Fix select for non existing or empty tables

solution.pl

@@ -233,6 +233,17 @@ For example:
 findall(Values, selec(persons,[+id,+first],[],Values), Projections)
 returns: Projections = [[0, "Jeffrey"], [1, "Lorena"], [2, "Joseph"], ...
 */
+selec(Table, _, _, _) :-
+	\+ tabl(Table, _),
+	throw("Table doesn't exist"),
+	!.
+
+selec(Table, _, _, _) :-
+	tabl(Table, Cols),
+	length(Cols, L),
+	\+ current_predicate(Table/L),
+	!.
+
 selec(Table, Selector, Cond, Projection) :-
 	tabl(Table, Cols),
 	row(Table, R),
@@ -246,8 +257,6 @@ selec(Table, *, Cond, Projection) :-
 	does_match(Cond, Table, R),
 	Projection = Cols/R.
 
-%base case
-selector(_,_,[],ColumnNames, ColumnValue, ColumnNames, ColumnValue):-!.
 /**
  * Row: A row from the table
  * Cols: name of the column of the table
@@ -257,6 +266,9 @@ selector(_,_,[],ColumnNames, ColumnValue, ColumnNames, ColumnValue):-!.
  * ColumnNames: List returning the column(s) name(s)
  * ColumnValue: List returning the row values for each column 
 */
+
+selector(_,_,[],ColumnNames, ColumnValue, ColumnNames, ColumnValue):-!.
+
 selector(Row, Cols, [+Selector|Selectors], AccCols, AccVals, ColumnNames, ColumnValue):-
 	(member(Selector, Cols) ->
 		%find the value corresponding to the column in the row