Overview of
M icrosoft Jet
I
n this c hapter, I’ll disc uss the Mic ro so ft Jet database system. I’ll explain its arc hitec ture, data types, and sec urity mo del. I’ll also c o ver ho w to use ADO with Jet, inc luding ho w to build a c o nnec tio n string and use the spec ial fac ilities built into Visual Basic to suppo rt designing Jet databases.Overview of Jet
There are perhaps mo re c o pies o f Mic ro so ft Jet installed o n PCs than any o ther database engine, due to the fac t that Jet is a c ritic al part o f Mic ro so ft Ac c ess, whic h is part o f the Mic ro so ft Offic e family. This is the ideal c ho ic e when yo u want to build a single-user database system o r share yo ur database o n a file server with a small gro up o f peo ple.
Mic ro so ft Jet c o ntains the fo llo wing key features:
✦Standard part o f Visual Basic Pro fessio nal Editio n and Enterprise Editio n
✦Limited versio n inc luded in the Visual Basic Learning Editio n.
✦Can be inc luded with yo ur Visual Basic applic atio n at no c harge
✦Requires little o r no maintenanc e to maintain
✦Do esn’t require a dedic ated database server
Jet and Access or Access and Jet: Since the Jet database is so tightly coupled to the Access application, m any people use the term s interchangeably. For instance, the Visual Data Manager tool refers to Access databases rather than Jet databases. I w ill refer to the database itself as Jet and the application developm ent tool as Access, unless I’m referring to a m enu item or com m and that specifically refers to Access rather than Jet.
M icrosoft Jet versions
Mic ro so ft Jet is just the database engine c o mpo nent fro m the Mic ro so ft Ac c ess database system. Jet bec ame available fo r the first time in 1992, when Ac c ess 1.0 was released. When Visual Basic Versio n 3.0 was released in 1993, Jet versio n 1.1 was inc luded. Eac h new versio n o f Ac c ess resulted in a new versio n o f the Jet database engine. When a new versio n o f Visual Basic was ready fo r release, it wo uld inc lude the latest versio n o f the Jet database, with a few mino r impro vements.
Bec ause the same database engine is used fo r bo th Ac c ess and Visual Basic , yo u c an build an applic atio n that inc ludes c o mpo nents in Ac c ess and in Visual Basic . Of c o urse, yo u will need to use c o mpatible versio ns o f the so ftware. In o ther wo rds, do n’t expec t to use an Ac c ess 1.0 database with Visual Basic 6.
Visual Basic 6 inc ludes versio n 3.51 o f the Jet database engine, whic h is basic ally the same as the versio n 3.5 that was shipped with Ac c ess 97. Ac c ess 2000 inc ludes versio n 4.0 o f the Jet engine, whic h c an also be used with Visual Basic . These are the two mo st c o mmo n versio ns o f the Jet engine yo u are likely to enc o unter as a Visual Basic pro grammer.
The tools don’t work: The tools included w ith Visual Basic 6 only w ork on version 3.5 and earlier Jet databases. If you plan to use a version 4.0 database, you m ust have Access 2000 installed on your system or be prepared to w rite your ow n code using ADOX or DAO to adm inistrator your database.Jet version 3.5
If yo u develo p VB6 applic atio ns with Jet, yo u are mo st likely using versio n 3.51. This is a very stable release, and it wo rks well with Ac c ess 97. It is signific antly faster than previo us versio ns o f the Jet engine and designed to be a little bit mo re friendly when ac c essing a database sto red o n a file server.
Key features o f Jet 3.5 inc lude:
✦Suppo rt fo r the mo st c o mmo n SQL statements, suc h as Se le ct
✦Database kept in a single file
✦Ability to sec ure info rmatio n based o n user name and sec urity gro up
✦Ability to link to remo te tables and treat them as lo c al tables
✦Database replic atio n with o ther database servers ✦Page-level lo c king
Jet version 4.0
Ac c ess 2000 intro duc ed a newly revised Jet database system. Mo st o f the c hanges made to Jet between Versio n 3.5 and 4.0 are transparent to the typic al user. These c hanges were made to impro ve Ac c ess’s integratio n with o ther Offic e 2000 applic a-tio ns. Ho wever, so me impro vements were made in the database engine itself:
✦Suppo rt fo r Unic o de data
✦Rec o rd-level lo c king
✦Migratio n utility to c o nvert versio n 3.5 and earlier databases to the versio n 4.0 fo rmat
Bigger is not always better: If you convert a database from Jet 3.5 to Jet 4.0 and the database contains a lot of character data, expect the size to double. This is due to the fact that the actual data in the database is stored using Unicode characters. So, even if you try to store sim ple ASCII characters, the Jet database w ill store them using their Unicode equivalents.
