Indexed Fields bo x and pressing the Delete key o n the keybo ard.
4. When yo u have finished adding yo ur index, press the OK butto n. The fo rm will
be c leared so that yo u may add ano ther index. If yo u are finished adding indexes
to this table, press the Clo se butto n to return to the Table Struc ture windo w.
Press the Clo se butto n o n the Table Struc ture windo w to return to the Database
Windo w.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Using the DAO and ADOX alternatives
Yo u c an also c reate database o bjec ts in a Jet database by using either DAO o r
ADOX (the ADO Extensio ns fo r Data Definitio n Language and Sec urity). While the
c o de to do this is very straightfo rward, it c an be lo ng and repetitive and no t easily
updated if yo ur database design is still in a state o f flux. Ho wever, having the ability
to c reate a new, empty database fro m within yo ur applic atio n c an be useful in a single user applic atio n. This allo ws a user to easily c reate multiple, independent data
files c o ntaining different sets o f data.
CrossReference
See Chapters 6 and 11 for m ore inform ation about the DAO and ADOX object
m odels, respectively.
M anaging Security
Every database system needs sec urity, even if yo u plan to allo w o nly o ne user to
ac c ess it. If yo u have Ac c ess installed, managing sec urity isn’t very diffic ult, and
this sec tio n disc usses ho w to use Ac c ess 2000 to manage sec urity fo r yo ur Jet 4.0
database. Similar features exist in Ac c ess 97 in o rder to manage yo ur Jet 3.5
database.
The security definition file
The first step in implementing sec urity o n a Jet database is to ensure that yo u
have a wo rkgro up database ( SYSTEM.MDW file) . This file c o ntains info rmatio n o n
the users and gro ups that sec ure a Jet database. By default, the SYSTEM.MDW file is
c reated when yo u install Ac c ess 97 o r Ac c ess 2000. Typic ally, this file is lo c ated in
the \Windows\System direc to ry; ho wever, it c an be lo c ated anywhere o n yo ur system, sinc e a value in the Windo ws registry is used to lo c ate the file.
If yo ur applic atio n is running in a netwo rked enviro nment, where the database is
shared amo ng multiple users, yo u sho uld make sure that the c lient systems po int to
a shared c o py o f the SYSTEM.MDW file. Yo u c an do this in DAO by spec ifying the
SystemDB pro perty in the DBEngine o bjec t, o r by adjusting the windo ws registry
key to po int to the file.
M anaging users and groups
Sinc e the wo rkgro up file is independent o f any database file, yo u c an manage the
users and gro ups in Ac c ess witho ut o pening a database. Yo u simply use the User
and Gro up Ac c o unts dialo g bo x, whic h has the ability to view and c hange info rmatio n abo ut existing users and gro ups, as well as c reate new users and gro ups.
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Part IX ✦ M icrosoft Jet
M anaging users
To see the info rmatio n abo ut an existing user, start Ac c ess 2000 and c ho o se To o ls ➪
Sec urity ➪ User and Gro up Ac c o unts to display the User and Gro up Ac c o unts dialo g
bo x (see Figure 30-11). Cho o se the user that yo u wish to view by selec ting the user
fro m the Name dro p-do wn bo x.
Figure 30-11: Displaying the User
and Group Accounts dialog box.
Note
Hidden secrets: While you can’t see the passw ord for a user, you can press the
Clear Passw ord button to rem ove the passw ord from the user if you are a m em ber
of the Adm ins group, or you can sw itch to the Change Logon Passw ord tab to
change the passw ord to a new value.
Yo u c an add a new user by pressing the New butto n in the User frame. This will display the New User/ Gro up dialo g bo x (see Figure 30-12). Enter the name o f the user
and a c o de phrase that will make the user name unique.
Figure 30-12: Creating a
new user or group.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Ousting Admin
Rem oving Adm in from pow er is one of the m ost im portant things you can do w hen you
im plem ent user-level security for your database. Unless you take the proper steps, it w ould be
easy for a disgruntled em ployee to subvert your entire security system . Even if you trust everyone that uses your database, you should still take this precaution. Som eone m ay take advantage of your open security and try to help you im prove your database. Even w ith the best of
intentions, and even if the individual has m ore know ledge about databases than you do, do
you really w ant som eone changing your application w ithout your know ledge? A healthy paranoia about security is very im portant and helps to prevent problem s in the long run.
