1DBI::FAQ(3) User Contributed Perl Documentation DBI::FAQ(3)
2
6 DBI::FAQ -- The Frequently Asked Questions for the Perl5 Database
7 Interface
8
10 perldoc DBI::FAQ
11
13 This document is currently at version 0.38, as of February 8th, 2000.
14
16 This document serves to answer the most frequently asked questions on
17 both the DBI Mailing Lists and personally to members of the DBI devel‐
18 opment team.
19
21 1.1 What is DBI, DBperl, Oraperl and *perl?
22 To quote Tim Bunce, the architect and author of DBI:
24 ``DBI is a database access Application Programming Interface (API)
26 for the Perl Language. The DBI API Specification defines a set
27 of functions, variables and conventions that provide a consistent
28 database interface independant of the actual database being used.''
29 In simple language, the DBI interface allows users to access multiple
31 database types transparently. So, if you connecting to an Oracle,
32 Informix, mSQL, Sybase or whatever database, you don't need to know the
33 underlying mechanics of the 3GL layer. The API defined by DBI will work
34 on all these database types.
35 A similar benefit is gained by the ability to connect to two different
37 databases of different vendor within the one perl script, ie, I want to
38 read data from an Oracle database and insert it back into an Informix
39 database all within one program. The DBI layer allows you to do this
40 simply and powerfully.
41 DBperl is the old name for the interface specification. It's usually
43 now used to denote perl4 modules on database interfacing, such as,
44 oraperl, isqlperl, ingperl and so on. These interfaces didn't have a
45 standard API and are generally not supported.
46 Here's a list of DBperl modules, their corresponding DBI counterparts
48 and support information. Please note, the author's listed here gener‐
49 ally do not maintain the DBI module for the same database. These email
50 addresses are unverified and should only be used for queries concerning
51 the perl4 modules listed below. DBI driver queries should be directed
52 to the dbi-users mailing list.
53 Module Name Database Required Author DBI
55 ----------- ----------------- ------ ---
56 Sybperl Sybase Michael Peppler DBD::Sybase
57 <mpeppler@itf.ch>
58 Oraperl Oracle 6 & 7 Kevin Stock DBD::Oracle
59 <dbi-users@perl.org>
60 Ingperl Ingres Tim Bunce & DBD::Ingres
61 Ted Lemon
62 <dbi-users@perl.org>
63 Interperl Interbase Buzz Moschetti DBD::Interbase
64 <buzz@bear.com>
65 Uniperl Unify 5.0 Rick Wargo None
66 <rickers@coe.drexel.edu>
67 Pgperl Postgres Igor Metz DBD::Pg
68 <metz@iam.unibe.ch>
69 Btreeperl NDBM John Conover SDBM?
70 <john@johncon.com>
71 Ctreeperl C-Tree John Conover None
72 <john@johncon.com>
73 Cisamperl Informix C-ISAM Mathias Koerber None
74 <mathias@unicorn.swi.com.sg>
75 Duaperl X.500 Directory Eric Douglas None
76 User Agent
77 However, some DBI modules have DBperl emulation layers, so, DBD::Oracle
79 comes with an Oraperl emulation layer, which allows you to run legacy
80 oraperl scripts without modification. The emulation layer translates
81 the oraperl API calls into DBI calls and executes them through the DBI
82 switch.
83 Here's a table of emulation layer information:
85 Module Emulation Layer Status
87 ------ --------------- ------
88 DBD::Oracle Oraperl Complete
89 DBD::Informix Isqlperl Under development
90 DBD::Ingres Ingperl Complete?
91 DBD::Sybase Sybperl Working? ( Needs verification )
92 DBD::mSQL Msqlperl Experimentally released with
93 DBD::mSQL-0.61
94 The Msqlperl emulation is a special case. Msqlperl is a perl5 driver
96 for mSQL databases, but does not conform to the DBI Specification. It's
97 use is being deprecated in favour of DBD::mSQL. Msqlperl may be down‐
98 loaded from CPAN via:
99 http://www.perl.com/cgi-bin/cpan_mod?module=Msqlperl
101 1.2. Where can I get it from?