Jet and DAO
Jet was designed aro und using the DAO( Data Ac c ess Objec ts) to ac c ess the c o
n-tents o f the database. DAO is a highly-struc tured o bjec t mo del that has evo lved alo ngside Jet sinc e its beginnings. In many ways, it is diffic ult to view the Jet database engine witho ut thinking o f the DAO o bjec t library.
See Chapter 6, “Accessing Databases from Visual Basic,” for explanations of the DAO, ADO, and RDO m odels.
There are mo re pro grams to day that use DAO, rather than ADO, to ac c ess Jet databases. Ho wever, I rec o mmend using ADO o ver DAO fo r two reaso ns. First the ADO o bjec t library o ffers a simpler interfac e to yo ur database, whic h translates into less c o de needed to do the same wo rk. Sec o nd, Mic ro so ft has dec ided that ADO is the way everyo ne sho uld ac c ess databases using Mic ro so ft develo pment to o ls suc h as Visual Basic . While this do esn’t mean that DAO is DOA, it do es mean that its days are numbered. Ac c ess 2000 already suppo rts ADO ac c ess to the Jet database, whic h inc identally allo ws yo u to use Ac c ess 2000 as a develo pment to o l fo r SQL Server databases. Also , Mic ro so ft has develo ped a c lient c o mputer friendly versio n o f SQL Server c alled the Mic ro so ft Database Engine ( MSDE) , whic h is 100% upwards c o mpatible with SQL Server — so mething that Jet is no t.
Cross-Reference
Jet 4.0, DAO 3.6: Even though the Jet database engine has been upgraded to ver-sion 4.0, the Data Access Objects (DAO) m ost often associated w ith the Jet database have been enhanced to version 3.6.
Jet utilities
Fo r the mo st part, there are very few utilities asso c iated with the Jet database, o ther than Ac c ess o f c o urse. Visual Basic inc ludes a to o l c alled the Visual Data Manager. This is an add-in to o l that allo ws yo u to design Jet databases. ( I’ll talk mo re abo ut the Visual Data Manager in Chapter 30) .
Understanding the Database Architecture
Mic ro so ft Jet is designed to be an SQL-based database management system that suppo rts a handful o f c o nc urrent users. It wo rks espec ially well with o nly a single user. While it c an easily be used with Visual Basic applic atio ns, muc h o f the arc hi-tec ture revo lves aro und suppo rting Ac c ess applic atio ns.Unlike mo st database systems, Mic ro so ft Jet relies o n shared ac c ess to a single file to c o o rdinate ac tivities amo ng multiple users. This means that the c o de that man-ages the database must run in eac h user’s address spac e and that yo u c an’t buffer data in memo ry, bec ause different pro grams c an’t share their memo ry. All sharing must be do ne at the file level, using file lo c king and o ther tec hniques. This really limits perfo rmanc e and means that yo u pro bably wo n’t even get c lo se to Jet’s 255 c o nc urrent user limit.
.M DB files
All o f the info rmatio n fo r the Jet database is sto red in a single file, c alled the .MDB
file. This file c o ntains all o f the data, inc luding tables and indexes, that c o mprise the Jet database. In additio n, fac ilities are inc luded that permit Ac c ess to sto re info rmatio n in the same file witho ut sto ring it in regular database tables.
Data is sto red in an .MDBfile using different types o f pages. The first page in the file is the datab ase header page, whic h c o ntains general info rmatio n ab o ut the datab ase, plus info rmatio n that trac ks user o peratio ns in a multi-user enviro n-ment. Tab le pages are used to ho ld info rmatio n sto red in a tab le, while index pages ho ld index info rmatio n. Lo ng value pages keep lo ng values suc h as Text and Memo fields, whic h c an exc eed the size o f a no rmal page. Finally, system tab le info rma-tio n is sto red in its o wn uniq ue type o f page.