Note
M ake personal ids personal: Given that you can easily substitute one w orkgroup
file for another, it is im portant that user ids and groups be m ade universally
unique. The key is supplying a value for the personal id that w ould not be easily
guessed.
M anaging groups
By switc hing to the Gro ups tab in the User and Gro up Ac c o unts dialo g bo x, yo u c an
see the list o f sec urity gro ups defined in the wo rkgro up file (see Figure 30-13). Yo u
c an c reate a new gro up by pressing the New butto n and entering the gro up and perso nal id as yo u did when yo u c reated a new user.
Figure 30-13: View ing the groups in
your w orkgroup file.
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Part IX ✦ M icrosoft Jet
M anaging permissions and ownership
Info rmatio n abo ut permissio ns to ac c ess a database o bjec t and the o wnership o f
this o bjec t is sto red in the system tables o f yo ur database. This info rmatio n is really
based o n SIDs and no t the ac tual user id o r gro up name. Thus, yo ur user id must
have already been authentic ated by using the wo rkgro up file.
M anaging permissions
Cho o se To o ls ➪ Sec urity ➪ User and Gro up Permissio ns fro m Ac c ess’s main menu
will display the User and Gro up Permissio ns dialo g bo x sho wn in Figure 30-14.
Figure 30-14: Editing a user or group’s
perm issions.
Changing ownership
Yo u c an switc h to the Change Owner tab o f the User and Gro up Permissio ns dialo g
bo x to c hange the o wner o f a database o bjec t to a different user id o r gro up (see
Figure 30-15). The Objec t and Current Owner lists sho w all o f the o bjec ts in yo ur
database that matc h the o bjec t selec ted in the Objec t Type dro p-do wn bo x.
To c hange the o wnership o f the o bjec t, yo u must begin by selec ting the database
o bjec t yo u wish to c hange. Then yo u must dec ide if the new o wner will be a gro up
o r a user and c lic k o n the appro priate radio butto n. The list o f po ssible o wners will
be displayed in the New Owner dro p-do wn bo x. Cho o se the user id o r gro up and
press the Change Owner butto n to make the c hange.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Caution
Don’t forget the database: The database itself is an object that is ow ned by a user
id or a group. By default, Adm in ow ns the database. You should change this to the
Adm ins group. Otherw ise, the Adm in user id is still all-pow erful.
Figure 30-15: Changing the ow nership
of a database object.
Thoughts on Creating Jet Databases
Jet based Visual Basic applications are usually targeted at either individuals or sm all organizations w ith only a handful of users. This m eans that m any of the approaches that I have
discussed throughout the book m ay not apply. For instance, security in a sm all application
is not nearly the problem it is in a larger application. For a single user application, the prim ary aim of security is to prevent the user from accidentally deleting inform ation needed to
run the application. Even w ith a handful of users, you are m ore concerned w ith identifying
the actions of individual users rather than protecting the database from m alicious actions.
Because of these differences, you m ight w ant to add som e features that you w ouldn’t consider in larger applications. For instance, you m ay w ant to add a feature in your application
to create a new copy of the database. This w ould let the user create m ultiple databases for
different projects.
Another feature you m ight consider adding is a w ay to create a backup copy of the
database. This m eans that the user has a w ay to m ake sure that their data is save in case of
a system problem . Of course, creating a w ay to backup the database im plies that a w ay to
restore it w ill be necessary at som e point.
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Summary
In this c hapter yo u learned:
✦ ho w to c reate databases, tables using Visual Basic to o ls fo r Jet 3.5 databases.
✦ ho w to use Ac c ess 2000 to c reate databases and tables fo r Jet 4.0.
✦ ho w to use Ac c ess 2000 to define sec urity in a Jet 4.0 database.
