Fix proper set result field names when using join clause

Closes cznic#137
Context cznic#136

This PR make sure the field names for results nested select are set properly.

The observation from postgresql  shows that  for the following sql.

```sql
      BEGIN TRANSACTION;
		CREATE TABLE department (
		    DepartmentID   integer,
			DepartmentName text
		);

		INSERT INTO department (DepartmentID,DepartmentName) VALUES
			(31, 'Sales'),
			(33, 'Engineering'),
			(34, 'Clerical'),
			(35, 'Marketing');

		CREATE TABLE employee (
			LastName     text,
			DepartmentID integer
		);

		INSERT INTO employee VALUES
			('Rafferty', 31),
			('Jones', 33),
			('Heisenberg', 33),
			('Robinson', 34),
			('Smith', 34),
			('Williams', NULL);

     COMMIT;
```

```
gernest=# select * from (select * from employee) as a,(select * from department ) as d  order by d.DepartmentID  desc;
  lastname  | departmentid | departmentid | departmentname
------------+--------------+--------------+----------------
 Rafferty   |           31 |           35 | Marketing
 Robinson   |           34 |           35 | Marketing
 Williams   |              |           35 | Marketing
 Jones      |           33 |           35 | Marketing
 Smith      |           34 |           35 | Marketing
 Heisenberg |           33 |           35 | Marketing
 Jones      |           33 |           34 | Clerical
 Williams   |              |           34 | Clerical
 Robinson   |           34 |           34 | Clerical
 Heisenberg |           33 |           34 | Clerical
 Rafferty   |           31 |           34 | Clerical
 Smith      |           34 |           34 | Clerical
 Smith      |           34 |           33 | Engineering
 Jones      |           33 |           33 | Engineering
 Williams   |              |           33 | Engineering
 Heisenberg |           33 |           33 | Engineering
 Robinson   |           34 |           33 | Engineering
 Rafferty   |           31 |           33 | Engineering
 Robinson   |           34 |           31 | Sales
 Jones      |           33 |           31 | Sales
 Heisenberg |           33 |           31 | Sales
 Rafferty   |           31 |           31 | Sales
 Smith      |           34 |           31 | Sales
 Williams   |              |           31 | Sales
(24 rows)
```

Previously the names were set to empty strings .

all_test.go

@@ -17,6 +17,7 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"reflect"
 	"runtime"
 	"runtime/debug"
 	"strconv"
@@ -3501,3 +3502,67 @@ COMMIT;
 	}
 	wg.Wait()
 }
+
+func TestIssue136(t *testing.T) {
+	RegisterMemDriver()
+	db, err := sql.Open("ql-mem", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	tx, err := db.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = tx.Exec(`
+create table category (key int, name string);
+create table condition (key int, name string);
+create table product (key int, catkey int, condkey int);
+
+insert into category values (1, "foo"), (2, "hello");
+insert into condition values (1, "bar"), (2, "baz");
+insert into product values (1, 1, 1), (2, 2, null), (3, null, 2), (4, null, null);
+	`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = tx.Commit()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	rows, err := db.Query(`
+select *
+from
+  (select
+   product.key as product_key,
+   category.name as category_name,
+   product.condkey as product_condkey
+   from
+     product
+   left join category on category.key == product.catkey)
+left join condition on condition.key == product_condkey;
+	`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rows.Close()
+	c, _ := rows.Columns()
+	e := []string{
+		"product_key",
+		"category_name",
+		"product_condkey",
+		"condition.key",
+		"condition.name"}
+	if !reflect.DeepEqual(e, c) {
+		t.Errorf("expected %v got %v", e, c)
+	}
+	for rows.Next() {
+		var pk, pck, ck sql.NullInt64
+		var cn, cndn sql.NullString
+		err = rows.Scan(&pk, &cn, &pck, &ck, &cndn)
+		if err != nil {
+			t.Error(err)
+		}
+	}
+}

ql.go

@@ -1765,23 +1765,18 @@ func (r *joinRset) plan(ctx *execCtx) (plan, error) {
 				if f != "" && nm != "" {
 					f = fmt.Sprintf("%s.%s", nm, f)
 				}
-				if nm == "" {
-					f = ""
-				}
 				fields = append(fields, f)
 			}
 		}
 		rsets[i] = q
 	}
 
-	if len(rsets) == 1 {
-		return rsets[0], nil
-	}
-
-	if len(rsets) == 0 {
+	switch len(rsets) {
+	case 0:
 		return nil, nil
+	case 1:
+		return rsets[0], nil
 	}
-
 	right := len(rsets[len(rsets)-1].fieldNames())
 	switch r.typ {
 	case crossJoin:

testdata.log

@@ -404,7 +404,7 @@ SELECT * FROM (SELECT * FROM employee;), (SELECT * FROM department;);
 │   └Output field names ["LastName" "DepartmentID"]
 │   ┌Iterate all rows of table "department"
 │   └Output field names ["DepartmentID" "DepartmentName"]
-└Output field names ["" "" "" ""]
+└Output field names ["LastName" "DepartmentID" "DepartmentID" "DepartmentName"]
 