103 The Comprehensive Perl Archive Network resources should be used for
105 retrieving up-to-date versions of the DBI and drivers. CPAN may be
106 accessed via Tom Christiansen's splendid CPAN multiplexer program
107 located at:
108 http://www.perl.com/CPAN/
110 For more specific version information and exact URLs of drivers, please
112 see the DBI drivers list and the DBI module pages which can be found
113 on:
114 http://dbi.perl.org/
116 http://www.symbolstone.org/technology/perl/DBI
117 This list is automatically generated on a nightly basis from CPAN and
119 should be up-to-date.
120 1.3. Where can I get more information?
122 There are a few information sources on DBI.
124 "Programming the Perl DBI"
126 "Programming the Perl DBI" is the official book on the DBI written
127 by Alligator Descartes and Tim Bunce and published by O'Reilly &
128 Associates. The book was released on February 9th, 2000.
129 The table of contents is:
131 Preface
133 1. Introduction
134 From Mainframes to Workstations
135 Perl
136 DBI in the Real World
137 A Historical Interlude and Standing Stones
138 2. Basic Non-DBI Databases
139 Storage Managers and Layers
140 Query Languages and Data Functions
141 Standing Stones and the Sample Database
142 Flat-File Databases
143 Putting Complex Data into Flat Files
144 Concurrent Database Access and Locking
145 DBM Files and the Berkeley Database Manager
146 The MLDBM Module
147 Summary
148 3. SQL and Relational Databases
149 The Relational Database Methodology
150 Datatypes and NULL Values
151 Querying Data
152 Modifying Data Within Tables
153 Creating and Destroying Tables
154 4. Programming with the DBI
155 DBI Architecture
156 Handles
157 Data Source Names
158 Connection and Disconnection
159 Error Handling
160 Utility Methods and Functions
161 5. Interacting with the Database
162 Issuing Simple Queries
163 Executing Non-SELECT Statements
164 Binding Parameters to Statements
165 Binding Output Columns
166 do() Versus prepare()
167 Atomic and Batch Fetching
168 6. Advanced DBI
169 Handle Attributes and Metadata
170 Handling LONG/LOB Data
171 Transactions, Locking, and Isolation
172 7. ODBC and the DBI
173 ODBC -- Embraced and Extended
174 DBI -- Thrashed and Mutated
175 The Nuts and Bolts of ODBC
176 ODBC from Perl
177 The Marriage of DBI and ODBC
178 Questions and Choices
179 Moving Between Win32::ODBC and the DBI
180 And What About ADO?
181 8. DBI Shell and Database Proxying
182 dbish -- The DBI Shell
183 Database Proxying
184 A. DBI Specification
185 B. Driver and Database Characteristics
186 C. ASLaN Sacred Site Charter
187 Index
188 The book should be available from all good bookshops and can be
190 ordered online either <I>via</I> O'Reilly & Associates
191 http://www.oreilly.com/catalog/perldbi
193 or Amazon
195 http://www.amazon.com/exec/obidos/ASIN/1565926994/dbi
197 POD documentation
199 PODs are chunks of documentation usually embedded within perl pro‐
200 grams that document the code ``in place'', providing a useful
201 resource for programmers and users of modules. POD for DBI and
202 drivers is beginning to become more commonplace, and documentation
203 for these modules can be read with the "perldoc" program included
204 with Perl.
205 The DBI Specification
207 The POD for the DBI Specification can be read with the:
208 perldoc DBI
210 command. The Specification also forms Appendix A of "Program‐
212 ming the Perl DBI".
213 Oraperl
215 Users of the Oraperl emulation layer bundled with DBD::Oracle,
216 may read up on how to program with the Oraperl interface by
217 typing:
218 perldoc Oraperl
220 This will produce an updated copy of the original oraperl man
222 page written by Kevin Stock for perl4. The oraperl API is fully
223 listed and described there.
