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TDualQuery

History

TDualQuery was derived from David Berneda's 16 bit QBE component. Unfortunately, the BDE handles query execution somewhat differently in Windows 95 so the source will not compile with 32 bit Delphi. Grant Carpenter made an attempt to fix the problem, and, indeed, produced a component which would execute a QBE query for 32 bit Delphi. However, some of the other functionality of the component was sacrificed for the substantial gain.

After a considerable amount of digging around in the BDE, I was able to modify the original component that it would execute in a 32 bit environment and still allow answer table specification and params. I added some additional properties and methods to make it a little easier to use. Finally, since the BDE will happily execute either a QBE or SQL query if you tell it which one it is looking at, I set up the component to deal with either type of Query.

Properties and Methods

Functionality

Query

This control will execute any query which you can design in the BDE which will run with local SQL. You must set the QueryLanguage property appropriately. You can also use the setQueryLanguage(qtQueryLanguage) procedure. Query Languages allowed are QBE (qtQBE) and SQL (qtSQL) The Query is entered with a memobox or you can use SetQuery(tStrings).

Parameters

Since params are dealt with via a simple string search and replace, virtually any text can be a parameter. You are limited to 10 parameters of 255 characters each. Parameters are entered in a memobox, one parameter per line and with the param and its replacement separated by a comma. This is CASE and CHARACTER sensitive. Do not put spaces around the commas. You can change QParams directly or use setParams(TStrings).

AnswerTable

This much needed feature is not available in regular queries. You can specify an answer table name as well as its type. If the table exists, it will be deleted, if possible. In order to delete a table, however, it must be opened exclusively. If something else has the table open, this is impossible and you will encounter an error.

RequestLive

Request that the rows and columns returned by the query be updatable.

BlankAsZero

Return a blank for any field which is zero.

AuxTables

Choose whether to use auxiliary tables or not. If you set this to false, the query can run faster but you will not be able to undo an insert, delete, or update query.

Known Problems

Failure to Set Cursor on Non-Result Set Queries

You will get an EDatabaseError on Insert, Update, and Delete Queries because there is no result set and the database engine will try to set a cursor anyway. This has been a problem with the QBE queries and I have been unable to find a fix for it. TDataset will always try to set a cursor. The TQuery equivalent command is that of an 'open' and 'execSQL' is not available.

Unused Parameters

The component embodies all of the properties and function of TDataset but some might not be relevant here. Unfortunately, you cannot hide properties in descendants.

Get the Component

TDualQuery.zip contains the pascal file, a dcr for the component and this HTML page. The file is simple enough that the details of properties and methods can be easily gleaned from the source file. Feel free to email me with problems you might find with this component. I will try to fix them if I can do so. Suggestions and fixes are also welcome.

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