Implement constant selection queries

Signed-off-by: Stefano Scafiti <stefano.scafiti96@gmail.com>

embedded/sql/engine_test.go

@@ -3153,6 +3153,23 @@ func TestQuery(t *testing.T) {
 			require.Equal(t, expectedRows[i].ValuesByPosition, row.ValuesByPosition)
 		}
 	})
+
+	t.Run("constant selection query", func(t *testing.T) {
+		_, err := engine.queryAll(
+			context.Background(),
+			nil,
+			"SELECT *",
+			nil,
+		)
+		require.ErrorContains(t, err, "SELECT * with no tables specified is not valid")
+
+		assertQueryShouldProduceResults(
+			t,
+			engine,
+			"SELECT 1, true, 'test'",
+			"SELECT * FROM (VALUES (1, true, 'test'))",
+		)
+	})
 }
 
 func TestJSON(t *testing.T) {

embedded/sql/parser_test.go

@@ -898,6 +898,19 @@ func TestSelectStmt(t *testing.T) {
 		expectedOutput []SQLStmt
 		expectedError  error
 	}{
+		{
+			input: "SELECT 1, true, 'test'",
+			expectedOutput: []SQLStmt{
+				&SelectStmt{
+					targets: []TargetEntry{
+						{Exp: &Integer{1}},
+						{Exp: &Bool{true}},
+						{Exp: &Varchar{"test"}},
+					},
+					ds: &valuesDataSource{rows: []*RowSpec{{}}},
+				},
+			},
+		},
 		{
 			input: "SELECT id, title FROM table1",
 			expectedOutput: []SQLStmt{

embedded/sql/proj_row_reader.go

@@ -36,6 +36,10 @@ func newProjectedRowReader(ctx context.Context, rowReader RowReader, tableAlias
 			return nil, err
 		}
 
+		if len(cols) == 0 {
+			return nil, fmt.Errorf("SELECT * with no tables specified is not valid")
+		}
+
 		for _, col := range cols {
 			targets = append(targets, TargetEntry{
 				Exp: &ColSelector{

embedded/sql/sql_grammar.y

@@ -717,6 +717,16 @@ select_stmt: SELECT opt_distinct opt_targets FROM ds opt_indexon opt_joins opt_w
                 offset: $13,
             }
     }
+|
+    SELECT opt_distinct opt_targets
+    {
+        $$ = &SelectStmt{
+            distinct: $2,
+            targets: $3,
+            ds: &valuesDataSource{rows: []*RowSpec{{}}},
+        }
+    }
+;
 
 opt_all:
     {