224 Drivers
226 Users of the DBD modules may read about some of the private
227 functions and quirks of that driver by typing:
228 perldoc <driver>
230 For example, the DBD::mSQL driver is bundled with driver-spe‐
232 cific documentation that can be accessed by typing
233 perldoc DBD::mSQL
235 Frequently Asked Questions
237 This document, the Frequently Asked Questions is also available
238 as POD documentation! You can read this on your own system by
239 typing:
240 perldoc DBI::FAQ
242 This may be more convenient to persons not permanently, or con‐
244 veniently, connected to the Internet. The DBI::FAQ module
245 should be downloaded and installed for the more up-to-date ver‐
246 sion.
247 The version of DBI::FAQ shipped with the "DBI" module may be
249 slightly out of date.
250 POD in general
252 Information on writing POD, and on the philosophy of POD in
253 general, can be read by typing:
254 perldoc perlpod
256 Users with the Tk module installed may be interested to learn
258 there is a Tk-based POD reader available called "tkpod", which
259 formats POD in a convenient and readable way. This is available
260 via CPAN as the module called Tk::POD and is highly recom‐
261 mended.
262 Driver and Database Characteristics
264 The driver summaries that were produced for Appendix B of "Program‐
265 ming the Perl DBI" are available online at:
266 http://dbi.perl.org/
268 http://www.symbolstone.org/technology/perl/DBI
269 in the driver information table. These summaries contain standard‐
271 ised information on each driver and database which should aid you
272 in selecting a database to use. It will also inform you quickly of
273 any issues within drivers or whether a driver is not fully compli‐
274 ant with the DBI Specification.
275 Rambles, Tidbits and Observations
277 http://dbi.perl.org/tidbits
278 http://www.symbolstone.org/technology/perl/DBI/tidbits
279 There are a series of occasional rambles from various people on the
281 DBI mailing lists who, in an attempt to clear up a simple point,
282 end up drafting fairly comprehensive documents. These are quite
283 often varying in quality, but do provide some insights into the
284 workings of the interfaces.
285 Articles
287 A list of articles discussing the DBI can be found on the DBI WWW
288 page at:
289 http://dbi.perl.org/
291 http://www.symbolstone.org/technology/perl/DBI
292 These articles are of varying quality and age, from the original
294 Perl Journal article written by Alligator and Tim, to more recent
295 debacles published online from about.com.
296 README files
298 The README files included with each driver occasionally contains
299 some useful information ( no, really! ) that may be pertinent to
300 the user. Please read them. It makes our worthless existences more
301 bearable. These can all be read from the main DBI WWW page at:
302 http://dbi.perl.org/
304 http://www.symbolstone.org/technology/perl/DBI
305 Mailing Lists
307 There are three mailing lists for DBI:
308 dbi-announce@perl.org -- for announcements, very low traffic
310 dbi-users@perl.org -- general user support
311 dbi-dev@perl.org -- for driver developers (no user support)
312 For information on how to subscribe, set digest mode etc, and
314 unsubscribe, send an email message (the content will be ignored)
315 to:
316 dbi-announce-help@perl.org
318 dbi-users-help@perl.org
319 dbi-dev-help@perl.org
320 Mailing List Archives
322 US Mailing List Archives
323 http://outside.organic.com/mail-archives/dbi-users/
324 Searchable hypermail archives of the three mailing lists, and
326 some of the much older traffic have been set up for users to
327 browse.
328 European Mailing List Archives
330 http://www.rosat.mpe-garching.mpg.de/mailing-lists/PerlDB-Interest
331 As per the US archive above.
333
335 2.1. Compilation problems or "It fails the test!"
336 First off, consult the README for that driver in case there is useful
338 information about the problem. It may be a known problem for your given
339 architecture and operating system or database. You can check the README
340 files for each driver in advance online at:
341 http://dbi.perl.org/
343 http://www.symbolstone.org/technology/perl/DBI
344 If it's a known problem, you'll probably have to wait till it gets
346 fixed. If you're really needing it fixed, try the following:
347 Attempt to fix it yourself
349 This technique is generally not recommended to the faint-hearted.
350 If you do think you have managed to fix it, then, send a patch file
351 ( context diff ) to the author with an explanation of:
352 * What the problem was, and test cases, if possible.