✦
✦
✦
C H A P T E R
Creating
Database
Objects with
M icrosoft Jet
I
n this c hapter, I’m go ing to sho w yo u ho w to use the Visual
Data Manager add-in to Visual Basic to c reate a Mic ro so ft
Jet database and vario us o bjec ts, suc h as tables and indexes,
inside it. I’ll also c o ver ho w to c reate users and gro ups, and
ho w to grant them permissio n to ac c ess database o bjec ts.
Introducing the Visual
Data M anager
The Visual Data Manager ( also c alled VisData) is a Visual
Basic add-in that was written to make it easier to design a
Jet datab ase witho ut b uilding yo ur o wn applic atio n o r using
Mic ro so ft Ac c ess. To run the Visual Data Manager, c ho o se
Add-Ins ➪ Visual Data Manager fro m the main menu. This
will start the applic atio n.
Tip
You got it: The Visual Data Manager utility w as w ritten in
Visual Basic 5 and show s w hat you can do w ith a little bit
of w ork. The com plete source code to the Visual Data
Manager is included in the Samples\VisData directory
of Visual Basic.
Sinc e the Visual Data Manager is based o n DAO, yo u c an use it
to o pen many o ther types o f databases besides Jet databases.
Yo u c an ac c ess dBase, Fo xPro , and Parado x databases, as well
as Exc el wo rksheets and vario us types o f text files. Yo u c an
even o pen an ODBC c o nnec tio n to a remo te database, suc h as
✦
✦
✦
✦
In This Cha pter
Intro ducing Visual
Data Manag er
Co nstructing
databases, tables,
and indexes
Manag ing security
fo r users and g ro ups
✦
✦
✦
✦
664
Part IX ✦ M icrosoft Jet
SQL Server o r Orac le, tho ugh depending o n yo ur c ho ic e o f database, so me o f the
c apabilities may no t be suppo rted.
Note
Why doesn’t it open the database I created with Access 2000?: Visual Data
Manager uses DAO 3.5 to access databases. To access a Jet 4.0 database created
w ith Access 2000, you m ust use DAO 3.6. While you could recom pile the Visual
Data Manager application using DAO 3.6, it’s probably just easier to use Access to
design your database.
Opening an existing database
To o pen an existing database, c ho o se File ➪ Open DataBase ➪ Mic ro so ft Ac c ess
fro m the Visual Data Manager main menu. This will display a File Open dialo g bo x.
Simply c ho o se the file c o ntaining yo ur database and press the Open butto n. After
yo u’ve o pened yo ur database, yo u’ll see a fo rm like the o ne sho wn in Figure 30-1.
By default, the Visual Data Manager o pens yo ur database and displays two multiple
do c ument interfac e (MDI) windo ws. The first windo w (Database Windo w) displays
yo ur database’s struc ture, while the sec o nd windo w (SQL Statement) allo ws yo u to
enter and run an SQL statement.
Figure 30-1: View ing an open database.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Viewing Database Information
The Visual Data Manager allo ws yo u to see the struc ture o f yo ur database (see
Figure 30-2). The tree view displays eac h table and its attributes, inc luding c o lumns
and indexes. As yo u expand eac h entry in the table, yo u will eventually reac h the
details that wo uld no rmally sho w up in the Properties c o llec tio n asso c iated with
the o bjec t.
Figure 30-2: View ing the structure of your database.
Running an SQL query
Ano ther useful to o l inc luded in the Visual Data Manager is the ability to exec ute an
SQL query against the c urrent database. Yo u c an enter the query direc tly in the SQL
Statement windo w (see Figure 30-3) o r use the Query Builder to c reate the query
(see Figure 30-4). Either way, when yo u run the query, yo ur results will be displayed
o ne ro w at a time in the Results windo w sho wn in Figure 30-5.
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Figure 30-3: Entering an SQL statem ent in the SQL statem ent w indow.
Using the Query Builder
Cho o sing Utility ➪ Query Builder fro m the Visual Data Manager main menu starts
the Query Builder (see Figure 30-4). This utility assists yo u with building a Select
statement. Yo u begin building the Select statement by c ho o sing the table yo u wish
to start with in the Tables sec tio n o f the fo rm, and the c o lumns yo u want to display
in the Fields to Sho w sec tio n o f the fo rm. If yo u do n’t selec t any c o lumns, all o f the
c o lumns will be returned.