 ---- 75
 SELECT * FROM (SELECT * FROM employee;) AS e, (SELECT * FROM department;) ORDER BY e.LastName, e.DepartmentID;
@@ -413,9 +413,9 @@ SELECT * FROM (SELECT * FROM employee;) AS e, (SELECT * FROM department;) ORDER
 │   └Output field names ["LastName" "DepartmentID"]
 │   ┌Iterate all rows of table "department"
 │   └Output field names ["DepartmentID" "DepartmentName"]
-└Output field names ["e.LastName" "e.DepartmentID" "" ""]
+└Output field names ["e.LastName" "e.DepartmentID" "DepartmentID" "DepartmentName"]
 ┌Order by e.LastName, e.DepartmentID,
-└Output field names ["e.LastName" "e.DepartmentID" "" ""]
+└Output field names ["e.LastName" "e.DepartmentID" "DepartmentID" "DepartmentName"]
 
 ---- 76
 SELECT * FROM (SELECT * FROM employee;), (SELECT * FROM department;) AS d ORDER BY d.DepartmentID DESC;
@@ -424,9 +424,9 @@ SELECT * FROM (SELECT * FROM employee;), (SELECT * FROM department;) AS d ORDER
 │   └Output field names ["LastName" "DepartmentID"]
 │   ┌Iterate all rows of table "department"
 │   └Output field names ["DepartmentID" "DepartmentName"]
-└Output field names ["" "" "d.DepartmentID" "d.DepartmentName"]
+└Output field names ["LastName" "DepartmentID" "d.DepartmentID" "d.DepartmentName"]
 ┌Order descending by d.DepartmentID,
-└Output field names ["" "" "d.DepartmentID" "d.DepartmentName"]
+└Output field names ["LastName" "DepartmentID" "d.DepartmentID" "d.DepartmentName"]
 
 ---- 77
 SELECT * FROM employee, (SELECT * FROM department;) ORDER BY employee.LastName;
@@ -435,9 +435,9 @@ SELECT * FROM employee, (SELECT * FROM department;) ORDER BY employee.LastName;
 │   └Output field names ["LastName" "DepartmentID"]
 │   ┌Iterate all rows of table "department"
 │   └Output field names ["DepartmentID" "DepartmentName"]
-└Output field names ["employee.LastName" "employee.DepartmentID" "" ""]
+└Output field names ["employee.LastName" "employee.DepartmentID" "DepartmentID" "DepartmentName"]
 ┌Order by employee.LastName,
-└Output field names ["employee.LastName" "employee.DepartmentID" "" ""]
+└Output field names ["employee.LastName" "employee.DepartmentID" "DepartmentID" "DepartmentName"]
 
 ---- 78
 SELECT * FROM (SELECT * FROM employee;) AS e, (SELECT * FROM department;) AS d WHERE e.DepartmentID == d.DepartmentID ORDER BY d.DepartmentName, e.LastName;
@@ -879,9 +879,9 @@ SELECT * FROM employee AS e, (SELECT * FROM department;) ORDER BY e.LastName;
 │   └Output field names ["LastName" "DepartmentID"]
 │   ┌Iterate all rows of table "department"
 │   └Output field names ["DepartmentID" "DepartmentName"]
-└Output field names ["e.LastName" "e.DepartmentID" "" ""]
+└Output field names ["e.LastName" "e.DepartmentID" "DepartmentID" "DepartmentName"]
 ┌Order by e.LastName,
-└Output field names ["e.LastName" "e.DepartmentID" "" ""]
+└Output field names ["e.LastName" "e.DepartmentID" "DepartmentID" "DepartmentName"]
 
 ---- 155
 SELECT * FROM employee AS e, (SELECT * FROM department;) AS d ORDER BY e.LastName;

testdata.ql

@@ -811,7 +811,7 @@ FROM
 	SELECT *
 	FROM department
 );
-|"", "", "", ""
+|"LastName", "DepartmentID", "DepartmentID", "DepartmentName"
 [Williams <nil> 35 Marketing]
 [Williams <nil> 34 Clerical]
 [Williams <nil> 33 Engineering]
@@ -849,7 +849,7 @@ FROM
 	FROM department
 )
 ORDER BY e.LastName, e.DepartmentID;
-|"e.LastName", "e.DepartmentID", "", ""
+|"e.LastName", "e.DepartmentID", "DepartmentID", "DepartmentName"
 [Heisenberg 33 35 Marketing]
 [Heisenberg 33 34 Clerical]
 [Heisenberg 33 33 Engineering]
@@ -887,7 +887,7 @@ FROM
 	FROM department
 ) AS d
 ORDER BY d.DepartmentID DESC;
-|"", "", "d.DepartmentID", "d.DepartmentName"
+|"LastName", "DepartmentID", "d.DepartmentID", "d.DepartmentName"
 [Rafferty 31 35 Marketing]
 [Jones 33 35 Marketing]
 [Heisenberg 33 35 Marketing]
@@ -922,7 +922,7 @@ FROM
 		FROM department
 	)
 ORDER BY employee.LastName;
-|"employee.LastName", "employee.DepartmentID", "", ""
+|"employee.LastName", "employee.DepartmentID", "DepartmentID", "DepartmentName"
 [Heisenberg 33 35 Marketing]
 [Heisenberg 33 34 Clerical]
 [Heisenberg 33 33 Engineering]
@@ -1654,7 +1654,7 @@ FROM
 	employee AS e,
 	( SELECT * FROM department)
 ORDER BY e.LastName;
-|"e.LastName", "e.DepartmentID", "", ""
+|"e.LastName", "e.DepartmentID", "DepartmentID", "DepartmentName"
 [Heisenberg 33 35 Marketing]
 [Heisenberg 33 34 Clerical]
 [Heisenberg 33 33 Engineering]