What built-in routine can I make use of to cast data of type LVARCHAR to data of type TEXT?
The larger context: I have a table with a column that has been defined as LVARCHAR(4096). Now a developer wishes to change the data type of this column to TEXT. Ideally this would be done with:
ALTER TABLE foo MODIFY bar TEXT;
...but in such a case the following error is puked to the screen:
ALTER TABLE can not modify column (bar) type. Need a cast from the current type to the new type.
I have read up on the CREATE CAST construction, but I cannot begin to think what on earth the proper conversion function would look like. Without a function, Informix will not allow the CREATE CAST to work. That is, if I do, simply:
CREATE CAST (LVARCHAR AS TEXT)
...Informix tells me that a cast function is required (which makes sense).
Beware, Informix developers: if you inadvertently run into a problem like this, there is no way to get out of it using SQL or DDL alone. Let me repeat that.
If you have a VARCHAR or an LVARCHAR column that you need to migrate to be a TEXT column, and if you cannot afford to lose data in that column, there is no way to do this in SQL or DDL.
Instead, you must write a program that does the conversion for you inside the database driver, in memory. In my case, I used JDBC mutable result sets and copied the column to a new column, letting the JDBC driver perform the conversion, then dropped the old column, and renamed the new column back to the old column. This general pattern is the only way to migrate existing character data into a TEXT column.
#Storm: Which version of IDS/ODBC are you using? AFAIK, IDS 9 or 10 can't do that without using specific embedded C in server (See boulder site), but in no way you can do that directly through SQL. Blob related functions or so.
Othe way is by using UNLOAD / LOAD.
In my scenario, we have lots of problems: no admin rights to enterprise server, as we are service providers, we only can use database, but cannot modify structure. We cannot modify TEXT fields only by launching queries.
Related
I am coding a cpp project with the database "postgreSQL".
I created a table in my database its type is character varying(40).
Now I need to SELECT these data FROM the table in my cpp project. I knew that I should use the library libpq, this is the interface of "postgreSQL" for c/cpp.
I have succeeded in selecting data from the table. Now I am considering if it's possible to get the data type of this table. For example, here I want to get character varying(40).
You need to use PQftype.
As described here: http://www.idiap.ch/~formaz/doc/postgreSQL/libpq-chapter17861.htm
And just take a look here about decoding return values: http://www.postgresql.org/message-id/da7021e0608040738l3b0880a1q5a76b838937f8c78#mail.gmail.com
You must also use PQfsize to get field size.
To protect against sql injection, I read in the introduction to ColdFusion that we are to use the cfqueryparam tag.
But when using stored procedures, I am passing my variables to corresponding variable declarations in SQL Server:
DROP PROC Usr.[Save]
GO
CREATE PROC Usr.[Save]
(#UsrID Int
,#UsrName varchar(max)
) AS
UPDATE Usr
SET UsrName = #UsrName
WHERE UsrID=#UsrID
exec Usr.[get] #UsrID
Q: Is there any value in including cfSqlType when I call a stored procedure?
Here's how I'm currently doing it in Lucee:
storedproc procedure='Usr.[Save]' {
procparam value=Val(form.UsrID);
procparam value=form.UsrName;
procresult name='Usr';
}
This question came up indirectly on another thread. That thread was about query parameters, but the same issues apply to procedures. To summarize, yes you should always type query and proc parameters. Paraphrasing the other answer:
Since cfsqltype is optional, its importance is often underestimated:
Validation:
ColdFusion uses the selected cfsqltype (date, number, etcetera) to validate the "value". This occurs before any sql is ever sent to
the database. So if the "value" is invalid, like "ABC" for type
cf_sql_integer, you do not waste a database call on sql that was never
going to work anyway. When you omit the cfsqltype, everything is
submitted as a string and you lose the extra validation.
Accuracy:
Using an incorrect type may cause CF to submit the wrong value to the database. Selecting the proper cfsqltype ensures you are
sending the correct value - and - sending it in a non-ambiguous format
the database will interpret the way you expect.
Again, technically you can omit the cfsqltype. However, that
means CF will send everything to the database as a string.
Consequently, the database will perform implicit conversion
(usually undesirable). With implicit conversion, the interpretation
of the strings is left entirely up to the database - and it might
not always come up with the answer you would expect.
Submitting dates as strings, rather than date objects, is a
prime example. How will your database interpret a date string like
"05/04/2014"? As April 5th or a May 4th? Well, it depends. Change the
database or the database settings and the result may be completely
different.
The only way to ensure consistent results is to specify the
appropriate cfsqltype. It should match the data type of the target
column/function (or at least an equivalent type).
