Snappy DBExpress SQL Executor method

Question

There is no easy solution for this

All tests performed with Delphi XE3 and MySQL

Behavior Explanation

At designtime you will get the correct parameter data types as long as the parameters depend on a table field from the FROM table without aliases

SELECT id FROM items WHERE id = :id

but it will also fail if not

SELECT id FROM items WHERE id/2 = :id

and this will also fail

SELECT i.* FROM items i WHERE i.id = :id

At runtime both will result in parameters with datatype ftUnknown.

Parameters are setup in private method

// Delphi XE3
Data.SqlExpr.TCustomSQLDataSet.SetParameterFromSQL

Inside this method the tablename is extracted and only

if csDesigning in ComponentState then

a temporary DataSet is created with

SELECT * FROM <tablename> WHERE 0 = 1

Every parameter is checked against the fieldnames from this dataset and if names match the parameter datatype is set.

Thats the reason why you get ftUnknown for your parameters at runtime.

To solve that Delphi does the same you tried with your solution, but dbexpress fails sometime. The Setter for parameters is located at

Data.DB.TParam.SetAsVariant

and value 1234567 has the variant type varLongword and parameter datatype will be set to ftLongword and causes this error.

Workaround

As a workaround you can set parameter datatype to ftString/ftWideString, as this will work in most cases.

procedure TTestClass.ExecuteSql(const ASql : String; const ParamVals : Array Of Variant);
var
  i : integer;
  Qry : TSqlQuery;
begin
  Qry := NewQuery(ASql);
  with Qry do
  try
    for i := Low(ParamVals) to High(ParamVals) do
    begin
      Qry.Params[i].DataType := ftWideString;
      Qry.Params[i].Value := ParamVals[i];
    end;
    ExecSql();
  finally
    Free;
  end;
end;

To get a better solution you get a procedure to set the parameter datatype to ftString/ftWidestring only for critical variant types (like your method SetParamValues but more general)

procedure SetParamValues( const AParams : TParams; const AValues : array of Variant );
var
  LIdx : Integer;
  LParam : TParam;
  LValue : Variant;
begin
  for LIdx := 0 to Pred( AParams.Count ) do
  begin
    LParam := AParams[LIdx];
    LValue := AValues[LIdx];
    // only handle the critical parts
    case VarType( LValue ) of
      varByte, varLongword : LParam.DataType := ftWideString;
    end;
    // all other will be set here
    LParam.Value := LValue;
  end;
end;

Solution: the hard way

As i first stated there is no easy solution for this. For a full functional solution you have to parse the whole WHERE statement

SELECT a.*, b*
FROM table1 a
JOIN table2 b ON a.id = b.id
WHERE a.id = :id AND b.count / 2 = :halfcount

and build a query from this

SELECT a.id as Param1, b.count / 2 as Param2
FROM table1 a
JOIN table2 b ON a.id = b.id
WHERE 0 = 1

to get the expected datatype.

Solution: the long way

IMHO this is a bug and should be reported to EMBA ...

Solution: the expensive way

I made a test with UniDAC and everything is the same as dbExpress. Parameter datatype is ftUnknown and setting parameter value will set the datatype to ftLongword.

But there is one special case: You will not get an error and your query is processed as you expect.