Question

I have three tables objects, (primary key object_ID) flags (primary key flag_ID) and object_flags (cross-tabel between objects and flags with some extra info).

I have a query returning all flags, and a one or zero if a given object has a certain flag:

SELECT
  f.*,
  of.*,
  of.objectID IS NOT NULL AS object_has_flag,
FROM
  flags f
  LEFT JOIN object_flags of
    ON (f.flag_ID = of.flag_ID) AND (of.object_ID = :objectID);

In the application (which is written in Delphi), all rows are loaded in a component. The user can assign flags by clicking check boxes in a table, modifying the data.

Suppose one line is edited. Depending on the value of object_has_flag, the following things have to be done:

• If object_has_flag was true and still is true, an UPDATE should be done on the relevant row in objects_flags.
• If object_has_flag was false but is now true, and INSERT should be done
• If object_has_flag was true, but is now false, the row should be deleted

It seems that this cannot be done in one query https://stackoverflow.com/questions/7927114/conditional-replace-or-delete-in-one-query.

I’m using MyDAC’s TMyQuery as a dataset. I have written separate code that executes the necessary queries to save changes to a row, but how do I couple this to the dataset? What event handler should I use, and how do I tell the TMyQuery that it should refresh instead of post?

EDIT: apparently, it is not completely clear what the problem is. The standard UpdateSQL, DeleteSQL and InsertSQL cannot be used because sometimes after editing a line (not deleting it or inserting a line), an INSERT or DELETE has to be done.

Answer 1

It seems that the easiest way is to use the BeforePost event, and determine what has to be done using the OldValue and NewValue properties of several fields.