There are fo ur system tables:
✦MSysACESho lds sec urity permissio ns.
✦MSysObje ctsho lds info rmatio n abo ut eac h database o bjec t, suc h as a table o r index, plus info rmatio n o n Ac c ess o bjec ts, suc h as fo rms and repo rts.
✦MSysQue rie sho lds info rmatio n abo ut predefined queries in the database. ✦MSysRe lationshipsho lds info rmatio n abo ut predefined relatio nships between
tables in the databases.
In additio n to these tables, o ther tables will sto re info rmatio n abo ut o bjec ts used o nly in Ac c ess applic atio ns.
You didn’t hear this from me: Microsoft considers the system tables in Access to be undocum ented. This basically m eans that Microsoft reserves the right to change the organization and num ber of system tables w ithout notifying anyone. Therefore, you shouldn’t attem pt to use these tables as part of your application.
.LDB files
When so meo ne o pens an .MDBfile fo r the first time, an .LDBfile is c reated in the same direc to ry as the .MDBfile. After the last user c lo ses the database, the .LDBfile will auto matic ally be deleted. This file c o ntains lo c king info rmatio n fo r the .MDBfile. Eac h user that has a c o nnec tio n to the database has an entry in the .LDBfile. This entry c o ntains the user name and the name o f the c o mputer that is being used.
When the user lo c ks a piec e o f data in the .MDBfile, an extended byte range lo c k is plac ed o n the .LDBfile. With a little c reativity, this type o f lo c k pro vides bo th shared and exc lusive ac c ess to data in the .MDBfile witho ut the o verhead o f using physic al lo c ks o n the .MDBfile itself.
.M DW Files
The sec urity info rmatio n fo r a Jet database is sto red in an .MDWfile, whic h is also kno wn as a wo rkgro up file. This file c o ntains info rmatio n abo ut users and gro ups, inc luding their lo gin passwo rds. Onc e a user is validated, they will be assigned an internal sec urity identifier ( SID) . The permissio ns that asso c iate the SID with the vario us database o bjec ts are sto red inside the .MDBfile.
M DA or M DW: Depending on the version of Access you have installed, the
SYSTEM.MDAfile m ay also be know n as SYSTEM.MDW.
Database objects
As yo u might expec t, a Jet database c an ho ld many o f the same database o bjec ts that are fo und in o ther database management systems. Ho wever, due to the nature o f the target audienc e, so me o f the database o bjec ts yo u wish to use may no t be available. While tables and indexes are present in Jet, views and triggers are no t.
Key database o bjec ts inc lude:
✦Table s, whic h sto re user data in fields.
✦Inde xe s, whic h pro vide quic k ways to lo c ate info rmatio n in a table.
✦Re lationships, whic h are used to define relatio nships between tables and to enfo rc e referential integrity.
✦Que rie s, whic h are used to define standard queries ( similar to views) against the database.
✦Use rs and Groups, whic h are used to asso c iate individuals with database per-missio ns.
Linked databases
One o f the nic er fac ilities in Jet is the ability to c reate a link to a table in ano ther database in suc h a way that the pro grammer thinks the table is lo c al to the Jet database. Besides linking to o ther Jet databases, yo u c an also link to these databases:
✦dBase
✦Fo xPro ✦Parado x
✦Exc el
✦Exc hange and Outlo o k data files ( read ac c ess o nly)
✦So me text file fo rmats, inc luding c o mma-separated value files ✦Any ODBC-c o mpliant database, suc h as SQL Server o r Orac le8
Link-be-gone: In the past, the only w ay to access a rem ote database w as to cre-ate a link using a Jet database. With Visual Basic 6, both DAO and ADO m ake this feature unnecessary. You can now access any ODBC-com patible database directly w ith DAO and the ODBCDirect interface, or you can use the m ore flexible OLE DB w ith ADO. (Of course, you can alw ays use RDO, but ADO m akes that tool even m ore obsolete than DAO).
Database capacities
Table 29-1 lists so me o f the key c apac ities and limitatio ns o f the Jet database.
