Executing queries using DAO - delphi

Question

I want to execute a list of queries against an Access database using DAO. The "Database.Execute()" method seems suited for this with the remark that it can only execute "action queries", that is queries which don't return a result set (MSDN reference). For queries which return records "Database.OpenRecordset()" can be used. Both methods throw exceptions if the wrong type of query is passed.
Having both action queries and select queries in the list how can I decide upfront which will return records and which will not?

Answer 1

Note that the ADO Execute method can be used regardless of whether or not the query returns a resultset.
But do you really want to execute all 'queries'? What if they contain SQL DDL? Consider you created a PROCEDURE using this SQL DDL code:
CREATE PROCEDURE qryDropMe AS DROP PROCEDURE qryDropMe;
;)

Answer 2

Access maintains a hidden system table called MSysObjects which includes a field (Flags) which stores a value indicating the query type. You could try the following function with each of the query names from your list and use the return value to determine whether to use Database.Execute() or Database.OpenRecordset()
The function requires read permission for MSysObjects. I have heard reports than some Access 2007 users are denied permission to read MSysObjects. However, I haven't encountered that problem with Access 2007.
I tested several query types to determine the Flags values. If one of your queries is a type I didn't test, the function will return the Flags value as unrecognized. You can modify the function to include that Flags type.
The only DDL query I tested was DROP TABLE (Flags = 96).
Also, please be aware that not all "SELECT ... FROM ..." queries are select queries for your purpose (returning a recordset). A query such as "SELECT fields INTO newtable FROM oldtable;" does not return records, and the Access UI classifies it as a Make Table query.
Public Function QueryType(ByVal pQueryName As String) As String
Dim lngFlags As Long
Dim strType As String
Dim strCriteria As String
strCriteria = "[Name] = """ & pQueryName & """ And [Type] = 5"
lngFlags = DLookup("Flags", "MSysObjects", strCriteria)
Select Case lngFlags
Case 0
strType = "Select"
Case 16
strType = "Crosstab"
Case 32
strType = "Delete"
Case 48
strType = "Update"
Case 64
strType = "Append"
Case 80
strType = "Make Table"
Case 96
strType = "Drop Table"
Case 128
strType = "Union"
Case Else
strType = "Flags " & CStr(lngFlags) & " unrecognized"
End Select
QueryType = strType
End Function

Answer 3

Inspired by #HansUp's answer I investigated a bit more the QueryDef structure provided by the DAO interface. The structure has a "Type" property which I can use to differentiate between different query types (MSDN). I ended up with the following implementation:
function TAccessDatabase.SQLExec(AName, AQuery: String): Integer;
var
I: Integer;
QDef: QueryDef;
QDefExists: Boolean;
begin
Result := 0;
// Lookup querydef in the database
QDefExists := False;
for I := 0 to DB.QueryDefs.Count - 1 do
begin
QDef := DB.QueryDefs[I];
if QDef.Name = AName then
begin
QDefExists := True;
break; //-->
end;
end;
// Create query def if it doesn't exists
if not QDefExists then
begin
QDef := DB.CreateQueryDef(AName, AQuery);
// Refresh is required to get the correct QDef.Type_
DB.QueryDefs.Refresh;
end;
// Execute the query only if it is not a SELECT
if QDef.Type_ <> dbQSelect then
begin
db.Execute(AQuery, dbInconsistent);
Result := DB.RecordsAffected;
end;
end;
Thank you all for the helpful answers and remarks.

Answer 4

Why don't you catch the exception thrown and analyse it?
Do you have any way to use a beginTrans/Rollback instruction? You could then send your SQL command, collect the errors, then rollback your transactions, having your database left unchanged.
What about using ADO connection, somewhat smarter than the ADO one, where the connection hold an 'errors' collection and returns some other data like number of records affected?

Answer 5

This information applies to the type of the query. So:
All queries that execute a SELECT ... FROM ... are select queries
All INSERT, UPDATE, DELETE are action queries
You just have to inspect the query sql command text to look if it starts with any of the above keywords and act accordingly.