354 * What you needed to do to fix it. Please make sure you mention
356 everything.
357 * Platform information, database version, perl version, module
359 version and DBI version.
360 Email the author Do NOT whinge!
362 Please email the address listed in the WWW pages for whichever
363 driver you are having problems with. Do not directly email the
364 author at a known address unless it corresponds with the one
365 listed.
366 We tend to have real jobs to do, and we do read the mailing lists
368 for problems. Besides, we may not have access to <insert your
369 favourite brain-damaged platform here> and couldn't be of any
370 assistance anyway! Apologies for sounding harsh, but that's the way
371 of it!
372 However, you might catch one of these creative genii at 3am when
374 we're doing this sort of stuff anyway, and get a patch within 5
375 minutes. The atmosphere in the DBI circle is that we do appreciate
376 the users' problems, since we work in similar environments.
377 If you are planning to email the author, please furnish as much
379 information as possible, ie:
380 * ALL the information asked for in the README file in the prob‐
382 lematic module. And we mean ALL of it. We don't put lines like
383 that in documentation for the good of our health, or to meet
384 obscure README file standards of length.
385 * If you have a core dump, try the Devel::CoreStack module for
387 generating a stack trace from the core dump. Send us that too.
388 Devel::CoreStack can be found on CPAN at:
389 http://www.perl.com/cgi-bin/cpan_mod?module=Devel::CoreStack
391 * Module versions, perl version, test cases, operating system
393 versions and any other pertinent information.
394 Remember, the more information you send us, the quicker we can
396 track problems down. If you send us no useful information, expect
397 nothing back.
398 Finally, please be aware that some authors, including Tim Bunce,
400 specifically request that you do not mail them directly. Please
401 respect their wishes and use the email addresses listed in the
402 appropriate module "README" file.
403 Email the dbi-users Mailing List
405 It's usually a fairly intelligent idea to cc the mailing list any‐
406 way with problems. The authors all read the lists, so you lose
407 nothing by mailing there.
408
410 3.1 What's the difference between ODBC and DBI?
411 In terms of architecture - not much: Both define programming inter‐
413 faces. Both allow multiple drivers to be loaded to do the actual work.
414 In terms of ease of use - much: The DBI is a 'high level' interface
416 that, like Perl itself, strives to make the simple things easy while
417 still making the hard things possible. The ODBC is a 'low level' inter‐
418 face. All nuts-bolts-knobs-and-dials.
419 Now there's an ODBC driver for the DBI (DBD::ODBC) the "What's the dif‐
421 ference" question is more usefully rephrased as:
422 Chapter 7 of "Programming the Perl DBI" covers this topic in far more
424 detail and should be consulted.
425 3.2 What's the difference between Win32::ODBC and DBD::ODBC?
427 The DBI, and thus DBD::ODBC, has a different philosophy from the
429 Win32::ODBC module:
430 The Win32::ODBC module is a 'thin' layer over the low-level ODBC API.
432 The DBI defines a simpler 'higher level' interface.
433 The Win32::ODBC module gives you access to more of the ODBC API. The
435 DBI and DBD::ODBC give you access to only the essentials. (But, unlike
436 Win32::ODBC, the DBI and DBD::ODBC do support parameter binding and
437 multiple prepared statements which reduces the load on the database
438 server and can dramatically increase performance.)
439 The Win32::ODBC module only works on Win32 systems. The DBI and
441 DBD::ODBC are very portable and work on Win32 and Unix.
442 The DBI and DBD::ODBC modules are supplied as a standard part of the
444 Perl 5.004 binary distribution for Win32 (they don't work with the
445 older, non-standard, ActiveState port).
446 Scripts written with the DBI and DBD::ODBC are faster than Win32::ODBC
448 on Win32 and are trivially portable to other supported database types.
449 The DBI offers optional automatic printing or die()ing on errors which
451 makes applications simpler and more robust.
452 The current DBD::ODBC driver version 0.16 is new and not yet fully sta‐
454 ble. A new release is due soon [relative to the date of the next TPJ
455 issue :-] and will be much improved and offer more ODBC functionality.