By default, all o f the ro ws in the table will be returned. Ho wever, yo u c an use the
Field Name, Operato r, and Value fields to selec t a value o r range o f values fo r a spec ific c o lumn. List Po ssible Values will po pulate the Value dro p-do wn bo x with all o f
the values fo r that spec ific c o lumn in the table.
Onc e yo u’ve identified a field, o perato r, and value, yo u sho uld press the And into
Criteria butto n o r the Or into Criteria butto n to inc lude this expressio n in the
Criteria sec tio n at the bo tto m o f the fo rm. Then yo u c an selec t o ther c riteria by
selec ting a new c o lumn, o perato r, and value c o mbinatio n. Pressing the And into
Criteria butto n will insert an And o peratio n between the two c riteria, while pressing the Or into Criteria butto n will insert an Or o peratio n.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Figure 30-4: Building a query using the Query Builder.
Yo u c an c ho o se to inc lude Group By and Order By c lauses by using the Gro up By
and Order By dro p-do wn bo xes. If yo u have selec ted multiple tables, pressing the Set
Table Jo ins butto n will display a dialo g bo x that helps yo u selec t the jo in c o lumns.
The To p N Value field allo ws yo u to limit the number o f ro ws retrieved by spec ifying
an abso lute number o f ro ws o r a relative perc entage value.
When all o f the info rmatio n is entered into the fo rm, yo u c an press the Sho w butto n
to see the Select statement yo u just built. Pressing Save will save the Select statement as a QueryDef in the database fo r use with Ac c ess, while pressing Co py will
save a c o py o f the Select statement to the c lipbo ard so that yo u may paste it into a
different applic atio n. Pressing Clo se will c lo se the Query Builder windo w and plac e
the query yo u built in the SQL Statement windo w.
Getting your results
If yo u pressed the Enter key o r c lic ked o n the Exec ute butto n in the SQL Statement
windo w, o r c lic ked o n the Run butto n in the Query Builder, the Select statement will
be exec uted and the results will be returned in the Results windo w, as sho wn earlier
in Figure 30-5.
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Figure 30-5: Seeing the results from running your query.
One ro w at a time will be displayed in the fo rm. Clic king o n the sc ro ll bar at the
bo tto m o f the fo rm will mo ve yo u to ano ther ro w that was retrieved fro m the database. The butto ns at the to p o f the fo rm perfo rm the func tio ns yo u wo uld expec t.
Pressing the Add butto n will display an empty versio n o f the Edit fo rm in whic h yo u
may enter the info rmatio n yo u want to add to the database. Again, press Update to
save yo ur new ro w, o r Canc el to disc ard it. Pressing the Edit butto n will c hange the
c o nfiguratio n o f the fo rm slightly (see Figure 30-6). Yo u c an c hange any o f the values and press the Update butto n to save the c hanges to the database, o r press the
Canc el butto n to disc ard the c hanges. The Delete butto n is used to delete the c urrent ro w. Pressing the Clo se butto n will c lo se the Results windo w and return yo u to
the previo us windo w where yo u c reated the query. Clic king o n the So rt butto n will
display a message bo x pro mpting yo u to enter the so rt c o lumn, while c lic king o n
the Filter butto n will display ano ther message bo x pro mpting yo u to enter a filter
expressio n. The Mo ve butto n will allo w yo u to spec ify the number o f ro ws to be
mo ved. The Find butto n displays a dialo g bo x to help yo u find a partic ular ro w in
the query. Yo u must selec t a single field, a relatio nal o perato r, enter a value and
then c ho o se whether yo u want to find the first value, next value, previo us value o r
the last value in the table. Pressing the OK butto n will start the searc h and the
rec o rd will be displayed in the main fo rm.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Figure 30-6: Editing a row in your database.
Constructing Databases, Tables, and Indexes
Pro bably the mo st c o mmo n use fo r the Visual Data Manager utility is to c reate Jet
database o bjec ts witho ut using Ac c ess and witho ut building yo ur o wn applic atio n
pro gram.
