Null field not inserting NULL from empty TEdit - mysql

Question

I add new order details to a table in my db.
One of the columns there is named email, in the form I created to add the new order in my delphi application and subsequently the db, I use TEdits to pass the data along.
This is the code:
procedure TForm2.actAddComandaExecute(Sender: TObject);
begin
if dbmodule.comenziConnection.Connected then
begin
if addOrderForm.ShowModal=mrok then
begin
dbmodule.comenziQuery.SQL.Clear;
dbmodule.comenziQuery.SQL.Add( 'insert into `r33758pi_tipotask`.`comenzi` ( stare, client, telefon, email, detalii, livrare, pret, user, status, observatii ) values ( :stare, :client, :telefon, :email, :detalii, :livrare, :pret, :user, :status, :observatii ) ' ) ;
dbmodule.comenziQuery.Params.ParamByName( 'stare' ).AsString := addOrderForm.ComboBoxEx1.Text ;
dbmodule.comenziQuery.Params.ParamByName( 'client' ).AsString := addOrderForm.Edit2.Text ;
dbmodule.comenziQuery.Params.ParamByName( 'telefon' ).AsString := addOrderForm.Edit3.Text ;
dbmodule.comenziQuery.Params.ParamByName( 'email' ).AsString := addOrderForm.Edit4.Text ;
dbmodule.comenziQuery.Params.ParamByName( 'detalii' ).AsString := addOrderForm.Edit5.Text ;
dbmodule.comenziQuery.Params.ParamByName( 'livrare' ).AsString := addOrderForm.ComboBoxEx6.Text ;
dbmodule.comenziQuery.Params.ParamByName( 'pret' ).AsString := addOrderForm.Edit7.Text ;
dbmodule.comenziQuery.Params.ParamByName( 'user' ).AsString := addOrderForm.ComboBoxEx8.Text ;
dbmodule.comenziQuery.Params.ParamByName( 'status' ).AsString := addOrderForm.ComboBoxEx9.Text ;
dbmodule.comenziQuery.Params.ParamByName( 'observatii' ).AsString := addOrderForm.Edit10.Text ;
dbmodule.comenziQuery.ExecSQL ;
end;
end;
end;
If I leave Edit4 empty on the form everything inserts but the field email for that order doesn't have a Null value, it shows just as being empty - not null but no data either.
The column email is set as Null by default in db so that's not the problem.
Screenie from workbench:
There should be 2 NULL showing up between the values there but it's just empty.
Any ideas why?
Using Rad Studio 10 Seattle and dbExpress components
EDIT
comenziQuery is a TSQLQuery
dbmodule is the name of a data module that holds the db components
the dataset is a TSimpleDataSet
the comenziConnection is the name of a TSQLConnection
For any future readers, please do read all the answers comments also, great stuff in there.

Answer 1

An empty string is not the same thing as a NULL string. When it comes to SQL, you need to understand the difference.
You need to add some sort of logic to write string values vs null values, such as...
if addOrderForm.Edit5.Text <> '' then
dbmodule.comenziQuery.Params.ParamByName('detalii').AsString := addOrderForm.Edit5.Text
else
dbmodule.comenziQuery.Params.ParamByName('detalii').Value := NULL;
This way, if the edit control is empty, a NULL will get written to the table field instead of an empty string.
As suggested, this can further be wrapped inside a common function to save you on writing a ton of code:
function NullIfEmpty(const S: string): Variant;
begin
if S <> '' then
Result := S
else
Result := NULL;
end;
And then use it like...
dbmodule.comenziQuery.Params.ParamByName('detalii'):=
NullIfEmpty(addOrderForm.Edit5.Text);

Answer 2

The problem you are having is due to using ParamByName( 'email' ).AsString, which will set the email column to a blank string. If you want it to remain null I would use code like this to clear the parameter,
if Trim(addOrderForm.Edit4.Text) = '' then
dbmodule.comenziQuery.Params.ParamByName('email').Clear
else
dbmodule.comenziQuery.Params.ParamByName('email').AsString := Trim(addOrderForm.Edit4.Text);

Answer 3

You should use TDB* components which handle automatically such problems.