456 To summarise: The Win32::ODBC module is your best choice if you need
458 access to more of the ODBC API than the DBI gives you. Otherwise, the
459 DBI and DBD::ODBC combination may be your best bet.
460 Chapter 7 of "Programming the Perl DBI" covers this topic in far more
462 detail and should be consulted.
463 3.3 Is DBI supported under Windows 95 / NT platforms?
465 Finally, yes! Jeff Urlwin has been working diligently on building DBI
467 and DBD::ODBC under these platforms, and, with the advent of a stabler
468 perl and a port of MakeMaker, the project has come on by great leaps
469 and bounds.
470 The DBI and DBD::Oracle Win32 ports are now a standard part of DBI, so,
472 downloading DBI of version higher than 0.81 should work fine as should
473 using the most recent DBD::Oracle version.
474 3.4 Can I access Microsoft Access or SQL-Server databases with DBI?
476 Yes, use the DBD::ODBC driver.
478 3.5 Is the a DBD for <insert favourite database here>?
480 Is is listed on the DBI drivers page?
482 http://dbi.perl.org/
484 http://www.symbolstone.org/technology/perl/DBI
485 If not, no. A complete absence of a given database driver from that
487 page means that no-one has announced any intention to work on it, not
488 that such a driver is impossible to write.
489 A corollary of the above statement implies that if you see an announce‐
491 ment for a driver not on the above page, there's a good chance it's not
492 actually a DBI driver, and may not conform to the specifications.
493 Therefore, questions concerning problems with that code should not
494 really be addressed to the DBI Mailing Lists.
495 3.6 What's DBM? And why should I use DBI instead?
497 Extracted from ``DBI - The Database Interface for Perl 5'':
499 ``UNIX was originally blessed with simple file-based ``databases'', namely
501 the dbm system. dbm lets you store data in files, and retrieve
502 that data quickly. However, it also has serious drawbacks.
503 File Locking
505 The dbm systems did not allow particularly robust file locking
507 capabilities, nor any capability for correcting problems arising through
508 simultaneous writes [ to the database ].
509 Arbitrary Data Structures
511 The dbm systems only allows a single fixed data structure:
513 key-value pairs. That value could be a complex object, such as a
514 [ C ] struct, but the key had to be unique. This was a large
515 limitation on the usefulness of dbm systems.
516 However, dbm systems still provide a useful function for users with
518 simple datasets and limited resources, since they are fast, robust and
519 extremely well-tested. Perl modules to access dbm systems have now
520 been integrated into the core Perl distribution via the
521 AnyDBM_File module.''
522 To sum up, DBM is a perfectly satisfactory solution for essentially
524 read-only databases, or small and simple datasets. However, for more
525 scaleable dataset handling, not to mention robust transactional lock‐
526 ing, users are recommended to use a more powerful database engine via
527 DBI.
528 Chapter 2 of "Programming the Perl DBI" discusses DBM files in detail.
530 3.7 What database do you recommend me using?
532 This is a particularly thorny area in which an objective answer is dif‐
534 ficult to come by, since each dataset, proposed usage and system con‐
535 figuration differs from person to person.
536 From the current author's point of view, if the dataset is relatively
538 small, being tables of less than 1 million rows, and less than 1000
539 tables in a given database, then mSQL is a perfectly acceptable solu‐
540 tion to your problem. This database is extremely cheap, is wonderfully
541 robust and has excellent support. More information is available on the
542 Hughes Technology WWW site at:
543 http://www.hughes.com.au
545 You may also wish to look at MySQL which is a more powerful database
547 engine that has a similar feel to mSQL.
548 http://www.tcx.se
550 If the dataset is larger than 1 million row tables or 1000 tables, or
552 if you have either more money, or larger machines, I would recommend
553 Oracle RDBMS. Oracle's WWW site is an excellent source of more infor‐
554 mation.