Creating a new database
The fo llo wing steps will sho w yo u ho w to use the Visual Data Manager to c reate a
new database:
1. Start the Visual Data Manager and c ho o se File ➪ New ➪ Mic ro so ft Ac c ess ➪
Versio n 7.0 MDB fro m the main menu. This will display the Selec t Mic ro so ft
Ac c ess Database to Create dialo g bo x ( see Figure 30-7) . Unless yo u have a
spec ific need fo r an o bso lete database, do n’t c reate a Versio n 2.0 MDB.
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Figure 30-7: Choosing the nam e of your new
database.
2. Using the Selec t Mic ro so ft Ac c ess Database to Create dialo g bo x, c ho o se a
name fo r yo ur database and press Save to c reate it.
Tip
An empty database is a good thing: Because your database file is really just
another disk file, I find it useful to keep an em pty copy of the database file around
w hen I’m developing an application. I can just copy the .MDB file to another file
and use that to test m y application, w ithout having to go through the process of
re-creating the database. Also, you m ay w ant to keep a populated database
around w ith a set of know n values. You can create a new copy of this file to test
your application, w hile leaving the original populated file untouched for the next
tim e you w ant to do som e testing.
Creating a table
After c reating a new database o r o pening an existing database, yo u c an easily c reate a new table by fo llo wing these steps:
1. While in the Visual Data Manager, selec t the Database windo w. Right c lic k
anywhere in the windo w and c ho o se New Table. This will display the Table
Struc ture windo w, as sho wn in Figure 30-8. No te that yo u c an use the same
windo w to display info rmatio n abo ut an existing table by right c lic king o n the
table’s name in the Database windo w and c ho o sing Design fro m the po p-up
menu.
2. Enter the name o f the table in the Table Name field. No te that yo ur table name
sho uld no t duplic ate any tables already defined in yo ur database.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Figure 30-8: Creating a new table w ith the Table
Structure w indow.
3. To add fields to the table, press the Add Field butto n. This will display the Add
Field dialo g bo x, as sho wn in Figure 30-9. Enter the name o f the c o lumn in the
Name field and selec t its data type in the Type dro p-do wn bo x. Yo u c an igno re
the OrdinalPo sitio n bo x, sinc e this value will be c o mputed when the field is
added. Yo u may also spec ify a validatio n rule in the Validatio nRule bo x. If yo u
do so , yo u may also indic ate the text that will be displayed when the validatio n
rule fails in the Validatio nText bo x. The value yo u enter in the DefaultValue text
bo x will be used, instead o f a Null value, whenever a new rec o rd is c reated.
When yo u have entered the info rmatio n fo r a field, press the OK butto n. The
fo rm will be c leared and yo u c an enter the next field. When yo u have finished
entering all o f the fields, press the OK butto n to save the last field and then
press the Clo se butto n to return to the Table Struc ture windo w.
Figure 30-9: Adding a field to your table.
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4. Yo u c an view the info rmatio n asso c iated with a field by c lic king o n the field’s
name in the Field List bo x. The fields to the right o f the Field List will be po pulated with the info rmatio n. Yo u c an c hange any o f the info rmatio n asso c iated
with a field, exc ept fo r its data type and related info rmatio n. If yo u wish to
c hange this info rmatio n, yo u must delete the field and add it bac k again.
5. When yo u have entered all o f the fields, yo u may press the Clo se butto n, o r
c o ntinue this pro c ess by adding an index.
Creating an index
In o rder to c reate an index, yo u must have an existing table. Yo u c an c reate an
index at the same time yo u c reate the table, o r yo u c an add an index to an existing
table by right c lic king o n the table name in the Database windo w and selec ting
design fro m the po p-up menu to display the Table Struc ture windo w sho wn in
Figure 30-9. Then fo llo w these steps:
1. Press the Add Index butto n. This will display the Add Index dialo g bo x, as
sho wn in Figure 30-10.
Figure 30-10: Adding an index to a table.
2. Enter the name o f the index in the Name field. Cho o se Primary, Unique, and
Igno reNulls as appro priate. Just remember that yo u may have o nly o ne
Primary index, and it sho uld always be unique.