I have my own data store mechanism for store data. but I want to implement standards data manipulation and query interface for end users,so I thought QT sql is suitable for my case.
but I still cannot understand how do I involved my indexes for sql query.
let say for example,
I have table with column A(int),B(int),C(int),D(int) and column A is indexed.assume I execute query like select * from Foo where A = 10;
How do I involved my index for search the results?.
You have written your own storage system and want to manipulate it using an SQL like syntax? I don't think Qt SQL is the right tool for that job. It offers connectivity to various SQL servers and is not meant for parsing SQL statements. Qt expects to "pass through" the queries and then somehow parse the result set and transform it into a Qt friendly representation.
So if you only want to have a Qt friendly representation, I wouldn't see a reason to go the indirection with SQL.
But regarding your problem:
In SQL, indexes are usually not stated in the queries, but during the creation of the table schema. But SQL server has a possibility to "hint" indexes, is that what you are looking for?
SELECT column_list FROM table_name WITH (INDEX (index_name) [, ...]);
I have a Borland builder c++ 6 application calling Oracle 10g database. Operating over a LAN. When the application in question makes a simple db select e.g.
select table_name from element_tablenames where element_id = 10023842
the following is recorded as happening in Oracle (from the performance logs)
select table_name
from element_tablenames
where element_id = 10023842
then immediately (and not from C++ source code but perhaps deeper)
select table_name, element_tablenames.ROWID
from element_tablenames
where element_id = 10023842
The select statement is only called once in the TADODbQuery object, yet two queries are being performed - one to parse and the other adds the ROWID for executon.
Over a WAN and many, many queries this is obviously a problem to the user.
Does anyone know why this might be happening, can someone suggest a solution?
Agree with Robert.
The ROWID uniquely identifies a row in a table so that the returned record can be applied back to the database with any changes (or as a DELETE).
Is there a way to identify a particular column (or set of columns) as a primary key so that it can be used to identify a row without using a ROWID.
I don't know exactly where the RowID is coming from, it could be either the TAdoQuery implementation or the Oracle Driver. But I am sure I found the reason.
From the Oracle docs:
If the database table does not contain a primary key, the ROWID must be selected explicitly when populating DataTable.
So I suspect your Table does not have a primary key, either add one or add the rowid.
Either way this will solve the duplicate query problem.
Since you are concerned about performance. In general
Using TAdoQuery you can set the CursorType to optimize different behaviors for performance. This article covers this from a TAdoQuery perspective. MSDN also has an article that covers it from from a general ADO Perspective. Finally the specifications from the Oracle Driver can be useful.
I would recommend setting the Cursor to either as they are the only supported by Oracle
ctStatic - Bi-directional query produced.
ctOpenForwardOnly - Unidirectional query produced, fastest but can't call Prior
You can also play with CursorLocation to see how it effects your speed.
I need to make a query like this:
SELECT PNPDeviceID FROM Win32_NetworkAdapter WHERE AdapterTypeId = 0
Trouble is, the AdapterTypeId column isn't always present. In this case, I just want everything, like so:
SELECT PNPDeviceID FROM Win32_NetworkAdapter
My WQL/SQL knowledge is extremely limited. Can anybody tell me how to do this in a single query?
EDIT:
A bit more background seems to be required: I am querying Windows for device information using WMI, which uses an SQL-like syntax. So, in my example, I am querying for network adapters that have an AdapterTypeId of 0.
That column is not always present however, meaning that if I enumerate through the returned values then "AdapterTypeId" is not listed.
EDIT 2:
Changed SQL to WQL; apparantly this is more correct.
I am assuming you mean the underlying schema is unreliable.
This is a highly unconventional situation. I suggest that you resolve the issue that is causing the column to not always be present, because to have the schema changing dynamically underneath your application is potentially (almost certainly) disastrous.
Update:
OK, so WQL lets you query objects with a SQL-like syntax but, unlike SQL, the schema can change underneath your feet. This is a classic example of a leaky abstraction, and I now hate WQL without ever having used it :).
Since the available properties are in flux, I am guessing that WQL provides a way to enumerate the properties for a given adapter. Do this, and choose which query to run depending upon the results.
After some Googling, there is an example here, which shows how to enumerate through the available properties. You can use this to determine if AdapterTypeId exists or not.
SELECT PNPDeviceID FROM Win32_NetworkAdapter WHERE AdapterTypeId = {yourDesire} OR AdapterTypeId IS NULL
I assume that you mean that this field is missing from the table.
Do you know before submitting the query if this field exists?
If yes then just create SQL dynamically, otherwise It think you will get syntax error in case of missing field
This is not an SQL question. SQL does not contemplate records with varying schemas in a single table source. Instead (as you mention) this is a different system using an "SQL-like" syntax. You'll have better luck if you recast the question using the actual product that you're trying to query, and information how that product deals with variable record structures is probably discussed in the documentation.