555 http://www.oracle.com
557 Informix is another high-end RDBMS that is worth considering. There are
559 several differences between Oracle and Informix which are too complex
560 for this document to detail. Information on Informix can be found on
561 their WWW site at:
562 http://www.informix.com
564 In the case of WWW fronted applications, mSQL may be a better option
566 due to slow connection times between a CGI script and the Oracle RDBMS
567 and also the amount of resource each Oracle connection will consume.
568 mSQL is lighter resource-wise and faster.
569 These views are not necessarily representative of anyone else's opin‐
571 ions, and do not reflect any corporate sponsorship or views. They are
572 provided as-is.
573 3.8 Is <insert feature here> supported in DBI?
575 Given that we're making the assumption that the feature you have
577 requested is a non-standard database-specific feature, then the answer
578 will be no.
579 DBI reflects a generic API that will work for most databases, and has
581 no database-specific functionality.
582 However, driver authors may, if they so desire, include hooks to data‐
584 base-specific functionality through the "func()" method defined in the
585 DBI API. Script developers should note that use of functionality pro‐
586 vided via the "func()" methods is very unlikely to be portable across
587 databases.
588
590 4.1 Is DBI any use for CGI programming?
591 In a word, yes! DBI is hugely useful for CGI programming! In fact, I
593 would tentatively say that CGI programming is one of two top uses for
594 DBI.
595 DBI confers the ability to CGI programmers to power WWW-fronted data‐
597 bases to their users, which provides users with vast quantities of
598 ordered data to play with. DBI also provides the possibility that, if a
599 site is receiving far too much traffic than their database server can
600 cope with, they can upgrade the database server behind the scenes with
601 no alterations to the CGI scripts.
602 4.2 How do I get faster connection times with DBD::Oracle and CGI?
604 Contributed by John D. Groenveld
606 The Apache "httpd" maintains a pool of "httpd" children to service
608 client requests.
609 Using the Apache mod_perl module by Doug MacEachern, the perl inter‐
611 preter is embedded with the "httpd" children. The CGI, DBI, and your
612 other favorite modules can be loaded at the startup of each child.
613 These modules will not be reloaded unless changed on disk.
614 For more information on Apache, see the Apache Project's WWW site:
616 http://www.apache.org
618 The mod_perl module can be downloaded from CPAN via:
620 http://www.perl.com/cgi-bin/cpan_mod?module=Apache
622 4.3 How do I get persistent connections with DBI and CGI?
624 Contributed by John D. Groenveld
626 Using Edmund Mergl's Apache::DBI module, database logins are stored in
628 a hash with each of these "httpd" child. If your application is based
629 on a single database user, this connection can be started with each
630 child. Currently, database connections cannot be shared between
631 "httpd" children.
632 Apache::DBI can be downloaded from CPAN via:
634 http://www.perl.com/cgi-bin/cpan_mod?module=Apache::DBI
636 4.4 ``When I run a perl script from the command line, it works, but,
638 when I run it under the "httpd", it fails!'' Why?
639 Basically, a good chance this is occurring is due to the fact that the
641 user that you ran it from the command line as has a correctly config‐
642 ured set of environment variables, in the case of DBD::Oracle, vari‐
643 ables like "ORACLE_HOME", "ORACLE_SID" or "TWO_TASK".
644 The "httpd" process usually runs under the user id of "nobody", which
646 implies there is no configured environment. Any scripts attempting to
647 execute in this situation will correctly fail.
648 One way to solve this problem is to set the environment for your data‐
650 base in a "BEGIN { }" block at the top of your script. Another tech‐
651 nique is to configure your WWW server to pass-through certain environ‐
652 ment variables to your CGI scripts.
653 Similarly, you should check your "httpd" error logfile for any clues,
655 as well as the ``Idiot's Guide To Solving Perl / CGI Problems'' and
656 ``Perl CGI Programming FAQ'' for further information. It is unlikely
657 the problem is DBI-related.
658 The ``Idiot's Guide To Solving Perl / CGI Problems'' can be located at:
660 http://www.perl.com/perl/faq/index.html
662 as can the ``Perl CGI Programming FAQ''. Read BOTH these documents
664 carefully!