3. Add fields to the index by do uble-c lic king o n the field name in Available
Fields. The field name will be added to the list o f field names in the Indexed
Fields field. If yo u make a mistake, yo u c an delete the field by simply selec ting
the text in the Indexed Fields bo x and pressing the Delete key o n the keybo ard.
4. When yo u have finished adding yo ur index, press the OK butto n. The fo rm will
be c leared so that yo u may add ano ther index. If yo u are finished adding indexes
to this table, press the Clo se butto n to return to the Table Struc ture windo w.
Press the Clo se butto n o n the Table Struc ture windo w to return to the Database
Windo w.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Using the DAO and ADOX alternatives
Yo u c an also c reate database o bjec ts in a Jet database by using either DAO o r
ADOX (the ADO Extensio ns fo r Data Definitio n Language and Sec urity). While the
c o de to do this is very straightfo rward, it c an be lo ng and repetitive and no t easily
updated if yo ur database design is still in a state o f flux. Ho wever, having the ability
to c reate a new, empty database fro m within yo ur applic atio n c an be useful in a single user applic atio n. This allo ws a user to easily c reate multiple, independent data
files c o ntaining different sets o f data.
CrossReference
See Chapters 6 and 11 for m ore inform ation about the DAO and ADOX object
m odels, respectively.
M anaging Security
Every database system needs sec urity, even if yo u plan to allo w o nly o ne user to
ac c ess it. If yo u have Ac c ess installed, managing sec urity isn’t very diffic ult, and
this sec tio n disc usses ho w to use Ac c ess 2000 to manage sec urity fo r yo ur Jet 4.0
database. Similar features exist in Ac c ess 97 in o rder to manage yo ur Jet 3.5
database.
The security definition file
The first step in implementing sec urity o n a Jet database is to ensure that yo u
have a wo rkgro up database ( SYSTEM.MDW file) . This file c o ntains info rmatio n o n
the users and gro ups that sec ure a Jet database. By default, the SYSTEM.MDW file is
c reated when yo u install Ac c ess 97 o r Ac c ess 2000. Typic ally, this file is lo c ated in
the \Windows\System direc to ry; ho wever, it c an be lo c ated anywhere o n yo ur system, sinc e a value in the Windo ws registry is used to lo c ate the file.
If yo ur applic atio n is running in a netwo rked enviro nment, where the database is
shared amo ng multiple users, yo u sho uld make sure that the c lient systems po int to
a shared c o py o f the SYSTEM.MDW file. Yo u c an do this in DAO by spec ifying the
SystemDB pro perty in the DBEngine o bjec t, o r by adjusting the windo ws registry
key to po int to the file.
M anaging users and groups
Sinc e the wo rkgro up file is independent o f any database file, yo u c an manage the
users and gro ups in Ac c ess witho ut o pening a database. Yo u simply use the User
and Gro up Ac c o unts dialo g bo x, whic h has the ability to view and c hange info rmatio n abo ut existing users and gro ups, as well as c reate new users and gro ups.
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M anaging users
To see the info rmatio n abo ut an existing user, start Ac c ess 2000 and c ho o se To o ls ➪
Sec urity ➪ User and Gro up Ac c o unts to display the User and Gro up Ac c o unts dialo g
bo x (see Figure 30-11). Cho o se the user that yo u wish to view by selec ting the user
fro m the Name dro p-do wn bo x.
Figure 30-11: Displaying the User
and Group Accounts dialog box.
Note
Hidden secrets: While you can’t see the passw ord for a user, you can press the
Clear Passw ord button to rem ove the passw ord from the user if you are a m em ber
of the Adm ins group, or you can sw itch to the Change Logon Passw ord tab to
change the passw ord to a new value.
Yo u c an add a new user by pressing the New butto n in the User frame. This will display the New User/ Gro up dialo g bo x (see Figure 30-12). Enter the name o f the user
and a c o de phrase that will make the user name unique.
