Re: ODBC C API question(s)

Venu Anuganti wrote:
> 1. There is no direct equivalent for the mysql_fetch_lengths; But you 
> could use SQLDescribeCol or SQLColAttribute(s) with SQL_ATTR_DESC_LENGTH 
> to get column length. But you need to iterate through all fields like..
> 
> SQLExecDirect("select..");
> SQLNumResultCols(&colCount);
> 
> for (i=1; i <= colCount; i++) {
>    SQLColAttribute(colCount, SQL_ATTR_DESC_LENGTH)..
> }
> 
> PS: its not a real example; but it is just a psedo code.
> 
> 2. SQLDriverConnect has wrong usage; either you need to have valid 
> window handle; else pass NULL for parameter 2.
> 
> 3. UIDs/PWDs are optional. If you provide it overwrites the DSN UID/PWD. 
> MySQL allows to store UID/PWD in DSN but few databases will not allow to 
> store PWD due to security; For example; Oracle will not let you specify 
> PWD in DSN.

Oracle might not let you store the PWD but quite a few drivers do (most 
encrypt the PWD some way). Just to add to what Venu said; the basic 
mechanics an ODBC driver should go through are:

1. pull all attributes out of the connection string
2. if there are not enough attributes defined to allow the driver to 
connect look for a DSN attribute (or FileDSN).
3. if the DSN attribute is found, pull any missing attributes from the 
DSN (which effectively means calling SQLGetPrivateProfileString).
4. if there are now enough attributes defined to connect, connect, else 
if a window handle is supplied, pop up a dialogue to get the remaining 
attributes.

Of course the last argument to SQLDriverConnect affects all this as it 
may be SQL_DRIVER_COMPLETE (fill in the missing attributes from the 
DSN), SQL_DRIVER_PROMPT (always throw up the dialogue) and 
SQL_DRIVER_NOPROMPT (never throw up the dialogue, so if insufficient 
attributes to connect it would be an error). Its actually a bit more 
complex than this but it is all described under SQLDriverConnect in the 
ODBC API.

Regarding the window handle issue, Venu is right - it should be NULL or 
a valid Window handle but this sort of goes out the window (pardon the 
pun) on UNIX when there is not really any such thing as a valid window 
handle. If you are using the unixODBC driver manager (which uses QT for 
the GUI by default) you might indeed pass 1 for the window handle to 
indicate to the driver that if it does support a GUI interface, it can 
use it (but the driver does not actually use the 1 as a window handle, 
it uses something like QApp (can't remember off the top of my head)). 
The Easysoft ODBC-ODBC Bridge can do this on Linux where QT is installed 
and you have the right libstdc++ but it is pretty difficult to do on 
other platforms due to libstdc++ issues and the way unixODBC/QT are 
built and in any case the app using the driver has to be a QT app.


> 4. For complete ODBC; check:
> 
> http://msdn.microsoft.com/library/default.asp?url=/library/en-us/odbc/htm/dasdkodbcoverview.asp 
> 
> http://msdn.microsoft.com/library/default.asp?url=/library/en-us/odbc/htm/odbcabout_this_manual.asp 
> 
> 
> 
> Patrick Galbraith wrote:
>> Hi all,
>>
>> I have been using the MySQL client API for quite some time now and am 
>> in the middle of utilising ODBC C API for a current project. I'm 
>> wondering if there is an equivalent call in ODBC for 
>> 'mysql_fetch_lengths' ?
>>
>> Also, I am wondering what the difference is between
>>
>>
>> SQLDriverConnect(dbc, (void *)1, "DSN=myodbc3patg", SQL_NTS,
>>       outstr, sizeof(outstr), &outstrlen,
>>             SQL_DRIVER_COMPLETE);
>>
>> This fails, and I don't know why.
>>
>>
>> But this works:
>>
>> SQLConnect(V_OD_hdbc, (SQLCHAR*) "myodbc3patg", SQL_NTS,
>>  (SQLCHAR*) "", SQL_NTS,
>>  (SQLCHAR*) "", SQL_NTS);
>>
>> What is the difference between the two? Is there any benefit/detriment 
>> to either? The first snippet is from Easysoft's site, the second from 
>> the tutorial on UNIX odbc.

I have explained the "(void *)1" above but in the example on our site 
for the tutorial it is not required and has confused you (I have asked 
for it to be changed to NULL).

I'd always use SQLDriverConnect over SQLConnect because it is a lot more 
flexible and the driver manager will map it to SQLConnect for REALLY OLD 
drivers that don't support SQLDriverConnect anyway.

>> I often see that people use a UID/Password with  ODBC  DSN. Why use a 
>> password and also a DSN? I thought the DSN provides that? If one uses 
>> a DSN with UID/Pass, does the UID/Password for the DSN get over-written?
>>
>> Also, where are some good sites for the ODBC C API, with examples, 
>> descriptions of the API calls? I have found info at Easysoft's website 
>> as well as UnixODBC website. It seems that so much ODBC programming is 
>> targeted for GUIs. Mine is all system-level that I want to accomplish.
>>
>> Kind regards, thanks in advance!
>>
>> Patrick
>>
> 

If you find areas not covered on our site that you think would be useful 
let me know and I'll attempt to get any holes filled in.

The strange thing is that many of our customers use ODBC in GUI 
environments (mostly Windows) and yet much of my use of ODBC has been on 
UNIX and from system tools or web servers. Being a lover of Perl I'd 
much sooner write in Perl than in C with the ODBC API but for speed the 
latter wins hands down in /a few/ circumstances (e.g. you can set the 
SQL_ATTR_ARRAY_SIZE attribute and fetch 1000's of rows in one go, which 
you can't do with DBD::ODBC and you can use cursors to move around a 
result-set which DBD::ODBC does not do - I hope Tim does not read this 
or I'll be getting a why don't you add support email (and I've got my 
excuses ready in case he does).

Martin
-- 
Martin J. Evans
Easysoft Limited
http://www.easysoft.com

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