function VarToSQL(AValue:Variant):string;
begin
  case VarType(AValue) of
    varString,varUnknown,varStrArg:Result:=''''+VarToStr(AValue)+'''';
    varSmallint,varInteger,varInt64,varWord,varLongWord:Result:=VarAsType(AValue,varInteger);
    varEmpty,varNull:Result:='NULL';
    varDouble,varCurrency:Result:=FloatForAccess(AValue);
    varDate:Result:=DateForMySQL(AValue);
  else
    Result:='NULL';
  end;
end;

где
Function FloatForAccess(N:Variant):String;
var s:string;
    i:integer;
begin
if VarIsNull(N) then begin
 Result:='NULL';
 exit;
 end;
s:=FloatToStr(N);
For i:= 1  to Length(S) do begin
    if s[i]=',' then s[i]:='.';
    end;
Result:=s;
end;

и
function DateForMySQL(D:Variant):string;//Под мускул
begin
if VarIsNull(D) then begin
 Result:='NULL';
 exit;
 end;
Result:=''''+FormatDateTime('yyyy"."mm"."dd',D)+'''';
end;