665 4.5 How do I get the number of rows returned from a "SELECT" statement?
667 Count them. Read the DBI docs for the "rows()" method.
669
671 5.1 Can I do multi-threading with DBI?
672 Perl version 5.005 and later can be built to support multi-threading.
674 The DBI, as of version 1.02, does not yet support multi-threading so it
675 would be unsafe to let more than one thread enter the DBI at the same
676 time.
677 It is expected that some future version of the DBI will at least be
679 thread-safe (but not thread-hot) by automatically blocking threads
680 intering the DBI while it's already in use.
681 For some OCI example code for Oracle that has multi-threaded "SELECT"
683 statements, see:
684 http://www.symbolstone.org/technology/oracle/oci/orathreads.tar.gz
686 5.2 How do I handle BLOB data with DBI?
688 Handling BLOB data with the DBI is very straight-forward. BLOB columns
690 are specified in a SELECT statement as per normal columns. However, you
691 also need to specify a maximum BLOB size that the <I>database han‐
692 dle</I> can fetch using the "LongReadLen" attribute.
693 For example:
695 ### $dbh is a connected database handle
697 $sth = $dbh->prepare( "SELECT blob_column FROM blobby_table" );
698 $sth->execute;
699 would fail.
701 ### $dbh is a connected database handle
703 ### Set the maximum BLOB size...
704 $dbh->{LongReadLen} = 16384; ### 16Kb...Not much of a BLOB!
705 $sth = $dbh->prepare( "..." );
707 would succeed <I>provided no column values were larger than the speci‐
709 fied value</I>.
710 If the BLOB data is longer than the value of "LongReadLen", then an
712 error will occur. However, the DBI provides an additional piece of
713 functionality that will automatically truncate the fetched BLOB to the
714 size of "LongReadLen" if it is longer. This does not cause an error to
715 occur, but may make your fetched BLOB data useless.
716 This behaviour is regulated by the "LongTruncOk" attribute which is
718 defaultly set to a false value ( thus making overlong BLOB fetches fail
719 ).
720 ### Set BLOB handling such that it's 16Kb and can be truncated
722 $dbh->{LongReadLen} = 16384;
723 $dbh->{LongTruncOk} = 1;
724 Truncation of BLOB data may not be a big deal in cases where the BLOB
726 contains run-length encoded data, but data containing checksums at the
727 end, for example, a ZIP file, would be rendered useless.
728 5.3 How can I invoke stored procedures with DBI?
730 The DBI does not define a database-independent way of calling stored
732 procedures.
733 However, most database that support them also provide a way to call
735 them from SQL statements - and the DBI certainly supports that.
736 So, assuming that you have created a stored procedure within the target
738 database, eg, an Oracle database, you can use $dbh->"do()" to immedi‐
739 ately execute the procedure. For example,
740 $dbh->do( "BEGIN someProcedure; END;" ); # Oracle-specific
742 You should also be able to "prepare" and "execute", which is the recom‐
744 mended way if you'll be calling the procedure often.
745 5.4 How can I get return values from stored procedures with DBI?
747 Contributed by Jeff Urlwin
749 $sth = $dbh->prepare( "BEGIN foo(:1, :2, :3); END;" );
751 $sth->bind_param(1, $a);
752 $sth->bind_param_inout(2, \$path, 2000);
753 $sth->bind_param_inout(3, \$success, 2000);
754 $sth->execute;
755 Remember to perform error checking, though! ( Or use the "RaiseError"
757 attribute ).
758 5.5 How can I create or drop a database with DBI?
760 Database creation and deletion are concepts that are entirely too
762 abstract to be adequately supported by DBI. For example, Oracle does
763 not support the concept of dropping a database at all! Also, in Oracle,
764 the database server essentially is the database, whereas in mSQL, the
765 server process runs happily without any databases created in it. The
766 problem is too disparate to attack in a worthwhile way.
767 Some drivers, therefore, support database creation and deletion through
769 the private "func()" methods. You should check the documentation for
770 the drivers you are using to see if they support this mechanism.