Table 29-1
Database Capacities
Item Capacity
Maxim um database size 1 gigabyte Maxim um characters per object nam e 64 Maxim um characters per user nam e 20 Maxim um characters per user passw ord 14 Maxim um characters per database passw ord 20 Maxim um num ber of concurrent users 255 Maxim um characters in a table nam e 64 Maxim um characters in a field nam e 64 Maxim um table size 1 gigabyte Maxim um num ber of fields per table 255 Maxim um num ber of indexes per table 32 Maxim um num ber of fields in an index 10 Maxim um num ber of fields in an Order By clause 10
Jet data types
Jet suppo rts a wide variety o f data types, as sho wn in Table 29-2.
Connecting to Jet with ADO
In o rder to c o nnec t to a Jet database using ADO, yo u c an use the fo llo wing c o nnec -tio n string:
Table 29-2
Jet Data Types
Jet Data Type Visual Basic Data Type Comments
Boolean Boolean Nullvalues can’t be used w ith a Bitfield Byte Byte Holds num bers from 0 to 255
Currency Currency Eight-byte scaled integer accurate to four decim al places
Date Date A date and tim e value ranging from 1 Jan 100 to 31 Dec 9999
Double Double A 64-bit floating-point num ber
Integer Integer Holds num bers from –32,768 to + 32,767 (16-bit integer)
Long Long Holds num bers from –2,147,483,648 to + 2,147,483,647 (32-bit integer) Memo String A text field containing up to 1.2 billion
characters
OLE Object Byte Array Holds pictures and other large, raw binary values up to 1.2 gigabytes in length Single Single A 32-bit floating-point num ber
Text String Variable length string up to 255 characters in length
If yo u spec ified user-level sec urity in yo ur database, yo u c an also inc lude the User ID=and Password=keywo rds if yo u want to inc lude that info rmatio n as part o f the c o nnec tio n string — o r yo u c an supply them as arguments to the Openmetho d.
Figure 29-1: Choosing the proper OLE DB Jet provider.
Figure 29-2: Specifying the nam e of the database, user nam e, and passw ord inform ation.
If yo u spec ified a database passwo rd, yo u c an spec ify that value o n the All tab o f the Data Link Pro perties windo w. Simply selec t Jet OLEDB: Database Password
Figure 29-3: Specifying a database passw ord.
Jet security
Like o ther database systems, Jet inc o rpo rates its o wn sec urity subsystem. While yo u c an c ho o se to use it o r igno re it, do n’t igno re it if yo u plan to let mo re than o ne perso n ac c ess yo ur database. In fac t, yo u sho uld use it even if o nly o ne perso n will use yo ur database. It just might sto p them fro m c o rrupting the database by pre-venting them fro m o pening the database with any o ther applic atio n.
Share-level security
In share-level sec urity, the database is assigned a passwo rd. Anyo ne that kno ws the passwo rd c an ac c ess the database. While this isn’t as sec ure as user-level sec urity (disc ussed next), it will prevent a user fro m ac c essing yo ur database with Exc el o r Ac c ess. Yo u c an set the passwo rd using Ac c ess o r by using the DAO CreateDatabase
metho d in the Wo rkspac e o bjec t. Simply append the string ;pwd=MyPasswordto the end o f the Localeparameter to set the database passwo rd to MyPassword. Yo u then spec ify the passwo rd in the Connectparameter o f the DAO OpenDatabasemetho d. In ADO, yo u simply spec ify the passwo rd as part o f the Connectionstring by using the
User-level security
User-level sec urity wo rks like the sec urity systems in SQL Server and Orac le8i. A userID and passwo rd are c reated fo r eac h perso n who ac c esses the database. Then the userID is granted permissio n to ac c ess the vario us o bjec ts in the database. Gro ups c an be used with user-level sec urity to simplify administratio n. Yo u c an c reate a gro up and assign permissio ns to it, just as if it was ano ther user. Then yo u c an assign gro ups to vario us userIDs. When users lo g o n to the database, they will inherit all o f the per-missio ns asso c iated with the gro up o r gro ups that have been assigned to them, plus any individual permissio ns they may have been granted.