Figure 30-12: Creating a
new user or group.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Ousting Admin
Rem oving Adm in from pow er is one of the m ost im portant things you can do w hen you
im plem ent user-level security for your database. Unless you take the proper steps, it w ould be
easy for a disgruntled em ployee to subvert your entire security system . Even if you trust everyone that uses your database, you should still take this precaution. Som eone m ay take advantage of your open security and try to help you im prove your database. Even w ith the best of
intentions, and even if the individual has m ore know ledge about databases than you do, do
you really w ant som eone changing your application w ithout your know ledge? A healthy paranoia about security is very im portant and helps to prevent problem s in the long run.
Note
M ake personal ids personal: Given that you can easily substitute one w orkgroup
file for another, it is im portant that user ids and groups be m ade universally
unique. The key is supplying a value for the personal id that w ould not be easily
guessed.
M anaging groups
By switc hing to the Gro ups tab in the User and Gro up Ac c o unts dialo g bo x, yo u c an
see the list o f sec urity gro ups defined in the wo rkgro up file (see Figure 30-13). Yo u
c an c reate a new gro up by pressing the New butto n and entering the gro up and perso nal id as yo u did when yo u c reated a new user.
Figure 30-13: View ing the groups in
your w orkgroup file.
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M anaging permissions and ownership
Info rmatio n abo ut permissio ns to ac c ess a database o bjec t and the o wnership o f
this o bjec t is sto red in the system tables o f yo ur database. This info rmatio n is really
based o n SIDs and no t the ac tual user id o r gro up name. Thus, yo ur user id must
have already been authentic ated by using the wo rkgro up file.
M anaging permissions
Cho o se To o ls ➪ Sec urity ➪ User and Gro up Permissio ns fro m Ac c ess’s main menu
will display the User and Gro up Permissio ns dialo g bo x sho wn in Figure 30-14.
Figure 30-14: Editing a user or group’s
perm issions.
Changing ownership
Yo u c an switc h to the Change Owner tab o f the User and Gro up Permissio ns dialo g
bo x to c hange the o wner o f a database o bjec t to a different user id o r gro up (see
Figure 30-15). The Objec t and Current Owner lists sho w all o f the o bjec ts in yo ur
database that matc h the o bjec t selec ted in the Objec t Type dro p-do wn bo x.
To c hange the o wnership o f the o bjec t, yo u must begin by selec ting the database
o bjec t yo u wish to c hange. Then yo u must dec ide if the new o wner will be a gro up
o r a user and c lic k o n the appro priate radio butto n. The list o f po ssible o wners will
be displayed in the New Owner dro p-do wn bo x. Cho o se the user id o r gro up and
press the Change Owner butto n to make the c hange.
Chapter 30 ✦ Creating Database Objects with M icrosoft Jet
Caution
Don’t forget the database: The database itself is an object that is ow ned by a user
id or a group. By default, Adm in ow ns the database. You should change this to the
Adm ins group. Otherw ise, the Adm in user id is still all-pow erful.
Figure 30-15: Changing the ow nership
of a database object.
Thoughts on Creating Jet Databases
Jet based Visual Basic applications are usually targeted at either individuals or sm all organizations w ith only a handful of users. This m eans that m any of the approaches that I have
discussed throughout the book m ay not apply. For instance, security in a sm all application
is not nearly the problem it is in a larger application. For a single user application, the prim ary aim of security is to prevent the user from accidentally deleting inform ation needed to
run the application. Even w ith a handful of users, you are m ore concerned w ith identifying
the actions of individual users rather than protecting the database from m alicious actions.
Because of these differences, you m ight w ant to add som e features that you w ouldn’t consider in larger applications. For instance, you m ay w ant to add a feature in your application
to create a new copy of the database. This w ould let the user create m ultiple databases for
different projects.
Another feature you m ight consider adding is a w ay to create a backup copy of the
database. This m eans that the user has a w ay to m ake sure that their data is save in case of
a system problem . Of course, creating a w ay to backup the database im plies that a w ay to
restore it w ill be necessary at som e point.
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Summary
In this c hapter yo u learned:
✦ ho w to c reate databases, tables using Visual Basic to o ls fo r Jet 3.5 databases.
✦ ho w to use Ac c ess 2000 to c reate databases and tables fo r Jet 4.0.
✦ ho w to use Ac c ess 2000 to define sec urity in a Jet 4.0 database.
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✦
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