771 5.6 How can I "commit" or "rollback" a statement with DBI?
773 See the "commit()" and "rollback()" methods in the DBI Specification.
775 Chapter 6 of "Programming the Perl DBI" discusses transaction handling
777 within the context of DBI in more detail.
778 5.7 How are "NULL" values handled by DBI?
780 "NULL" values in DBI are specified to be treated as the value "undef".
782 "NULL"s can be inserted into databases as "NULL", for example:
783 $rv = $dbh->do( "INSERT INTO table VALUES( NULL )" );
785 but when queried back, the "NULL"s should be tested against "undef".
787 This is standard across all drivers.
788 5.8 What are these "func()" methods all about?
790 The "func()" method is defined within DBI as being an entry point for
792 database-specific functionality, eg, the ability to create or drop
793 databases. Invoking these driver-specific methods is simple, for exam‐
794 ple, to invoke a "createDatabase" method that has one argument, we
795 would write:
796 $rv =$dbh->func( 'argument', 'createDatabase' );
798 Software developers should note that the "func()" methods are non-por‐
800 table between databases.
801 5.9 Is DBI Year 2000 Compliant?
803 DBI has no knowledge of understanding of what dates are. Therefore, DBI
805 itself does not have a Year 2000 problem. Individual drivers may use
806 date handling code internally and therefore be potentially susceptible
807 to the Year 2000 problem, but this is unlikely.
808 You may also wish to read the ``Does Perl have a Year 2000 problem?''
810 section of the Perl FAQ at:
811 http://www.perl.com/CPAN/doc/FAQs/FAQ/PerlFAQ.html
813
815 The Perl5 Database Interface is FREE software. IT COMES WITHOUT WAR‐
816 RANTY OF ANY KIND. See the DBI README for more details.
817 However, some organizations are providing either technical support or
819 training programs on DBI. The present author has no knowledge as to the
820 quality of these services. The links are included for reference pur‐
821 poses only and should not be regarded as recommendations in any way.
822 Caveat emptor.
823 Commercial Support
825 The Perl Clinic
827 The Perl Clinic provides commercial support for Perl and Perl
828 related problems, including the DBI and its drivers. Support is
829 provided by the company with whom Tim Bunce, author of DBI and
830 DBD::Oracle, works and ActiveState. For more information on their
831 services, please see:
832 http://www.perlclinic.com
834 Training
836 Westlake Solutions
838 A hands-on class for experienced Perl CGI developers that teaches
839 how to write database-connected CGI scripts using Perl and DBI.pm.
840 This course, along with four other courses on CGI scripting with
841 Perl, is taught in Washington, DC; Arlington, Virginia; and on-site
842 worldwide upon request.
843 See:
845 http://www.westlake.com/training
847 for more details.
849
851 In this section, we present some miscellaneous WWW links that may be of
852 some interest to DBI users. These are not verified and may result in
853 unknown sites or missing documents.
854 http://www-ccs.cs.umass.edu/db.html
856 http://www.odmg.org/odmg93/updates_dbarry.html
857 http://www.jcc.com/sql_stnd.html
858
860 Alligator Descartes <http://www.symbolstone.org/descarte/contact.html>.
861 Portions are Copyright their original stated authors.
862
864 This document is Copyright (c)1994-2000 Alligator Descartes, with por‐
865 tions Copyright (c)1994-2000 their original authors. This module is
866 released under the 'Artistic' license which you can find in the perl
867 distribution.
868 This document is Copyright (c)1997-2000 Alligator Descartes. All rights
870 reserved. Permission to distribute this document, in full or in part,
871 via email, Usenet, ftp archives or http is granted providing that no
872 charges are involved, reasonable attempt is made to use the most cur‐
873 rent version and all credits and copyright notices are retained ( the
874 AUTHOR and COPYRIGHT sections ). Requests for other distribution
875 rights, including incorporation into commercial products, such as
876 books, magazine articles or CD-ROMs should be made to Alligator
877 Descartes <http://www.symbolstone.org/descarte/contact.html>.
878perl v5.8.8 2006-02-07 DBI::FAQ(3)