Finding the workgroup file
Sec urity info rmatio n is kept in a wo rkgro up file (.MDW) . Only o ne wo rkgro up file is required per system, and the fo llo wing windo ws registry key will be used to lo c ate it:
HKEY_LOCAL_SYSTEM\Software\Microsoft\Office\8.0\Access\Jet\3.5\ Engine\SystemDB
A typic al value fo r this key is:
C:\WINDOWS\SYSTEM\system.mdw
Of c o urse, if yo u are using a Jet 4.0 database with Ac c ess 2000 ( Versio n 9.0) , yo u will need to adjust the registry key to inc lude the appro priate versio n numbers.
Security with Access:In order to use security w ith a Jet database, you m ust have installed Access 97 (or Access 2000 for a Jet 4.0 database). The file SYSTEM.MDW
w ill autom atically be installed.
SIDs, user ids, personal ids, and passwords
All info rmatio n inside a database file is sec ured o n the basis o f a Sec urity ID, o r SID. A SID is derived by enc rypting the user id and a spec ial value kno wn as a perso nal id. If yo u c reate a user id and do n’t bo ther to assign a perso nal id, it is po ssible fo r so meo ne else to c reate ano ther wo rkgro up file with the same user id and their o wn passwo rd and user id to gain ac c ess to database reso urc es fo r whic h they do no t have permissio n.
The perso nal id is used to ensure that the SID is uniq ue and c an’t b e duplic ated, so yo u sho uld always c ho o se a value that is hard to duplic ate. Simply c ho o sing so meo ne’s first name as the user id and their last name fo r the perso nal id is no t a go o d idea.
By default, the user id Admin exists in all database systems with the same perso nal id. This means that Admin has the same SID no matter whic h c o mputer they use. The same is true fo r the Users gro up. Ho wever, this is no t true fo r the Admins gro up. The SID asso c iated with the Admins gro up is always unique fo r eac h wo rkgro up file.
Sinc e anyo ne in the Admins gro up has ac c ess to the entire database, the first thing yo u want to do is c reate a new user id and assign it to the Admins gro up. Then yo u sho uld remo ve the Admin user id fro m the Admins gro up to ensure that so meo ne do esn’t use their o wn wo rkgro up file to o verride yo ur database’s sec urity.
Permissions, SIDs, and ownership
The wo rkgro up file exists simply to return a SID fo r a partic ular user and ano ther SID fo r eac h gro up that the user is a member o f. This value is used as a key into so me o f the system tables inside the database file to determine whic h permissio ns the user has.
If the user has o verlapping permissio ns fro m multiple gro ups and/ o r explic it permis-sio n assignments, the least restric tive permispermis-sio n will apply. Thus, if a user has read and write permissio n to the Custo mers table via the Clerks gro up and has been explic itly granted read permissio n to the Custo mers table, the user will have read/ write ac c ess to the Custo mers table.
Whenever an o b jec t is c reated in the datab ase, the user id o f the individual that c reated the o b jec t is used as the o b jec t’s o wner. The o b jec t’s o wner always has
Microsoft Jet is a low -end database system designed prim arily for use w ith Microsoft Access. Much of the design revolves around features that are im portant to Access users, w hile features that m ight be useful to Visual Basic program m ers aren’t present. Jet is not upw ards-com patible w ith SQL Server and m ost other databases, due to these lim itations.
Jet is also lim ited to a m axim um of 255 concurrent users. How ever, its true m axim um is far less, depending on how frequently the database is accessed. While m any im provem ents have been m ade to m ake Jet m ore efficient, it is still lim ited by its file-oriented approach to synchronization for m ultiple users.
Summary
In this c hapter yo u learned:
✦abo ut the vario us versio ns o f the Jet database system. ✦that the to o ls in Visual Basic do no t suppo rt Jet 4.0.
✦that yo u c an use DAO 3.6 o r ADO with the appro priate pro vider to write pro -grams that wo rk with Jet 4.0.
✦abo ut the arc hitec ture o f the Jet database.
✦ho w to c o nnec t to a Jet database.
✦abo ut the sec urity c apabilities fo r a Jet database.
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This m akes it ideal for those applications that are cost-sensitive, such as sharew are, or part of a large distributed application w here you w ant to cache data on the local com puter.
